Method for increasing conversion efficiency for oxidation of an alkyl aromatic compound to an aromatic carboxylic acid

A method and system for increasing conversion efficiency of aromatic alkyl reactant to aromatic carboxylic acid product and for improving the quality of the product, are disclosed. The method and system provide for the continuous production of an aromatic carboxylic acid by the liquid phase, exothermic oxidation of an aromatic alkyl in a vaporizable solvent in an oxidation reactor. The reactor makes use of a vented, overhead condenser system and a separator system for condensation of vaporized reactor material, separation of the condensed solvent therefrom, and reflux of separated solvent back into the reactor. The improvement comprises combining the reactor liquid feedstream with the refluxed solvent upstream from the oxidation reactor.

FIELD OF THE INVENTION 
This invention relates generally to the continuous, liquid phase oxidation 
of an aromatic alkyl to an aromatic carboxylic acid. More particularly, 
the present invention concerns a method and system for increasing reactor 
conversion efficiency and for improving the aromatic carboxylic acid 
product quality as well. 
BACKGROUND OF THE INVENTION 
Liquid phase oxidation of an aromatic alkyl to an aromatic carboxylic acid 
is a highly exothermic chemical reaction. Volatilizable aqueous acidic 
solvents are used to contain the reaction mixture and to dissipate the 
heat of reaction. Conventionally, the oxidation of aromatic alkyls in the 
liquid phase to form aromatic carboxylic acids is generally performed in a 
vented, well-mixed oxidation reactor, with a substantial portion of the 
heat generated by the exothermic oxidation reaction being removed by 
evaporating directly from the reaction mixture a portion of the aqueous 
solvent and aromatic alkyl contained within the reactor. 
The materials vaporized as a result of the heat generated in the exothermic 
reaction, together with unreacted oxygen and other aqueous components that 
may be present, pass upwardly through the reactor and are withdrawn from 
the reactor at a point above the reaction mixture liquid level for the 
reactor. The vapors are passed upwardly and out of the reactor to an 
overhead reflux condenser system where the vaporized solvent, water and 
aromatic alkyl are condensed. The resultant condensate is thereafter 
separated, e.g., in a reflux splitter, into a portion having a relatively 
higher water concentration and a portion having a relatively lower water 
concentration. The separated portion having a relatively lower water 
concentration, now at a temperature less than the reactor contents' 
temperature, is refluxed back into the reactor by gravity. Conventionally, 
the refluxed portion of the condensate is returned directly to the reactor 
through a process line external to the reactor. The non-condensable gases, 
carried along with the vaporized reactor material, are vented. 
In operation, the reactor is fed by a liquid feed stream containing the 
aromatic alkyl, aqueous acidic solvent and an oxidation catalyst. An 
oxygen-containing gas is separately introduced into the reactor for 
oxidizing the aromatic alkyl to the aromatic carboxylic acid in the 
presence of the catalyst. 
The reaction mixture contained in the reactor typically comprises a 
suspension of crystalline aromatic carboxylic acid in liquid, 
volatilizable, aqueous acidic solvent as mother liquor. The mother liquor 
contains, in addition to dissolved catalyst, some dissolved aromatic 
carboxylic acid product and lesser amounts of partially-converted species 
of such product. The mother liquor can also include a minor amount of 
unreacted, aromatic alkyl. 
Aromatic carboxylic acid product quality is measured by optical density. At 
present, optical density of the obtained product limits the oxidation 
reactor operating temperature and pressure, as well as the reactor 
throughput and mother liquor recycle rate into the reactor. Because of the 
commercial importance of the oxidation of aromatic alkyls, however, it is 
highly desirable to improve the reactor conversion efficiency and quality 
of aromatic carboxylic acids produced by the oxidation of aromatic alkyls. 
The invention disclosed herein tends to diminish so-called reactor 
"entrance" effects, thought to be caused by an oxygen deficiency at the 
point where the reactor feedstream feeds the reactor. The invention 
disclosed herein also tends to minimize color-body generation, known to 
limit aromatic carboxylic acid plant operating flexibility and capacity. 
SUMMARY OF THE INVENTION 
The present invention is an improvement in a method and in a system for the 
continuous production of an aromatic carboxylic acid by liquid phase 
oxidation of an aromatic alkyl in an oxydation reactor. The improvement 
includes combining upstream from the reactor a liquid feed stream, 
containing an aromatic alkyl, with condensed acidic solvent medium that is 
refluxed back into the reactor, thereby providing a reflux-containing feed 
mixture, and then introducing the reflux-containing feed mixture into the 
oxidation reactor. A system embodying the present invention includes a 
liquid-liquid mixing means for effecting the "combining" step.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Aromatic carboxylic acid is produced in an oxidation reactor at an elevated 
temperature and pressure by liquid phase, exothermic oxidation of an 
aromatic alkyl by an oxygen-containing gas in a vaporizable, aqueous 
acidic solvent medium. Oxidation of the aromatic alkyl to the aromatic 
carboxylic acid takes place in the aqueous acidic solvent medium in the 
presence of an oxidation catalyst. The conversion of aromatic alkyl to 
aromatic carboxylic acid is exothermic. Heat generated in the oxidation 
reaction is at least partially dissipated by vaporization of a portion of 
the solvent, water, aromatic alkyl and other vaporizable constituents of 
the reaction mixture present in the oxidation reactor. Vaporized reaction 
mixture constituents are withdrawn from the oxidation reactor, condensed 
in an overhead condenser system, and separated in a reflux splitter or a 
similar device into condensate portions having different water 
concentrations. Condensate portion having a relatively lower water 
concentration, and thus a relatively higher acidic solvent concentration, 
is fed back into the oxidation reactor. 
A liquid feedstream for the oxidation reactor contains the aromatic alkyl, 
the acidic solvent medium, and an effective amount of an oxidation 
catalyst for effecting in the reactor a liquid phase oxidation of the 
aromatic alkyl, in the presence of oxygen, to the aromatic carboxylic 
acid. The improvement of the present invention comprises combining the 
refluxed condensate portion with the oxidation reactor liquid feed stream 
upstream from the oxidation reactor to produce a reflux-containing liquid 
feed mixture which is at a temperature below the reactor contents' 
temperature. The reflux-containing feed mixture is then introduced into 
the oxidation reactor. 
Referring to FIG. 1, an elongated, vertically-disposed, continuous 
stirred-tank oxidation reactor 10 for oxidizing an aromatic alkyl to an 
aromatic carboxylic acid is shown. The oxidation reaction is continuous 
and proceeds in the liquid phase. The reactor 10 includes an agitator 12 
which drives impeller blades 14, fixed to an agitator shaft 15. The 
reactor 10 further includes internal baffles (not shown). Each impeller 
blade 14 is rotated by the shaft 15 in a generally horizontal plane at a 
pre-selected rotational speed so that the contents of the reactor 10 are 
well mixed. 
The contents of the reactor 10 are subjected to an elevated pressure and 
temperature sufficient to maintain the contained volatilizable solvent and 
aromatic alkyl substantially in the liquid state. 
An aromatic alkyl, such as para-xylene, from a source 16, and a 
volatilizable aqueous acidic solvent medium, such as a catalyst-containing 
aqueous acetic acid solution, from a source 18, are combined to form a 
mixture. A liquid, reactor reflux stream from reflux splitter 48, 
contained in transfer pipe 32 and having a relatively higher acetic acid 
concentration than the non-refluxed condensate portion exiting via 
discharge pipe 48, is further combined with the formed mixture and is 
introduced into the reactor 10, via side inlet 36, as will be described in 
greater detail below. An oxygen-containing gas from a source 20 is 
introduced into the bottom of the reactor 10 via a gas inlet line 66. The 
oxygen-containing gas serves to oxidize the aromatic alkyl to an aromatic 
carboxylic acid in the presence of the catalyst. 
Localized pockets of relatively low oxygen concentration or relatively high 
aromatic alkyl or catalyst concentration, such as are in the vicinity of 
the reactor inlet or the reactor baffles, are thought to reduce conversion 
efficiency of aromatic alkyl to aromatic carboxylic acid. To counteract 
these so-called "entrance" and "other" effects, it has been discovered 
that, when the reactor feed stream containing the aromatic alkyl and the 
volatilizable aqueous acidic solvent medium (the solvent medium containing 
the oxidation catalyst) is combined with the liquid reactor reflux stream 
to produce a reflux-containing mixture and the reflux-containing mixture 
is then introduced into the reactor 10, the overall conversion efficiency 
of aromatic alkyl to aromatic carboxylic acid is increased and the product 
quality is improved as well. 
The prior art teaches recycling the reflux stream to the bottom of the 
reactor 10 and introducing the feedstream into the side of the reactor 10. 
The present invention, however, contemplates introducing the combined 
reflux-containing liquid feed mixture either at the bottom or the side of 
the reactor 10, as desired. 
Accordingly, in one embodiment of this invention, a liquid-liquid mixing 
means, such as the piping "T" connection 28 (FIG. 1), is provided. The 
aromatic alkyl is supplied to the "T" connection 28 by an inlet pipe 30 
which carries the aromatic alkyl feed stream from source 16 via pipe 22 
and the aqueous acidic solvent (containing the oxidation catalyst) from 
source 18 via pipe 24. The aromatic alkyl and aqueous acidic solvent 
mixture is further combined with the reactor reflux stream in "T" 
connection 28, with the reflux stream being introduced into "T" connection 
28 by transfer pipe 32. The resultant reflux-containing reactor feed 
exiting the "T" connection 28 is transferred via discharge pipe 34 into 
the reactor 10 either at side inlet 36, bottom inlet 38, or both, as 
desired. The reactor side-inlet 36 is located below the reactor liquid 
level D. The temperature of the reflux-containing feed mixture is less 
than the reactor temperature. 
The source of oxygen for the oxidation of this invention can vary. Air and 
oxygen-enriched gas such as oxygen-enriched air or gaseous oxygen can be 
used. The oxygen-containing gas, from whatever source, supplied to the 
reactor 10 provides sufficient oxygen to result in an exhaust gas-vapor 
mixture containing from about two to about eight volume percent oxygen 
(measured on a solvent-free basis) when the oxidation reactor is in 
operation. For example, when each alkyl substituent on the aromatic ring 
of the aromatic alkyl is a methyl group, a feed rate of the 
oxygen-containing gas sufficient to provide oxygen in the amount of from 
about 1.4 to about 2.8 moles per methyl group will provide such two to 
eight volume percent oxygen concentration in the gas-vapor mixture in the 
condenser 40. 
In operation, the minimum pressure at which the reactor 10 is maintained is 
that pressure which will maintain a substantial amount of the aromatic 
alkyl present in the liquid phase and at least about 70 percent of the 
volatilizable, aqueous acidic solvent in the liquid phase. When the 
aqueous acidic solvent is an acetic acid-water mixture, suitable gauge 
pressures in the reactor 10 can be up to about 35 kg/cm.sup.2 and 
typically are in the range of about 10 kg/cm.sup.2 to about 30 
kg/cm.sup.2. 
The process temperature employed is, on the one hand, low enough that the 
oxidation occurs with relatively low heat losses but, on the other hand, 
is high enough so that an economically desirable degree of conversion of 
the aromatic alkyl to the corresponding aromatic carboxylic acid is 
obtained. Process temperatures suitable for use in practicing the method 
of this invention generally are in the range of about 120.degree. C. to 
about 240.degree. C., preferably about 150.degree. C. to about 230.degree. 
C. Various narrower ranges may be preferred for a particular aromatic 
alkyl being oxidized. For example, when the aromatic alkyl is para-xylene, 
the preferred overall temperature range within the reactor 10 is about 
175.degree. C. to about 225.degree. C., and the preferred temperature of 
the reflux-containing liquid feed mixture is about 85.degree. C. 
The residence time of the reactor is defined as the quotient of the reactor 
liquid volume divided by the liquid feed-stream flow rate into the reactor 
10. Typically, in a commercial operation, the residence time in the 
reactor 10 is in the range of about 20 to about 90 minutes. 
Suitable aromatic alkyls for use in the method of this invention include 
toluene, ortho-, meta-, and para-xylene, and the trimethylbenzenes. The 
respective aromatic carboxylic acid products are benzoic acid, 
orthophthalic acid, isophthalic acid, terephthalic acid, and the 
benzenetricarboxylic acids. Preferably, the method of this invention is 
used to produce terephthalic acid, isophthalic acid, and trimellitic acid 
(1, 2, 4-benzenetricarboxylic acid). More preferably, the method of this 
invention is used to produce terephthalic acid. 
Suitable volatilizable, aqueous acidic solvents for use in the method of 
this invention can be aqueous solutions of any C.sub.2 -C.sub.6 fatty acid 
such as acetic acid, propionic acid, n-butyric acid, isobutyric acid, 
n-valeric acid, trimethylacetic acid, caproic acid, and mixtures thereof. 
The volatilizable acidic solvent preferably is aqueous acetic acid. When 
combined with the liquid feed, the volatilizable aqueous solvent 
preferably contains from about 0.5 to about 20 weight percent of water. 
However, after being combined with the reflux stream, the resultant 
reflux-containing liquid feed fed to reactor 10 can contain up to about 35 
weight percent of water. 
Suitable catalysts for present purposes include any catalyst system 
conventionally used for liquid phase oxidation of an aromatic alkyl. A 
suitable catalyst system preferably includes a mixture of cobalt, 
manganese and bromine compounds or complexes, that are soluble in the 
particular volatilizable, aqueous acidic solvent employed. When the 
catalyst system comprises soluble forms of cobalt, manganese or bromine, 
the cobalt (calculated as elemental cobalt) preferably is present in the 
range about 0.1 to about 10.0 milligram atoms (mga) per gram mole of the 
aromatic alkyl. Similarly, the manganese (calculated as elemental 
manganese) is preferably present in the ratio of about 0.1 to about 10.0 
mga per mga of the cobalt. Further, the bromine (calculated as elemental 
bromine) is preferably present in the ratio of from about 0.2 to about 1.5 
mga per mga of total cobalt and manganese (both on an elemental basis). 
In the method and system embodiments of this invention in which the 
catalyst system employed comprises a mixture of soluble forms of cobalt, 
manganese and bromine, and the solvent is aqueous acetic acid, each of 
cobalt and manganese can be provided in any of its known ionic or combined 
forms that are soluble in aqueous acetic acid solutions. For example, such 
forms can include cobalt and/or manganese carbonate, acetate tetrahydrate, 
and/or bromide. However, the desired catalysis cannot be effected by 
bromides of both cobalt and manganese. Rather, the catalysis can be 
effected by appropriate ratios of the bromide salts and other aqueous 
acetic acid-soluble forms containing no bromide; for example, the 
acetates. As a practical matter, and by way of non-limiting example, a 
0.1:1 to 10:1 ratio of manganese mga to cobalt mga is provided through use 
of the aqueous, acetic acid-soluble forms other than bromides; for 
example, both as acetate tetrahydrates. A 0.2:1 to 1.5:1 ratio of 
elemental bromine mga to total cobalt and manganese mga is provided by a 
source of bromine. Such bromine sources include elemental bromine 
(Br.sub.2), and ionic bromides (for example, HBr, NaBr, KBr, NH.sub.4 Br, 
etc.). 
Heat of reaction in the reactor 10 vaporizes the volatilizable solvent, 
water and reaction mixture contained therein. A substantial portion of the 
heat generated by the exothermic reaction in the reactor 10 is removed 
from the reaction mixture by vaporization of the aqueous solvent and, to a 
lesser extent, the aromatic alkyl. The vaporized material and any 
unreacted oxygen and other components of the oxygen-containing gas fed to 
the reactor 10 pass upwardly through the reactor 10 and are withdrawn from 
the reactor 10 via the exit pipe 42. The vaporized materials contained 
within pipe 42 are received into an overhead condenser system such as the 
condenser 40, are condensed, and are conveyed by a transfer line 44 into a 
reflux splitter 46 in which the condensed solvent phase is separated into 
two portions having different acid, and thus water, concentrations. Such 
liquid-liquid splitters are well-known in the art and will not be 
described herein. (See, e.g., Perry's Chemical Engineers' Handbook, 6th 
Ed., published 1984 by McGraw-Hill, at pages 21-64 through 21-83.) 
The amount of acidic solvent contained in the refluxed portion of the 
condensed solvent, being dictated by the operation of the reflux splitter 
46 and the overall plant economics, of course, can vary. However, a major 
portion of the water produced by the liquid phase oxidation of the 
aromatic alkyl is removed in the non-refluxed portion of the condensate in 
reflux splitter 46 via discharge pipe 48 from the reflux splitter 46 to 
storage means 50 for further use, or to waste, as desired. The 
non-refluxed portion of the condensate contains water, a relatively lower 
concentration of aromatic carboxylic acid, and a minor amount of aromatic 
alkyl. 
The refluxed portion of the condensate from the reflux splitter 46, 
containing aromatic alkyl, water, a relatively higher acidic solvent 
concentration, and aromatic carboxylic acid, is returned from reflux 
splitter 46 into reactor 10 via transfer line 32, and is combined with the 
aromatic alkyl in the "T" connection 28, as described above. A pump 54 can 
be used to assist flow of reflux through line 32 into the "T" connection 
28, if desired. In this manner, localized oxygen starvation in pockets of 
high aromatic alkyl and catalyst concentrations within the reactor 10 is 
avoided. 
To effect condensation, coolant is introduced into the condenser 40 through 
coolant inlet pipe 56 and exits via coolant discharge pipe 58. The 
condensate from condenser 40 flows generally downwardly and through 
transfer line 44, and upwardly into the splitter 46. Non-condensable 
gases, included with the vaporized reactor material introduced into the 
condenser 40, are vented from the separator 46 through a vent pipe 60 
which includes a flow-control valve 62. Preferably, the oxygen 
concentration from the vent gas is about three to about four percent 
oxygen by volume, but can be in the overall range of about two to about 
eight percent oxygen by volume. 
The reaction mixture, which typically comprises a suspension of crystalline 
aromatic carboxylic acid in liquid, volatilizable, aqueous acidic solvent 
mother liquor, is conventionally transferred by a discharge pipe 64 to 
suitable crystallizers (not shown). The discharge pipe 64 is located below 
the reactor liquid level D. The reactor feed pipe 36 is preferably located 
on the reactor 10 lower than the reactor discharge pipe 64 and is spaced 
about 180 degrees therefrom to minimize the likelihood of any aromatic 
alkyl, introduced by inlet 36, being in the reactor 10 for less than the 
desired residence time. 
In the reactor 10, the aromatic alkyl is oxidized by oxygen, usually 
introduced as air at the bottom of reactor 10 by inlet pipe 66, in the 
presence of the catalyst, to form the desired aromatic carboxylic acid and 
intermediates thereto. A product stream is withdrawn as an effluent stream 
from the reactor 10 via the discharge pipe 64. The product stream is 
thereafter treated using conventional techniques to separate its 
components and to recover the aromatic carboxylic acid contained therein, 
usually by crystallization. 
A further embodiment of the present invention is illustrated in FIG. 2. As 
between FIGS. 1 and 2, like reference numerals have been assigned to like 
parts or elements of the present invention. Further, for the sake of 
brevity, and because the function of many of the parts or elements 
appearing in FIG. 2 have been described in connection with FIG. 1, only 
those parts or elements of FIG. 2 which have not been discussed heretofore 
will be discussed at length hereinbelow. 
As shown in FIG. 2, a liquid-liquid mixing means 68 is used to combine the 
aromatic alkyl and aqueous acidic solvent from line 30 with the reflux 
from line 32 to provide the reflux-containing liquid feed mixture to be 
fed via side-inlet line 36 or bottom-inlet line 38 into the reactor 10. 
Such a liquid-liquid mixing means can be a so-called "static" mixer or any 
one of a large number of other flow or line mixers well-known in the art. 
(See, e.g., Perry's Chemical Engineers' Handbook, 6th Ed., at pages 21-57 
through 21-64.) Preferably, the choice of a liquid-liquid mixing means 68 
is such that it does not require the use of a pump such as the optional 
pump 54 in transfer pipe line 32. 
Such liquid-liquid mixing means can be a liquid-handling device 
conventionally referred to as a jet pump. A jet pump is a suitable 
liquid-handling device which makes use of the momentum of one fluid to 
move another. The preferred liquid-liquid mixing means, shown in FIG. 3, 
is a liquid-liquid ejector 70, a type of jet pump which is well-known in 
the art. 
The liquid-liquid ejector 70 shown in FIG. 3 includes an aromatic alkyl and 
aqueous solvent mixture inlet port 72, a reactor reflux suction port 74 
and a discharge port 76. Fluid momentum originating at the sources 16 
and/or 18 forces the aromatic alkyl and aqueous solvent mixture into and 
through the first venturi 78 which feeds the second venturi 80 thereby 
creating suction at suction port 74 and causing the reactor reflux stream 
to enter the suction port 74 and flow into the ejector fluid suction 
chamber 82. From the suction chamber 82 the reflux stream enters the 
fluid-mixing chamber 83 where the liquid-phase solvent reflux is combined 
and mixes with the mixture of aromatic alkyl and aqueous acidic solvent. 
The fluid momentum provided at sources 16 and 18 usually is sufficient to 
discharge the resultant mixture from the liquid-liquid ejector 70 via 
discharge port 76 and through side inlet 36 or bottom inlet 38 into the 
reactor 10, as desired. To facilitate clean-out of the liquid-liquid 
ejector 70, first and second threaded clean-out plugs 84 and 86 are 
provided. 
Combining the reactor-reflux stream with the reactor-feed stream increases 
the ratio of acidic solvent to aromatic alkyl in the resulting combined 
feed stream. It has been found that this tends to increase overall 
conversion efficiency of aromatic alkyl to aromatic carboxyl acid as well. 
The prior art method of recycling reflux from the reflux splitter 46 into 
the reactor 10 teaches introducing the reactor feed stream (containing the 
aromatic alkyl and aqueous solvent) at a reactor location point spaced 
from the reflux return point. When the aromatic alkyl is para-xylene and 
the aqueous solvent is aqueous acetic acid, the ratio of acetic acid to 
para-xylene in the reactor feed under the prior art scheme is about 3:1 
(volumetric basis). In contradistinction, when the aromatic alkyl and 
aqueous acidic solvent mixture is combined with the reactor reflux, as 
discussed above in connection with the present invention, the refluxed 
portion of the condensate has a relatively higher acetic acid 
concentration and the ratio of acetic acid to para-xylene in the resultant 
reactor feed stream is about 14:1 (volumetric basis). 
The foregoing description exemplifies preferred embodiments of the present 
invention. Still other variations and rearrangements of component parts 
are possible without departing from the spirit and scope of this invention 
and will readily present themselves to one skilled in the art.