Title: Process for producing carboxylic acids
Abstract: Improved process for producing aromatic carboxylic acids by catalytic liquid phase oxidation of a corresponding precursor in a suitable solvent comprising feeding the reactants to a first oxidation reaction zone at high pressure and high solvent ratio, wherein uptake of oxygen is less than that required for full conversion of the precursor to its corresponding carboxylic acid, and then feeding the resulting reaction medium to a second oxidation reaction zone.
Patent Number: 6,949,673 Issued on 09/27/2005 to Housley, et al.
| Inventors:
|
Housley; Samuel Duncan (Yarm, GB);
Turner; John A (Stokesley, GB)
|
| Assignee:
|
E.I. Du Pont de Nemours and Company (Wilmngton, DE)
|
| Appl. No.:
|
393971 |
| Filed:
|
March 21, 2003 |
| Current U.S. Class: |
562/412; 562/77; 562/413; 562/414; 562/416 |
| Intern'l Class: |
C07C 051/16; C07C 051/25.5 |
| Field of Search: |
562/414,413,77,412,416
|
References Cited [Referenced By]
U.S. Patent Documents
| 3686293 | Aug., 1972 | Gualdi et al.
| |
| 4269805 | May., 1981 | Schoengen et al.
| |
| 4593122 | Jun., 1986 | Hashizume et al.
| |
| 5004830 | Apr., 1991 | Park et al.
| |
| Foreign Patent Documents |
| WO 98/3815/0 | Sep., 1998 | WO.
| |
| WO 99/3103/8 | Jun., 1999 | WO.
| |
| WO 99/5995/3 | Nov., 1999 | WO.
| |
Primary Examiner: Tsang; Cecilia J.
Assistant Examiner: Oh; Taylor Victor
Attorney, Agent or Firm: Krukiel; Charles E.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S. application Ser. No. 09/481,811
filed Jan. 12, 2000, U.S. application Ser. No. 09/757,458 filed Jan. 9, 2001, both
abandoned, and U.S. application Ser. No. 09/884,381 filed Jun. 19, 2001 now abandoned.
Claims
1. A process for producing terephthalic acid by catalytic liquid phase oxidation
of paraxylene in a solvent selected from an aliphatic carboxylic acid or a non-aliphatic
organic acid, said solvent optionally including water, said process comprising:
(a) feeding the solvent, an oxidation catalyst, the paraxylene precursor, and
a supply of oxygen to a first reaction zone to form a reaction medium in which
the solvent: precursor mass ratio is in the range from 10-20;1 and the operating
pressure is at least about 2,500 kPa;
(b) limiting the uptake of oxygen within the reaction medium in said first reaction
zone to a value which is less than that required for full conversion of the paraxylene
to terephthalic acid such that said terephthalic acid produced in the reaction
medium in the first reaction zone remains in solution;
(c) feeding the reaction medium to a second reaction zone while simultaneously
reducing the pressure of the reaction medium to a value in the range of from 500
kPa to less than 2,500 kPa;
(d) within the second reaction zone, vaporizing a portion of the solvent present
in the reaction medium and removing the vapor from the reactor overhead; and
(e) condensing the vapor from the second reaction zone and recycling some or
all of the condensate from the second zone to the first reaction zone.
2. The process of claim 1, wherein the uptake of oxygen within the reaction medium
in said first reaction zone is limited, to a value less than 70 percent of that
required for full conversion of the paraxylene to terephthalic acid.
3. The process of claim 1, wherein less than 10 percent by weight of terephthalic
acid precipitates as a solid in the first reaction zone.
4. The process of claim 1, wherein less than 1 percent by weight of the terephthalic
acid precipitates as a solid in the first reaction zone.
5. The process of claim 1, wherein there is no precipitation of terephthalic
acid as a solid in the first reaction zone.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an improved process for producing aromatic carboxylic
acids by catalytic liquid phase oxidation of a corresponding precursor in a suitable
solvent. More particularly, the present invention is an improved process for catalytic
liquid phase air oxidation of paraxylene to produce terephthalic acid which comprises
sequential steps of feeding the reactants to a first reaction zone at elevated
pressure wherein the temperature and the uptake of oxygen are controlled and any
terephthalic acid which forms remains in solution, and then feeding the resulting
reaction medium to a second reaction zone to complete the oxidation reaction.
Practically all terephthalic acid is produced on a commercial scale by
catalytic, liquid phase air oxidation of paraxylene. Commercial processes use acetic
acid as a solvent and a multivalent heavy metal or metals as catalyst. Cobalt and
manganese are the most widely used heavy metal catalysts, and bromine is used as
a renewable source of free radicals in the process.
Acetic acid solvent, air (molecular oxygen), paraxylene and catalyst are fed
continuously into an oxidation reactor that is maintained at from 150° C.
to 225° C. and from about 500 to 2,500 kPa (i.e., 5-25 atm). The feed solvent:paraxylene
mass ratio is typically less than 5:1. Air is added in amounts in excess of the
stoichiometric requirements for full conversion of the paraxylene to terephthalic
acid, to minimize formation of undesirable by-products, such as color formers.
The oxidation reaction is exothermic, and heat is removed by allowing the acetic
acid solvent to vaporize. The corresponding vapor is condensed and most of the
condensate is refluxed to the reactor, with some condensate being withdrawn to
control reactor water concentration (two moles of water are formed per mole of
paraxylene reacted). The residence time is typically 30 minutes to 2 hours, depending
on the process. Depending on oxidation reactor operating conditions, e.g., temperature,
catalyst concentration and residence time, significant degradation of the solvent
and precursor can occur, which, in turn, can increase the cost of operating the process.
The effluent, i.e., reaction product, from the oxidation reactor is a slurry
of crude terephthalic acid (TA) crystals which are recovered from the slurry by
filtration, washed, dried and conveyed to storage. They are thereafter fed to a
separate purification step or directly to a polymerization process. The main impurity
in the crude TA is 4 carboxybenzaldehyde (4-CBA), which is incompletely oxidized
paraxylene, although p-tolualdehyde and p-toluic acid can also be present along
with undesirable color formers. By conducting the oxidation reaction according
to the invention as described in greater detail below, it is possible to substantially
reduce the formation of impurities in the final TA product and effectively control
solvent and precursor degradation.
SUMMARY OF THE INVENTION
The present invention is an improved process for producing aromatic carboxylic
acids by catalytic liquid phase oxidation of a corresponding precursor in a suitable
solvent. In particular, the present invention is an improved process for the catalytic
liquid phase oxidation of paraxylene to produce terephthalic acid. The process
of the present invention comprises sequential steps of feeding the reactants, including
a suitable solvent, to a first reaction zone at elevated pressure wherein the temperature
and the uptake of oxygen are controlled and any terephthalic acid which forms remains
in solution, and then feeding the resulting reaction medium to a second oxidation
reaction zone.
In a preferred embodiment of the invention, the process comprises:
- (a) feeding a solvent, an oxidation catalyst, paraxylene and a supply
of oxygen to a first reaction zone to form a reaction medium in which the solvent:paraxylene
mass ratio is in the range of from 5-30:1 and the operating pressure is in the
range of at least about 2,500 kPa;
- (b) limiting the uptake of oxygen within the reaction medium in the
first reaction zone to a value which is less than that required for full conversion
of the paraxylene to terephthalic acid such that any terephthalic acid which forms
substantially remains in solution; and
- (c) feeding the reaction medium to a second reaction zone while simultaneously
reducing the pressure of the reaction medium to a value in the range of from about
500 kPa to less than 2,500 kPa.
The resulting terephthalic acid can be recovered from the reaction medium exiting
the second reaction zone by any convenient means.
While the preferred embodiment of the invention is described herein in terms
of an improved oxidation system for converting paraxylene to terephthalic acid,
it will be recognized that the invention is applicable to producing a range of
aromatic carboxylic acids, and particularly phthalic acids, by catalytic liquid
phase oxidation of a corresponding precursor in a suitable solvent. The invention
resides in the discovery that the conversion of the precursor to its corresponding
carboxylic acid can be substantially improved by carrying out the oxidation reaction
in at least two stages, or zones, which comprise:
(1) feeding a solvent, an oxidation catalyst, a precursor, and a supply of oxygen
to a relatively high pressure, e.g., at least about 2,500 kPa, first reaction zone
to form a reaction medium in which the solvent:precursor mass ratio is in the range
of from 5-30:1, preferably 10-20:1; and
(2) feeding the reaction medium from the first reaction zone to a second reaction
zone, where the oxidation reaction runs to completion, that is, substantially complete
conversion of the precursor to the corresponding carboxylic acid.
In addition to maintaining the solvent:precursor mass ratio as described, the
uptake of oxygen in the first reaction zone is limited to a value which is less
than that required for full conversion of the precursor to its corresponding carboxylic
acid. The corresponding carboxylic acid can have one or more acid groups, depending
on the precursor.
Oxygen uptake in the first reaction zone is controlled by one or more of the
following methods: (i) maintaining oxygen supply within a predetermined range;
(ii) maintaining catalyst concentration within a predetermined range; (iii) limiting
the residence time (defined as the reactor liquid volume divided by the reactor
feed rate) within the first reaction zone to less than about 6 minutes, but preferably
less than 4 minutes; and (iv) optionally removing heat from the reaction zone.
One aspect of the invention is limiting the uptake of oxygen within the reaction
medium in the first reaction zone to a value less than that required for full conversion
of the precursor to the corresponding aromatic carboxylic acid. Preferably, the
oxygen uptake within the reaction medium in the first reaction zone is less than
70 percent of the oxygen required for full conversion of the precursor to the corresponding
carboxylic acid.
Simultaneously while feeding the reaction medium to the second reactor
the pressure of the reaction medium is reduced to a value in the range of from
500 kPa to less than 2,500 kPa. The carboxylic acid which results can be recovered
from the final reaction medium, which is typically a slurry of acid crystals, by
conventional methods.
In one embodiment of the invention, a feed stream comprising a solvent and an
oxidation catalyst is prepared and oxygen is dissolved directly into the feed stream.
The oxygenated feed stream is then fed continuously and simultaneously with the
precursor into the first oxidation reaction zone, which is a plug flow reaction
zone. Immediately upon entering the first reaction zone the precursor, e.g., paraxylene,
is thoroughly mixed with the oxygenated solvent to initiate the reaction. By controlling
the oxygen supply, catalyst concentration, residence time in and/or temperature
of the first reaction zone, it is possible to control, i.e., limit, the uptake
of oxygen within the reaction medium to a value which is less than that required
for full conversion of the precursor to its corresponding carboxylic acid. The
reaction medium is then fed to a second, more conventional, reactor as described above.
The process of the invention is particularly applicable to producing terephthalic
acid by catalytic liquid phase oxidation of paraxylene in a solvent comprising
acetic acid and water.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified schematic diagram of the process of the invention according
to one embodiment.
FIG. 2 is a simplified schematic diagram of the process of the invention according
to a preferred embodiment.
FIG. 3 is a simplified schematic diagram of an alternative to the process diagram
shown in FIG. 2 wherein a pumped reactor recycle is illustrated.
DETAILED DESCRIPTION OF THE INVENTION
The present invention resides in the discovery that it is possible to produce
aromatic carboxylic acids with improved efficiency and quality compared to the
processes of the prior art. The present invention is characterized by a two-stage
process. The first stage is carried out in a first reaction zone at a relatively
high pressure, e.g., at least 2,500 kPa. The second stage is carried out in a second
reaction zone at a lower pressure than the first stage, e.g., from about 500 to
less than 2,500 kPa.
The first reaction zone of the process is characterized by a relatively high
solvent:precursor mass ratio in the range of from 5-30:1, and a relatively high
pressure, e.g., in the range of from at least 2,500 kPa up to 30,000 kPa or even
higher. Best results for the overall process have been observed when the solvent:precursor
mass ratio in the first reaction zone is in the range of from 10-20:1. For the
purposes of this disclosure, the solvent:precursor mass ratio is defined as follows:
(a) the solvent mass flow is the total solvent flow through the first reaction
zone, including any solvent recycled from the second reaction zone and downstream
vessels, but excluding solvent recycled within the first reaction zone
(b) the precursor mass flow is the total precursor flow to the process,
whether the precursor is exclusively fed to the first reaction zone or whether
a proportion is arranged to bypass the first reaction zone to be fed directly to
the second reaction zone.
The first reaction zone is optionally cooled to control the temperature of the
reaction medium as it exits the first reaction zone. Control of temperature, catalyst
concentration, reactor residence time, and/or maintaining the oxygen supply to
the first reaction zone within a predetermined range makes it possible to conveniently
limit the uptake of oxygen within the reaction medium to a value which is less
than that required for full conversion of the paraxylene to terephthalic acid.
Temperature control can be established, for example, by placing an internal
cooling coil or other cooling device within the first reaction zone, by employing
a cooling jacket to surround the reactor or by circulating the reaction medium
through a heat exchanger located externally from the reactor.
Catalyst control can be established by, for example, routing some of the
catalyst-containing mother liquor directly to the second reaction zone, bypassing
the first reaction zone.
The invention is characterized in that the terephthalic acid (TA) formed in the
first reaction zone remains substantially in solution. By "substantially," the
inventors mean that it is preferred to have very little, e.g., less than about
10 percent by weight of solid TA precipitate from solution in the first reaction
zone. It is more preferred to have only a trace, e.g., less than 1 percent of solid
TA precipitate from solution in the first reaction zone. It is most preferred to
avoid precipitation of solid terephthalic acid in the first reaction zone.
Formation of terephthalic acid in the first reaction zone is limited by
limiting oxygen uptake in the first reaction zone. Precipitation of terephthalic
acid is prevented by maintaining a high solvent:precursor mass ratio, by maintaining
a sufficiently high reaction medium temperature, and by selecting an appropriate
coolant (e.g., boiling water) and cooling means that avoids cold spots from forming
at any location within the reaction zone.
On exiting the first reaction zone, the pressure of the reaction medium is reduced
simultaneously as it is fed to a more conventional oxidation reactor. This reactor
could typically be a stirred tank reactor or a bubble column reactor, for example.
Pressure reduction can be conveniently accomplished by passing the reaction medium
through one or a plurality of pressure letdown valves positioned about the periphery
of the reactor. Best results have been obtained when the reaction medium is dispersed
rapidly upon entering the second reactor. Rapid dispersion can be achieved by using
established methods for dispersing paraxylene-containing feeds in conventional
reactors. In a stirred tank reactor, for example, this would include injecting
the reaction medium into the reactor below the liquid line in close proximity to
the discharge from an agitator impeller. Rapid dispersion of the reaction medium
can be achieved in a bubble column reactor by injecting the reaction medium in
close proximity to the air feeds.
Referring now to the drawings, FIG. 1 is a simplified schematic diagram
of the process of the invention according to one embodiment. As mentioned above,
the process will be described as it relates to the production of terephthalic acid,
although the invention is applicable to the production of a range of aromatic carboxylic
acids and mixtures thereof.
In the illustrated embodiment shown in FIG. 1, the process is carried out by
first
forming a feed stream
10 comprising solvent, i.e., acetic acid and water,
and-oxidation catalyst. In practice the feed stream will comprise a mixture comprising
(i) recycled solvent, recycled mother liquor and catalyst, line
11, (ii)
reactor condensate from the second reactor, line
12, and (iii) fresh acetic
acid make-up, line
13. The mixed feed stream will contain typical catalyst
components (e.g., Co, Mn, Br), at generally diluted concentrations from what would
normally be present when using a single conventional oxidation reactor. Optionally,
but not shown, control of catalyst concentration in the first reaction zone can
be achieved by bypassing some of the catalyst-containing mother liquor, line
11,
directly to second reactor
20.
The mixed feed stream will generally have a temperature in the range of from
130° C. to 160° C., based on the temperature of the various components
which form the feed stream. However, the temperature of the feed stream is not critical.
The pressure of feed stream
10 is raised via a suitable pump
14
to a value of at least about, but generally in excess of about, 2,500 kPa, and
the feed stream is introduced continuously and simultaneously into a first stirred
tank reactor
15 with paraxylene, via line
16, and a source of oxygen,
via line
17.
The supply of oxygen via line
17 can be air, oxygen-enriched air, oxygen
mixed with a gas such as, for example, carbon dioxide, or essentially pure oxygen.
When the source of oxygen includes nitrogen or another sparingly soluble carrier
gas, the extent of cooling in the first reaction zone and its operating pressure
are preferably chosen such that the vapor present in the first reaction zone is
fuel-lean, i.e., the hydrocarbon content of the vapor is below the Lower Explosive
Limit (LEL). When the source of oxygen includes a soluble carrier gas or when essentially
pure oxygen is used, the extent of cooling in the first reaction zone and its operating
pressure are preferably chosen such that there is no vapor phase present in the
first reaction zone. Optionally, but not shown in FIG. 1, some or all of the oxygen
can be pre-dissolved directly into feed
10 via a mixing device located downstream
of the feed pump
14.
The paraxylene feed
16 may optionally be pre-mixed with acetic acid solvent
and introduced into the system either upstream or downstream of feed pump
14.
Optionally, but not shown, a portion of paraxylene feed
16 may bypass reactor
15 and be fed directly to second reactor
20. The reaction medium
which results in the first reactor, without is bypassing any paraxylene to the
second reaction zone, has a solvent:paraxylene mass ratio in the range of from
5-30:1. Best results have been observed for this embodiment when the solvent:paraxylene
mass ratio is in the range of from 10-20:1.
In cases where a portion of the paraxylene feed, i.e., line
16 in FIG.
1 and line
31 in FIGS. 2 and 3, is arranged to bypass the first reactor
and is fed directly to second reactor
20, the effective solvent:paraxylene
mass ratio in the reaction medium in the first reactor will adjust upward in response
to that portion of the paraxylene which bypasses the first reactor, and the resulting
mass ratio may reach a value in the range of up to 100:1 or even higher. The paraxylene
feed, line
6, should be dispersed rapidly upon entering the first reactor.
This can be accomplished by using any of the established methods for rapidly dispersing
paraxylene-containing feeds in conventional reactors. In a stirred tank reactor
15, as shown in the embodiment of the invention illustrated in FIG. 1, this
would include injecting the feed in close proximity to the discharge from an agitator
impeller. Although a stirred tank reactor is shown in FIG. 1, other conventional
oxidation reactor configurations may also be used with satisfactory results.
The process is carried out in the presence of an oxidation catalyst system, which
can be homogeneous or heterogeneous. A homogeneous catalyst is normally used and
is selected from one or more heavy metal compounds, such as, for example, cobalt,
manganese and/or zirconium compounds. In addition, the catalyst will normally also
include an oxidation promoter such as bromine. The catalyst metals and oxidation
promoter largely remain in solution throughout the process and are recovered and
recycled, following product recovery, with fresh catalyst make-up as a solution.
The feed stream to the first reaction zone, line
10, contains typical
oxidation catalyst components (e.g., Co, Mn, Br), but diluted by a factor of about
3 to 5 relative to the catalyst concentration in recycled mother liquor from product
recovery, line
11. The catalyst concentration is subsequently raised to
more conventional catalyst concentration levels when and as solvent is vaporized
and removed overhead in the second reaction zone
20. The total catalyst
metals concentration in the first reaction zone will typically lie in the range
150 to 1,000 ppm w/w, whereas the catalyst metals concentration in the second reaction
zone will typically lie in the range 500 to 3,000 ppm w/w. When using a Co and
Mn metal catalyst system, the total catalyst metals concentration in the first
reaction zone should preferably be controlled at greater that about 200 ppm w/w
for good catalyst selectivity and activity.
The oxidation reaction is highly exothermic. Depending on the oxygen uptake and
solvent ratio and without a means of cooling the reaction, the heat of reaction
could raise the temperature of the first reaction medium to a relatively high value,
e.g. in excess of 300° C.
Optionally, the first reaction zone may include a cooling coil
18
or employ some other internal or external means for removing heat from the reactor
(and reaction medium). It is important that the temperature of the coolant is sufficient
to prevent cold spots from forming which can result in localized precipitation
of terephthalic acid (TA).
Maintaining the supply of oxygen to the first reaction zone within a
predetermined range and controlling the exit temperature, catalyst concentration
and residence time of the reaction medium makes it possible to limit the uptake
of oxygen within the reaction medium to a value which is less than that required
for full conversion of the paraxylene to TA. In a preferred embodiment of the present
invention, the oxygen uptake within the reaction medium in the first reaction zone
is a value which is less than about 70 percent of the oxygen required for full
conversion of the precursor to the corresponding carboxylic acid.
Thus, according to the invention, paraxylene is converted in first reactor
15 primarily to TA intermediates, such as p-tolualdehyde, p-toluic acid
and 4-CBA. Under the described process conditions, with effective exit temperature
control, the first reactor will not produce any solid TA.
The reaction medium exiting first reactor
15 is fed via line
19
to a second reactor, i.e., oxidation zone,
20, which, as shown, can be a
conventional, continuously stirred tank reactor. Simultaneously, the pressure of
the reaction medium is reduced to a value in the range of from about 500 kPa to
less than 2,500 kPa. As described above, pressure reduction can be conveniently
accomplished by passing the reaction medium through one or a plurality of pressure
letdown valves or nozzles
21 positioned about the periphery of reactor
20
whereby the reaction medium is dispersed rapidly by injection into an agitator
impeller region below the liquid line of the reactor.
Where the source of oxygen to the first reactor includes nitrogen or another
sparingly soluble carrier gas, spent or excess air from first reactor
15,
line
22, can be mixed with a fresh supply of air or oxygen-containing gas,
line
22a, and the resulting mixed feed gas stream introduced and
rapidly dispersed into the reaction medium in second reactor
20 by any convenient
means. Alternatively, spent or excess air from first reactor
15 can be fed
directly to condenser
24, as shown via dotted line
22b, with
exclusively fresh air or oxygen-containing gas being fed to second reactor
20.
Where the source of oxygen to the first reactor zone is essentially pure oxygen
there will be no spent air from first reactor
15. In this case, exclusively
fresh air or oxygen-containing gas is fed to second reactor
20. Where the
oxygen supply to first reactor
15 is from an air separation plant, the waste
nitrogen from the air separation plant may be mixed with the oxygen-containing
gas supply to second reactor
20.
TA will precipitate to form a slurry within reactor
20, and it can be
recovered
from the reactor system via line
23 using conventional methods. Reactor
overhead vapor from reactor
20, which will necessarily contain some acetic
acid and water, is condensed via condenser
24, and most of the condensate
is returned, i.e., recycled, via line
12 for feed stream make-up to first
reactor
15. A proportion of the acetic acid and water condensate stream
(so-called water draw off) is diverted to a solvent dehydration system to remove
the water of reaction. Optionally, but not shown, a portion of the condensate may
be returned to reactor
20, to the reactor headspace, via a reflux slinger,
and/or to the reaction zone, via a separate feed line or by mixing with the existing
feed stream, line
19. Optionally, but not shown, the overhead vapor from
reactor
20 may be fed to a rectifier column, with the bottom product, or
condensate, from the rectifier recycled, via line
12, for feed stream make-up
to first reactor
15.
FIG. 2 is a simplified schematic diagram of a preferred embodiment of the invention.
The first reaction zone, i.e., first reactor
30, according to this embodiment
is a plug flow reactor. The term "plug flow reactor" is used herein to define a
generally elongated, or tubular, reaction zone in which rapid and thorough radial
mixing of the reactants occurs as they flow through the tube or conduit. The invention,
however, is intended to embrace any reactor configuration which approximates to
a plug flow reaction zone.
As described above in connection with FIG. 1, feed stream
10 is a mixed
feed stream comprising (i) recycled solvent, recycled mother liquor and catalyst,
via line
11, (ii) second reactor condensate, via line
12, and (iii)
fresh acetic acid make-up, via line
13. Optionally, but not shown, control
of catalyst concentration in the first reaction zone can be achieved by bypassing
some of the catalyst-containing mother liquor, line
11, directly to second
reactor
20. The supply of oxygen in this embodiment, line
17a,
is essentially pure gaseous oxygen.
The mixed feed stream will generally have a temperature in the range of from
130° C. to 160° C., depending on the temperature of the makeup streams.
A temperature in the range of about 140° C. to 160° C. has been found
to be suitable for initiating the oxidation reactions.
The pressure of mixed feed
10 is raised to a value in the range of at
least, but generally in excess of about, 2,500 kPa by any suitable pumping means
14. The pressure is chosen to ensure that all of the gaseous oxygen, introduced
via line
17a, will be readily dissolved in the feed stream ahead
of first reactor
30 as shown. The mixed feed stream with dissolved oxygen
is then fed simultaneously and continuously into plug flow reactor
30 with
paraxylene being fed via line
31, and the reaction is initiated. The paraxylene
may optionally be pre-mixed with acetic acid solvent and the mixture fed via line
31. Optionally, but not shown, the paraxylene feed
31 may be pre-mixed
with mixed feed stream
10, upstream or downstream of feed pump
14,
but upstream of oxygen injector
33. Further optionally, but not shown, a
portion of paraxylene feed
31 may bypass reactor
30 and be fed directly
to second reactor
20. In cases where a portion of paraxylene feed
31
is fed directly to second reactor
20, the effective solvent: paraxylene
mass ratio in the reaction medium in the first reactor will adjust upwardly in
response to that portion of the paraxylene feed which bypasses the first reactor,
and the resulting mass ratio may reach a value in the range of up to 100:1 or even higher.
Molecular oxygen is dissolved in the mixed feed stream using any convenient
in-line mixing device
33 to achieve a concentration of dissolved oxygen
in the mixed feed stream of up to 5.0% w/w. Mixing device
33 could be an
in-line nozzle arranged to discharge oxygen directly into the feed stream. In-line
static mixers (not shown) can also be positioned upstream of first reactor
30
to facilitate mixing.
It is also possible according to the invention to stage the introduction of oxygen,
i.e., to introduce the oxygen at a plurality of locations along the length of first
reaction zone
30. By staging oxygen injection, the maximum local oxygen
concentration is reduced, and this, in turn, permits a reduction in reactor operating pressure.
In practice, feed stream
10 is fed into plug flow reactor
30 with
paraxylene to form a reaction medium in which the resulting solvent: paraxylene
mass ratio is at least about 5:1, although the solvent: paraxylene mass ratio can
be as high as 30:1 or even higher. In a preferred embodiment the solvent: paraxylene
mass ratio is in the range of 10-20:1.
Residence time of the reaction medium within plug flow reaction zone
30
is relatively short, i.e., less than 6 minutes.
The reactor
30 shown in FIG. 2 is a shell and tube design. The reaction
medium flows through the tubes, while a coolant, e.g., pressurized water (PW),
is introduced into the shell side where it boils and is removed as steam (S). A
small water purge (boiler blowdown, BB) is taken to control impurity/residue build-up
in the water system. The temperature of the reaction medium as it exits first reactor
30 is controlled by controlling the pressure of the produced steam, and
hence its temperature.
Controlling the process parameters as described according to the invention
makes it possible to limit the uptake of oxygen within the reaction medium in the
first reaction zone to a value which is less than that required for full conversion
of the paraxylene to TA. Thus, paraxylene is converted in first reactor
30
primarily to TA intermediates, such as p tolualdehyde, p toluic acid and 4 CBA.
Under the described process conditions, the first reactor will not produce any
solid TA.
Although a shell and tube reactor design is shown in FIG. 2, reactor
30
can be any suitable reactor design with optional heat removal and optional multiple
oxygen injection. For example, the reactor can have multiple tube passes, with
oxygen injection into the reaction medium upstream of each tube pass. Alternatively,
a pumped circulating loop reactor can be employed, with oxygen injection into the
loop and heat removal from the loop as illustrated in FIG.
3. Optionally,
but not shown, the reactor can comprise an un-cooled (adiabatic) reaction vessel
with heat removal from a suitable cooling device downstream of the reaction vessel
or in the circulating loop
The reaction medium exiting first reactor
30 is fed via line
19
as described above in connection with the process embodiment shown in FIG. 1, to
a second reactor, i.e., oxidation zone,
20.
*
