Process for the production of aromatic carboxylic acids

Disclosed is an improved process for the continuous production of aromatic carboxylic acids by the liquid-phase oxidation of an alkyl aromatic compound with an oxygen-containing gas in the presence of oxidation catalyst which effectively utilizes the heat of reaction in the process of removing excess water generated from the reaction and minimizes the loss of solvent used as the carrier for the reaction catalyst. Operation of the process is improved by removing reactor off-gas directly into a water removal column for distillation. A portion of distillate condensed from the overhead aqueous vapors of the water removal column is refluxed to the fractionating zone of the water removal column. A bottoms liquid of partially de-watered process solvent obtained from the water removal column is sprayed into the reactor above the phase separation of the gas/liquid contents thereby enriching the water content of the reactor off-gas to improve the efficiency of the water removal column without additional heat input beyond that of the heat of reaction.

TECHNICAL FIELD 
This invention pertains to an improved process for the continuous 
production of aromatic polycarboxylic acids by the liquid-phase oxidation 
of alkyl aromatic hydrocarbons with molecular oxygen in the presence of an 
oxidation catalyst or catalyst system. More particularly, this invention 
pertains to such oxidation processes carried out in a columnar oxidation 
reactor provided with means to effectively remove the excess water 
generated from the process with minimal solvent loss utilizing the energy 
of oxidation. 
BACKGROUND OF THE INVENTION 
The liquid-phase oxidation of an alkyl aromatic hydrocarbon to an aromatic 
carboxylic acid is a highly exothermic reaction commonly carried out in a 
vented, intimately-mixed, columnar oxidation reactor. The oxidation 
process comprises continuously feeding separately or in admixture an alkyl 
aromatic hydrocarbon, fresh and/or recycled solvent generally in aqueous 
solution, and catalyst components to the reactor to which a molecular 
oxygen-containing gas also is fed, normally at or near the bottom of the 
reactor. This process gas rises through the liquid contents of the reactor 
resulting in vigorous agitation of the reaction mixture and providing 
intimate contact between the alkyl aromatic hydrocarbon and the process 
solvent having dissolved therein the catalyst or catalyst components. The 
aromatic carboxylic acid produced is removed continuously through a lower 
exit port located at or near the base of the reactor as a slurry in the 
solvent which also contains soluble catalyst components. After separation 
of the aromatic carboxylic acid product, the solvent is returned to the 
reactor. 
Oxygen-depleted process gas, along with minor amounts of solvent 
decomposition products, is removed through an upper exit port located at 
or near the top of the reactor. The heat of reaction is also removed 
through the upper exit port by vaporization of the process solvent and 
water generated by the reaction. The oxygen-depleted process gas and the 
vaporized process solvent and water comprise the reactor off-gas which is 
typically condensed by means of one or more condensers to separate the 
solvent and water for recycling to the reactor. The condensed aqueous 
solvent may be subjected to a water removal step prior to recycling. 
The described production system can be utilized in the manufacture of 
aromatic carboxylic acids at excellent production rates relative to the 
volume of the reactor. One significant problem presented by the production 
system is the efficient removal of the excess water generated by the 
reaction since the water concentration must be held at an acceptable 
level, typically below 10 percent, for the reaction to continue at a 
reasonable rate. The reaction produces one mole of water per mole of 
carboxyl moiety produced. In addition, there are other side reactions 
which release water, i.e. the direct oxidation of the solvent to form 
by-products, and water may be added to the process for other reasons such 
as scrubbing off-gas for solvent recovery. Typically, water is removed by 
conventional distillation methods. 
Another problem is the effective removal of the heat of reaction to control 
vaporization of the reactants in the reactor. A widely practiced form of 
heat removal is to cool the reactor off-gas in a condenser and return the 
cold liquid to the reactor, as disclosed in U.S. Pat. No. 4,777,287. 
Alternatively, the heat of reaction has been.removed by circulating a 
portion of the product-containing liquid at the bottom of the reactor 
through a heat exchanger and returning it to the reactor, as disclosed in 
U.S. Pat. No. 4,855,492. 
To resolve the two aforementioned problems, the energy created by the heat 
of reaction has been utilized in the removal of excess water. In a side 
distillation process, the energy produced from condensation of the reactor 
off-gas has been used to generate steam, which is then used as part of the 
heat input to the side distillation column. However, this method does not 
effectively utilize the heat of reaction. Additional heat sources are 
generally required to accomplish distillation and additional heat 
exchangers must be added to the process. 
Direct distillation of the reactor off-gas to remove water has 
conventionally been employed utilizing the heat of reaction. However, 
process limitations exist. Since the amount of distillate reflux 
determines the purity of the overhead distillate and the heat input to the 
distillation process determines the amount of reflux which the process can 
accommodate, the heat of reaction fixes both the amount of reflux and the 
purity of the overhead distillate. The heat of reaction alone is generally 
insufficient to obtain a desirable overhead purity which minimizes solvent 
loss. Therefore, direct distillation requires additional heat input. 
Another effective method of water removal is by azeotropic distillation. 
U.S. Pat. No. 3,402,184 discloses reactor oxidation vapors being sent 
directly to an azeotropic distillation column in which benzene is the 
entraining medium. U.S. Pat. No. 4,250,330 discloses condensed solvent 
being sent to an isobutyl-acetate azeotropic distillation process. These 
methods, however, typically require expensive distillation equipment and 
additional heat exchange equipment. The process is more complicated and 
expensive since the entraining medium must be purchased, handled, 
recovered and replenished due to loss and degradation. 
Thus, there exists a need for a method to remove the excess water generated 
from the reaction by effectively using the heat from the energy of 
oxidation without requiring additional heat input or equipment while also 
minimizing solvent loss. 
SUMMARY OF THE INVENTION 
The present invention provides for the effective utilization of the heat of 
reaction in the process of removing excess water generated from the 
reaction while minimizing solvent loss. A distilled waste water of low 
solvent content is produced and the reactor water level is sufficiently 
maintained to allow for optimal production rates and product quality. 
These and other advantages are afforded by carrying out the oxidation of 
an alkyl aromatic hydrocarbon in a columnar reactor wherein the reactor 
off-gas is fed directly into a water removal column. A portion of the 
bottoms liquid from the water removal column comprising partially 
de-watered solvent is sprayed as reflux into the reactor through one or 
more spray nozzles. The spray reflux enters the reactor above the phase 
separation of the gas/liquid reaction mixture. A portion of distilled 
condensed water generated from the water vapor at the top of the water 
removal column is returned as reflux to the water removal column. 
Our invention thus provides a method for the continuous production of an 
aromatic polycarboxylic acid in a pressurized oxidation reactor by 
liquid-phase, exothermic oxidation of an alkyl aromatic hydrocarbon with 
an oxygen-containing gas in the presence of an oxidation catalyst and 
aqueous, C.sub.2 -C.sub.6 aliphatic, monocarboxylic acid solvent which 
comprises the steps of: 
(1) continuously feeding to a reactor alkyl aromatic hydrocarbon, aqueous, 
monocarboxylic acid solvent having oxidation catalyst dissolved therein, 
and an oxygen containing gas; 
(2) continuously removing from the lower portion of the reactor 
product-containing liquid comprising aromatic polycarboxylic acid and the 
aqueous, monocarboxylic solvent having the oxidation catalyst dissolved 
therein; 
(3) continuously removing from the upper portion of the reactor and feeding 
directly into a lower portion of a water removal column reactor off-gas 
comprising oxygen-depleted gas and vaporized aqueous, mono-carboxylic acid 
solvent; 
(4) continuously removing from the lower portion of the water removal 
column a bottoms liquid containing partially de-watered monocarboxylic 
acid solvent; 
(5) returning to the reactor at least a portion of the bottoms liquid 
obtained in step (4) in the form of a spray above the phase separation of 
the gas/liquid contents of the reactor; 
(6) continuously removing from the water removal column overhead aqueous 
vapors having minimal monocarboxylic acid solvent therein; 
(7) condensing the aqueous vapors into a distillate product; and 
(8) returning to the fractionating zone of the water removal column at 
least a portion of the distillate product obtained in step (7).

DETAILED DESCRIPTION OF THE INVENTION 
While the present invention is susceptible to embodiment in various forms, 
there is shown in the accompanying FIG. 1 and hereinafter described in 
detail a preferred embodiment of the invention. The present disclosure is 
to be considered as an exemplification of the invention without limitation 
to the specific embodiment illustrated. 
Referring to the accompanying FIG. 1, reactor feed mixture is introduced 
via conduit 10 into oxidation reactor 12. The reactor feed mixture 
comprises an alkyl aromatic hydrocarbon, an aqueous, C.sub.2 to C.sub.6 
monocarboxylic aliphatic acid solvent, and a suitable oxidation.catalyst 
which is typically dissolved in the solvent. The aliphatic, carboxylic 
acid solvent feed typically contains up to about 10 weight percent water. 
If desired, the alkyl aromatic compound, and/or aliphatic acid solvent 
containing catalyst components may be fed to reactor 12 at a plurality of 
points along the side of the reactor. An oxygen-containing gas under 
pressure is introduced near the bottom of the reactor 12 via conduit 14. 
The preferred oxygen-containing gas is air. The flow rate of the 
oxygen-containing gas to reactor 12 is controlled to maintain between 
about 2 and 9 volume percent oxygen (calculated on a dry, solvent free 
basis) in the off-gas which exits the reactor via conduit 16. The 
reactants in reactor 12 are maintained at an elevated pressure sufficient 
to maintain a contained, volatilizable reaction medium substantially in 
the liquid state at the reaction temperature. 
Reactor 12 is a columnar, pressurized, oxidation vessel wherein 
liquid-phase exothermic oxidation of the alkyl aromatic hydrocarbon by the 
oxygen-containing gas takes place in the presence of the oxidation 
catalyst. The reaction medium contained by reactor 12 thus comprises the 
oxygen-containing gas, the alkyl aromatic hydrocarbon that is to be 
oxidized to an aromatic carboxylic acid product, the catalyst, and the 
aqueous, C.sub.2 to C.sub.6 monocarboxylic aliphatic acid solvent. 
Utilizing the method of the present invention, the amount of water within 
the reactor does not generally exceed about 8 to 10 weight percent based 
on the weight of the water and the aliphatic, carboxylic acid. 
During the course of the oxidation reaction, exothermic heat of reaction 
and water generated by the oxidation of the alkyl aromatic compound are 
removed from the reactor 12 by vaporization of a portion of the liquid 
reaction medium. These vapors, known as reactor off-gas, comprise the 
aqueous solvent at about five (5) to thirty (30) weight percent water and 
oxygen-depleted process gas containing minor amounts of decomposition 
products including catalyst residue. The reactor off-gas passes upwardly 
through the reactor 12 and is introduced via conduit 16 into a lower 
portion of a water removal column 18 for distillation. The water removal 
column may be a distillation column having a fractionating zone of either 
a plurality of trays or a suitable packing for effecting mass transfer and 
may have twenty-five (25) or more equilibrium stages and a refluxed top 
section. 
Overhead aqueous vapors exit the upper portion of the water removal column 
18 through conduit 20 into a condenser 22. The composition of the 
condensible components of the aqueous vapors collected in the condenser 
22, known as the distillate, is above about ninety-nine (99) percent 
water. A portion of the distillate is returned as reflux to the 
fractionating zone of the water removal column 18 via conduits 23 and 4. 
The other portion of the distillate is removed for disposal via conduits 
23 and 26. The reflux ratio by weight ranges from four (4) to seven (7) 
parts reflux distillate to one (1) part disposal distillate. An additional 
stream of water (not shown) containing minor amounts of acid solvent 
generated from other water processes such as pump seals, vent scrubbers 
and water washing may be fed to the water removal column 18. The 
non-condensible components are vented via conduit 28 or may be transported 
to a pollution control device for further treatment if desired. 
A distilled bottoms liquid containing partially dewatered monocarboxylic 
aliphatic acid solvent of about four (4) to twelve (12) weight percent 
water exits the lower portion of the water removal column 18 via conduit 
30. A portion of the partially de-watered solvent is recycled directly to 
the reactor 12 via conduit 32. This amount ranges from ten (10) to one 
hundred (100) percent depending on the amount of partially de-watered 
solvent utilized for washing catalyst from a product-containing liquid of 
the reactor 12 as described below. In accordance with our invention, the 
partially de-watered solvent is fed to the reactor 12 by means of spray 
head 34 located below exit conduit 16 and above the phase separation of 
the gas/liquid contents of the reactor 12. The spray head 34 is designed 
to distribute the partially de-watered solvent in a finely divided form, 
e.g., droplets, over a substantial portion, preferably over all, of the 
surface of the phase separation of the gas/liquid reaction mixture. The 
particular means employed to feed the partially de-watered solvent in the 
form of a spray to the reactor is not critical so long as it provides 
liquid-gas contact at the top of the reactor. Thus, the spray may be 
created by means of a single spray head as shown in the FIG. 1 or by a 
plurality of spray nozzles. 
The reactor 12 continuously produces a product-containing liquid of an 
aromatic carboxylic acid that is continuously withdrawn as a slurry in the 
aqueous, monocarboxylic aliphatic acid solvent, which also contains 
dissolved catalyst. The product-containing liquid exits the bottom portion 
of the reactor 12 and is conveyed via conduit 36 to a suitable 
solid/liquid separation system 38. A second portion of the partially 
de-watered solvent as supplied from the bottom of the water removal column 
via conduits 30 and 40 is used in the separation system 38 for washing 
catalyst from the product-containing liquid. The liquid phase recovered 
from separation system 38 comprising solvent with dissolved catalyst 
components and water is recycled to the bottom portion of reactor 12 via 
conduit 42. The solids phase contains the product of the process, aromatic 
carboxylic acid compound, and is transported for recovery via conduit 44. 
In distillation processes, high purity distillate products are typically 
desired. The purity or richness of the distillate is determined by the 
amount of reflux, i.e. higher reflux ratio, richer distillate. However, if 
the reflux is increased, the amount of heat to operate the distillation 
process must also be increased. Thus, the amount of reflux that the 
distillation process can accommodate, as well as the purity of the 
distillate, is limited by the heat of reaction unless additional heat 
input is provided. With the improvement of the present invention, that is 
the introduction into the reactor of a spray of partially de-watered 
solvent obtained as the bottoms liquid from the water removal column, the 
efficiency of the distillation process is increased over conventional 
distillation processes to provide a distillate of above ninety-nine (99) 
percent water accomplished utilizing only the exothermic heat of reaction. 
Such level of distillate purity is generally not attainable without 
additional heat sources which also increase cost. It is the interaction 
between the water removal column and the reactor through the spray and 
reactor off-gas that causes the reactor to act as a reboiler for the water 
removal column, thus eliminating the need for additional heat input to 
effect distillation. 
The spray effects a higher concentration of water and a lower concentration 
of aliphatic acid solvent in the reactor off-gas due to the vaporization 
of the water in the spray. The atomized water from spraying readily 
vaporizes into the reactor off-gas limiting any increase in the water 
concentration of the reaction medium, which must be maintained at a 
certain level for the reaction to proceed. By lowering the concentration 
of solvent fed to the water removal column, less energy is needed for 
separation and the reflux ratio can be increased without additional heat 
input to effect a richer concentration of water in the distillate. The 
heat of reaction now provides the necessary energy to result in the water 
rich distillate. Thus, the operating efficiency of the water removal 
system is increased since more water generated from the oxidation reaction 
may exit the process as distillate through conduit 26 with minimal solvent 
loss at no additional cost related to increased heat input. The valuable 
aliphatic acid solvent is confined to the process in the bottoms liquid of 
the water removal column rather than rejected to waste. 
Furthermore, the spray knocks down entrained solids and vaporized material 
which produce solids upon cooling, thereby preventing the fouling or 
plugging of process equipment such as lines, condensers and distillation 
columns. It also decreases the concentration of bromine-containing 
compounds in vapor conduit 16 which decreases the rate of corrosion of 
process equipment. The spray also permits the use of higher pressures and 
higher levels of the gas/liquid reaction mixture within the reactor which 
increase the overall efficiency of the process. 
Examples of suitable alkyl aromatic hydrocarbons useful as reactor 
feed-mixture components or ingredients in the method of the present 
invention and their respective aromatic carboxylic acid products are as 
follows: 
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hydrocarbon acid 
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toluene benzoic acid 
o-xylene orthophthalic acid 
m-xylene isophthalic acid (IPA) 
p-xylene terephthalic acid (TPA) 
1,2,3-trimethyl benzene 
hemimellitic acid 
1,2,4-trimethyl benzene 
trimellitic acid 
1,2,5-trimethyl benzene 
trimesic acid 
2,6- and 2,7-dimethyl 
2,6- and 2,7-naphthalene 
naphthalene dicarboxylic acids. 
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The method is particularly well suited for the production of TPA, IPA, 
trimellitic acid, trimesic acid and the naphthalenedicarboxylic acids. 
Suitable aqueous aliphatic acid solvents useful in the method of this 
invention are those that are readily volatilizable at the reaction 
temperatures. Among such solvents are aqueous solutions of C.sub.2 to 
C.sub.6 mono-carboxylic acids, e.g., acetic acid, propionic acid, 
n-butyric acid, isobutyric acid, n-valeric acid, trimethylacetic acid, 
caproic acid, and mixtures thereof. Preferably, the volatilizable 
monocarboxylic aliphatic acid solvent is acetic acid. 
The catalyst systems which may be employed in the oxidation process include 
any catalyst system conventionally used for liquid-phase oxidation of an 
alkyl aromatic hydrocarbon. A suitable catalyst system may include a 
mixture of cobalt, manganese and bromine compounds or complexes, soluble 
in the particular volatilizable aqueous solvent employed. 
In a specific example of the improved process of the present invention, 
p-xylene at a rate of 1000 parts by weight per hour was fed to the reactor 
12 and oxidized to produce medium purity TPA at a rate of 1600 parts by 
weight per hour. The reactor used was a vertical bubble column having a 
height:diameter ratio of 13.3. All amounts of material were measured in 
parts by weight. 
Aqueous acetic acid containing dissolved catalyst was fed at the rate of 
about 30 parts per hour and p-xylene was fed at the rate of about 1000 
parts per hour via conduit 10 to the reactor 12. Air with 0.5% water was 
fed via conduit 14 at a rate of about 4900 parts per hour air. The 
oxidation reaction medium filled approximately 85% of the volume of the 
reactor. The temperature of the vigorously mixed reaction medium was about 
140.degree. to 160.degree. C. and the pressure was controlled at about 75 
psia. Typical operating temperatures and pressures range respectively from 
about 120.degree. to 180.degree. C. and from about 50 to 175 psia. TPA 
slurried in acetic acid was removed from the reactor via conduit 36 at the 
rate of about 1600 parts per hour. 
A reactor off-gas stream comprising oxygen--depleted air, acetic acid and 
water was removed continuously via a port located at the top of the 
reactor and transported via conduit 16 to the lower portion of the water 
removal column 18. The water concentration in conduit 16 was about 9.4% by 
weight based on condensibles. A bottom liquid of partially de-watered 
acetic acid with a water concentration of about 6% by weight was removed 
via conduit 30 from the water removal column 18 at a rate of about 15,800 
parts per hour acetic acid. A portion of the partially de-watered solvent 
was fed to the reactor via conduit 32 and spray head 34 at a rate of about 
9000 parts per hour acetic acid. The remainder of the partially de-watered 
solvent was fed to the separation system 38 via conduit 40. 
Overhead aqueous vapors continuously exited the upper portion of the water 
removal column through conduit 20 into a condenser 22. The condensible 
liquid components comprising 99.5% by weight water and 0.5% by weight 
acetic acid exited the condenser 22 through conduit 23 at a rate of about 
3600 parts per hour. A portion of the condensate was refluxed to the water 
removal column 18 via conduit 24 at a rate of about 3200 parts per hour. 
The remainder of the condensate exited the system via conduit 26 at a rate 
of about 470 parts per hour. The resulting reflux ratio of the water 
removal column was about 6.8. The non-condensibles exited the condenser 22 
via conduit 28 at a rate of about 4100 parts per hour. 
The invention has been described in detail with particular reference to 
preferred embodiments thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.