Process for purification of aromatic polycarboxylic acids

The present invention provides a process for purifying crude aromatic polycarboxylic acids having one or more condensed rings, wherein two or more carboxylic acid groups can be at any position of the aromatic ring or rings. The process uses solvents consisting of two families of organic compounds as major solvent: a monocyclic compound containing two hetero-atoms and alkylamine compound, and two families of co-solvent: water and alcohol, to separate impurities from the crude acids. The product purity from the instant invention can be achieved to a level that is significantly better than the current state of the art.

BACKGROUND OF THE INVENTION 
The purification process in the instant invention applies to aromatic 
polycarboxylic acids having one or more condensed aromatic rings with two 
or more carboxylic acid groups at any position of the aromatic ring or 
rings. Typical examples of one-ring polycarboxylic acids are terephthalic 
acid, isophthalic acid and trimellitic acid; two ring aromatic 
polycarboxylic acids, 2,6-naphthlene dicarboxylic acid, 2,7-naphthalene 
dicarboxylic acids; three ring aromatic polycarboxylic acids, 
2,3,6-anthracene tricarboxylic acid, etc. The background of terephthalic 
acid purification process is discussed first because of its largest 
commercial production quantity and it is the most difficult to purify due 
to its low solubility in most solvents, high boiling point, and 
similarities in physical and chemical properties with impurities present. 
Those skilled in the art would recognize that most principles discussed 
below for terephthalic acid are also applicable to other aromatic 
polycarboxylic acids. 
Terephthalic acid has been well established as a starting material for 
manufacture of polyester fibers, films, and molding resin. However, the 
presence of its major impurities: p-toluic acid, benzoic acid, or 
4-carboxybenzaldehyde (4-CBA), even in minute amounts, will adversely 
affect the quality of the polyester product from polymerization of 
terephthalic acid with ethylene glycol (MEG) into poly-ethylene 
terephthalate (PET). The impurities, such as monoffinctional p-toluic acid 
and benzoic acid, act as polymerization terminators that slow down the 
polymerization rate and decrease the average molecular weight of the 
polymer. Some other impurities, such as 4-CBA, cause undesirable coloring 
of the polymer as a consequence of their thermal instability during 
polymerization. 
The purity specification for technical-grade terephthalic acid is 98.5+ wt 
%. However this purity is not high enough to be used as raw material for 
polyester production. Before polymer-grade, or pure terephthalic acid 
(PTA), can be commercially produced in 1965, technical-grade terephthalic 
acid was mainly used to produce polymer-grade dimethyl terephthalate (DMT) 
for its easier purification from crystallization and distillation. Either 
DMT or PTA is reacted with MEG to form bis(2-hydroxyethyl) terephthalate 
(BHET) which is condensation-polymerized to PET. 
Polymer-grade terephthalic acid, i.e. PTA, must conform to many 
specifications to be suitable for the production of polyester fibers, 
films, and molding resin. Although no industry standards have been 
established officially, most polymer-grade terephthalic acids have maximum 
25 ppm of residual 4-CBA and 150 ppm of p-toluic acid. Residual benzoic 
acid is generally low and not specified. However, significant amount of 
benzoic acid may still be present in some polymer grade terephthalic 
acids. 
Currently, almost all technical-grade terephthalic acid is produced by 
catalytic liquid-phase air oxidation of para-xylene. This and other 
similar reactions producing crude aromatic polycarboxylic acids are 
referred to from time to time as oxidation reaction or oxidation reactions 
in this patent. Mid-Century process is the most widely adopted process 
which uses acetic acid as a solvent to assist slurry mixing and 
circulation; heavy metals, e.g., cobalt and manganese, as catalysts; and a 
bromine-containing compound as promoter. Reaction conditions are generally 
in the range of 175-230.degree. C. and 1500-3000 kPa. A variant of the 
process uses acetaldehyde as oxidation promoter that runs at 
120-175.degree. C. and 700-1400 kPa. Some currently obsolete commercial 
processes are: 
HNO.sub.3 oxidation of para-xylene(PX). The process was used by du Pont in 
the U.S. and ICI in the United Kingdom. 
Henkel I and II processes that rearrange benzoic acid or phthalic anhydride 
into terephthalic acid using naphthalene or toluene as starting material. 
These processes were used by Teijin, Kawasaki, and Mitsubishi in Japan. 
Compared to DMT, advantages of polymer-grade terephthalic acid as a 
feedstock for PET are its lower cost, no methanol as by-product, lower 
investment and energy costs, higher unit productivity, and purer polymer 
because less catalyst is needed for the polymerization process. These 
factors, together with competitive marketing pressures, have induced a 
number of companies into developing processes that produce polymer-grade 
terephthalic acid since 1965. The success in the removal of impurities 
from technical-grade terephthalic acid has made polymer-grade terephthalic 
acid as a major and often the preferred feedstock for PET. 
To produce polymer-grade terephthalic acid, separate purification processes 
have been developed to remove 4-CBA, p-toluic acid, and benzoic acid. The 
PTA process is separated into two sections: oxidation reaction section and 
purification section. The oxidation reaction section is for the production 
of technical grade or crude terephthalic acid (CTA) which is then 
introduced to purification section for removal of impurities. As discussed 
above, CTA production is generally produced by a liquid phase oxidation 
process. Terephthalic acid is also present as a major constituent in 
intermediate streams of the DMT production processes. 
With slight variations, the prior art teaches that the purification section 
removes 4-CBA from terephthalic acid by chemically converting 4-CBA into 
p-toluic acid through hydrogenation reaction using charcoal supported 
noble metals, such as platinum, palladium, and so on, as the catalysts. 
P-toluic acid (converted from 4-CBA or existing in terephthalic acid as 
contaminant) is generally removed by recrystallizing terephthalic acid 
from water at elevated temperatures and pressures. An alternative is 
continuing further oxidation of 4-CBA to terephthalic acid. 
However, this type of purification method, either by hydrogenation or 
oxidation, although efficient, can only handle relatively small amount of 
4-CBA present in CTA initially. To meet the final polymer-grade PTA 
product specification, 4-CBA, the principal impurity present in CTA, is 
generally limited to less than 1.0 wt %, preferably less than 0.5 wt %, to 
avoid overloading the purification section of the process. 
To achieve this purpose of reducing the amount of impurities introduced 
into the purification section, either the equipment has been modified to 
run at higher severity, or additional processing steps are added after the 
oxidation reaction step, such as a secondary oxidation step or reslurrying 
CTA in fresh acetic acid. Because higher severity increases the combustion 
rate of para-xylene, the CTA feedstock, and acetic acid, the preferred 
solvent, to CO and CO.sub.2, the overall yield of the desired product and 
production efficiency are both reduced. Using acetic acid as solvent and 
operating under severe condition require reactors and some other parts of 
the process to use expensive corrosion resistant material, such as 
titanium. This requirement increases initial capital investment 
significantly. Adding more processing steps likewise requires higher 
capital investments. Therefore, most prior arts teach the use of a 
relatively cumbersome purification procedure and high-cost equipment to 
remove as little as 0.5 wt % of impurities from terephthalic acid. 
In searching for alternative methods to produce polymer-grade terephthalic 
acid, earlier patents disclosed that terephthalic acid could be purified 
by crystallization in organic solvents. A partial list of those solvents 
is given below 
1. N,N-dimethylacetamide, or N,N-diemthylformamide, or their mixtures with 
water or methanol (U.S. Pat. No. 2,811,548); 
2. Pyridine with isopropylamine and others, recrystallizing in ethylene 
glycol, and acidifying in acid water (U.S. Pat. No. 2,829,160); 
3. N-formyl morpholine, or N-formnyl piperidine (U.S. Pat. No. 2,849,483); 
4. Ammonia with methanol and acetone (U.S. Pat. No. 2,862,963). 
These disclosed organic solvents, however, have several disadvantages. They 
are unable to produce the required high purity terephthalic acid. They are 
either unstable themselves or tend to form additional products with 
terephthalic acid. It is also difficult to separate the residual solvent 
included in the crystals of the product. 
On the other hand, the thermally more stable and chemically much less 
reactive solvents such as acetic acid, acetic anhydride (U.S. Pat. No. 
3,574,727) and water, suffer from low solubility of terephthalic acid and 
lack of selectivity between terephthalic acid and 4-CBA. With this type of 
solvents, expensive hydrogenation process is required to convert 4-CBA 
into p-toluic acid most of which can be later removed by recrystallization 
in water (U.S. Pat. No. 3,584,039). 
The manufacturing processes of isophthalic and phthalic acid that have the 
two carboxylic acids located at meta and ortho positions are similar to 
the manufacturing process of terephthalic acid. Liquid-phase oxidation 
production facilities often can be used interchangeably between 
terephthalic acid and isophthalic acid. Phthalic acid produced by this 
liquid process has significantly higher yields than those from vapor-phase 
oxidation processes with higher capital costs. 
The manufacturing process of benzenetricarboxylic acid is also similar to 
the terephthalic acid process. Trimellitic acid is produced commercially 
in large volume in the U.S. mainly by liquid-phase air oxidation of 
pseudocumene. It is dehydrated to trimellitic anhydride, a preferred form 
commercially. Trimellitate esters have many superior properties than 
phthalic acid esters in certain applications. For example, trimellitate 
esters are used as plasticizers for poly-vinyl chloride, especially if 
permanency is required, e.g., in high temperature wire insulation. Other 
important uses of trimellitate esters are in alkyd resins, amide-imide 
polymers, and epoxy curing. 
The manufacturing process of aromatic polycarboxylic acids with two 
condensed rings, such as naphthalene dicarboxylic acids (NDA), is also 
similar to terephthalic acid process. 2,6- or 2,7-NDA can be produced by 
the oxidation of 2,6- or 2,7-dialkyl naphthalene respectively with air or 
oxygen enriched air, in the presence of cobalt, manganese, and bromine. 
Relative to PTA, 2,6-NDA imparts greater structure stability to the 
resulting polymers at the same molecular level. Since the crude NDA also 
contains impurities, such as trimellitic acid, bromo-2,6-NDA, 2-naphthoic 
acid, 2-formyl-6-naphthoic acid, a similar purification process is 
required. 
To improve NDA purity, a number of Japanese patents described the methods 
of dissolving the crude NDA in an aqueous solution of alkali, then 
subjecting the solution to such treatment as oxidation, hydrogenation, 
decoloring by adsorption, and so on, and followed by acidifying the 
resulting solution, thereby obtaining the purified NDA (JP-A48-68554, 
JP-B-52-20993, JP-A-50-105639, and JP-A-50-16024). However, the above 
methods suffer several drawbacks, that large amounts of acid and alkali 
have to be used, that an inorganic salt is produced, and that waste water 
is discharged in large quantities. 
Organic solvents were also disclosed for purifying crude NDA as described 
by JPA-62-230747. An organic solvent selected from N,N-dimethylformamide 
(DMF), N,N-dimethylacetamide, and dimethyl sulfoxide (DMSO), treating the 
solution with active carbon, and then recrystallizing the purified NDA. 
However, the solubility of NDA in DMF or DMSO is low, so large quantity of 
solvent has to be used. Furthermore, it was found, in a higher NDA 
recovery mode, almost no improvement in color was achieved in the purified 
NDA product. Toxicity of the solvents is also a major concern. In 
addition, they are difficult to recover due to their high boiling points 
(U.S. Pat. No. 5,344,969). 
An aqueous solution of alkylamines such as dimethylamine, was used to 
dissolve crude NDA and the purified NDA was precipitated by removing 
dimethylamine from the solution by distillation (JP-A-50-142542). However, 
a large portion of water in the aqueous solution is lost along with the 
amine because the amine evaporates as an azeotropic mixture with water. 
NDA recovery by the method is low because complete removal of the amine 
from the aqueous solution is extremely difficult. 
Another alternative was to use alkylamines and alcohols to dissolve crude 
NDA. The purity and color of NDA were improved by precipitating NDA solids 
from the solution by one of the following precipitation method (U.S. Pat. 
No. 5,344,969): 
1. Cooling to precipitate an amine salt of NDA, and the amine is then 
recovered from the amine salt by heating, thereby to obtain NDA having 
high purity. 
2. Cooling to precipitate an amine salt of NDA, and the amine is then 
treated with an acid, thereby to obtain NDA having high purity. 
3. Adding an acid to the solution, thereby to obtain NDA having high 
purity. 
The prior arts of purification by crystallization used either a pure 
organic solvent or a mixture of solvents at constant composition to 
dissolve crude aromatic polycarboxylic acids at high temperature. The 
solution was then cooled to precipitate solids and leave impurities in the 
solution to purify the acids. These processes mainly took advantage of 
differences in solubility at different temperature to dissolve the crude 
acids and precipitate the solids. Since the solubility of the crude acids 
in these solvents is generally insignificant around room temperature 
(25.degree. C.), the processes were focused to find a solvent or a solvent 
mixture having high solubility for the crude acids at high temperature. 
The solvent of course has to meet additional requirements, such as 
non-reactive with the crude acids, easy to be recovered, and extremely low 
amount of residual solvent to be remained in the purified product, etc. 
SUMMARY OF THE INVENTION 
It was unexpectedly found in this invention that the solubilities of the 
crude acids and impurities vary significantly with the composition of a 
mixture of solvents at both of low and high temperature. To dissolve crude 
acids, the higher of the solubility is the better. To precipitate solids, 
the lower of the solubility is the better. It was found, in some cases at 
room temperature, that the solubility of a solvent mixture at a 
composition can be as high as around 60 wt % that is a level difficult to 
be reached by processes in prior arts. By simply changing the composition 
alone, the solubility can be reduced to insignificant level. In the 
instant invention, the process takes the advantage of differences in 
solubility at different composition, in addition to temperature, to 
improve product purity and reduce the costs of dissolution and 
precipitation. 
The present invention provides a process for purifying crude aromatic 
polycarboxylic acids having one or more condensed rings, wherein two or 
more carboxylic acid groups can be at any position of the aromatic ring or 
rings. The process comprises: (1) dissolving the crude aromatic 
polycarboxylic acid in a mixed solvent selected from the group consisting 
of N,X-monocyclic compound, alkylamine compound, water, alcohol, acid, or 
other co-solvent; (2) optionally pre-treating the solution with 
filtration, or activated carbon or other proper absorbents; (3a) 
crystallizing by changing the solvent composition or temperature and then 
dissolving the crystallized salt into an acid solvent, or (3b) 
crystallizing by changing the solvent composition or temperature and then 
heating the crystallized salt to decompose the salt, or (3c) crystallizing 
by direct adding an acid solvent to solution; (4) filtering and optionally 
washing with a mixed solvent to obtain a purified crystalline product that 
meets or exceeds the current industrial standard at lower capital and 
operating costs; (5) before drying, optionally re-dissolving the purified 
crystalline product into a mixed solvent to re-crystallize by flashing or 
evaporation, with or without cooling, to further improve product 
qualities; and (6) drying. 
These steps may be carried out with or without a purge of an inert or 
non-oxidizing gas. Some of these steps may be repeated to further improve 
product purity. 
The instant invention has the following major advantages: 
1. Purification cost by crystallization is lower than the cost by chemical 
reactions. The capital and operation costs for product purification are 
significantly lower than the current commercial purification processes. 
2. The product purity obtained from this invention can meet and exceed the 
current industrial standard. It is even possible to reach to the 
ultra-pure level that the impurities almost cannot be detected by the 
current HPLC measuring method. This may allow manufacturing of polymers 
that are not possible to obtain with the current levels of aromatic 
polycarboxylic acid purity. 
3. This new approach can process crude aromatic polycarboxylic acids having 
much higher impurity content. This capability introduces several 
synergistic effects that include relaxing requirements in oxidation 
reaction section to reduce operation and capital investment costs, 
allowing to process the crude acids from intermediate streams in 
esterification processes, such as DMT or NDC esters, recovering aromatic 
polycarboxylic acids present as byproduct and/or impurity in some process 
streams, or further purifying pure aromatic polycarboxylic acids to ultra 
pure level. 
DETAILED DESCRIPTION OF THE INVENTION 
Crude Aromatic Polycarboxylic Acids 
The aromatic polycarboxylic acids that may be purified in accordance with 
the present invention are those having one or more condensed rings, 
wherein two or more carboxylic acid groups can be at any position of the 
aromatic ring or rings. The one-ring aromatic dicarboxylic acids include 
terephthalic acid, isophthalic acid, and orthophthalic acid. Other 
one-ring aromatic polycarboxylic acids include trimellitic acid, 
hemimellitic acid, trimesic acid, pyromellitic acid, mellitic acid, etc. 
The two-ring aromatic polycarboxylic acids include 2,6-naphthalene 
dicarboxylic acid, 2,7-naphthalene dicarboxylic acid, 1,7-naphthalene 
dicarboxylic acid, and 1,8-naphthalene dicarboxylic acid. Other two-ring 
aromatic polycarboxylic acids include 2,3,6-naphthalene tricarboxylic 
acid, 1,4,5,8-naphthalene tetracarboxylic acid, 2,3,6,7-naphthalene 
tetracarboxylic acid, etc. The three-condensing ring aromatic 
polycarboxylic acids include 2,6-anthracene dicarboxylic acid, 
2,7-anthracene dicarboxylic acid, 2,8-anthracene dicarboxylic acid, 
2,9-anthracene dicarboxylic acid, or 1,9-anthracene dicarboxylic acid. 
Other three-ring aromatic polycarboxylic acids include 2,3,6-anthracene 
tricarboxylic acid, 1,4,5,8-anthracene tetracarboxylic acid, 
2,3,6,7-anthracene tetracarboxylic acid, etc. 
Most crude aromatic dicarboxylic acids are obtained by oxidation processes 
of poly-substituted aromatic compounds with air or oxygen enriched air. 
These processes have been described by numerous patents and publications. 
For example, the oxidation of xylene isomers produces one-ring aromatic 
dicarboxylic acids; the oxidation of the di-substituted naphthalenes, such 
as dimethylnaphthalenes, produces aromatic dicarboxylic acids with two 
condensed rings; and the oxidation of the substituted anthracenes produces 
three-ring aromatic dicarboxylic acids. These oxidation processes are 
generally carried out in a solvent, e.g., a lower aliphatic monocarboxylic 
acid or water, in the presence of a catalyst, i.e., one comprising heavy 
metals such as cobalt, manganese, or their mixture, and in the presence of 
a promoter such as bromine. Acetic acid is the most popular lower 
aliphatic monocarboxylic acid that used as solvent for the oxidation 
process. 
The oxidation reaction is conducted usually at a temperature range of from 
about 170.degree. C. to about 300.degree. C., depending on the aromatic 
feed to the oxidation reactor, with the oxygen partial pressure in the gas 
phase being preferably from 0.2 to 20 Kg/cm.sup.2 in terms of absolute 
pressure. After completion of the oxidation reaction, the reaction mixture 
is cooled to around room temperature and the precipitated crude aromatic 
polycarboxylic acid is recovered. 
The other possible source of crude aromatic polycarboxylic acid is from the 
intermediate stream product in an aromatic polycarboxylic ester production 
process. For example, crude terephthalic acid can be recovered by 
filtration from the oxidizer effluent during the production of DMT; or 
crude 2,6-NDA can be recovered by filtration from the oxidizer effluent 
during the production of dimethyl-2,6-naphthalene dicarboxylate (NDC). The 
impurity content for terephthalic acid obtained from DMT process may be as 
high as 30 wt %. This is significantly higher than 0.5-1.0 wt % levels 
typically found in the catalytic liquid-phase air oxidation processes. 
Consequently, it will cause overloading problem in prior art purification 
processes by either oxidation or hydrogenation as discussed previously. 
However, the present invention is capable to purify such a kind of 
intermediate product to their polymer-grade specifications. 
Crude aromatic polycarboxylic acid can also be produced from HNO.sub.3 
oxidation of PX, or from Henkel I and II processes that use naphthalene, 
toluene, benzoic acid or phthalic anhydride as starting or intermediate 
material. This instant invention is also capable of purifying such 
materials. The major impurities in the more common crude aromatic 
polycarboxylic acids (CAPA) are listed below: 
______________________________________ 
Process Impurities 
______________________________________ 
Terephthalic 
Oxidation 4-CBA, p-toluic acid, benzoic acid 
acid 
Isophthalic Oxidation 3-CBA, m-toluic acid, benzoic acid 
acid 
Orthophthalic Oxidation 2-CBA, o-toluic acid, benzoic acid 
acid 
2,6-NDA Oxidation Trimellitic acid, bromo-2,6-NDA, 2- 
naphthoic acid, 2-formyl-6-naphthoic acid 
Terephthalic Esterification 4-CBA, p-toluic acid, benzoic acid, 
acid monomethyl terephthalate, dimethyl 
terephthalic acid, methyl p-toluate, 
dimethyl terephthalate, methylbenzoate 
2,6-NDA Esterification Trimellitic acid, bromo-2,6-NDA, 2- 
naphthoic acid, 2-formyl-6-naphthoic acid, 
2,6-NDC, 2-formyl-6-naphthoic acid, 
esters of trimellitic acid, etc. 
______________________________________ 
In addition to these listed impurities, some other trace compounds may be 
present in various CAPA production processes, such as ashes, metals, 
halides, color substances, p-cresol, 4-hydroxylmethyl benzoic acid, 
4-formyl-benzoic acid, 4-hydroxymethyl-benzaldehyde, methyl acetate, 
p-tolualdehyde, bromo-2,6-naphthalene dicarboxylic acid, 
2-formyl-6-napththoic acid, etc. Organic acid or inorganic acid, such as 
acetic acid, carried over from the oxidation reaction section may also be 
present. 
Solvents 
The following lists the solvents that are used in this invention. These 
solvents used may include a single solvent, a mixture, or an admixture of 
solvents that can be miscible or immiscible. The solvents are categorized 
into major solvents, co-solvents, and acid solvents. A mixed solvent is a 
combination of a major solvent, a co-solvent, and an acid solvent in any 
proportions. Detailed combinations are described at the appropriate 
sections of the specifications. 
Major Solvents 
Major solvents play the major role in dissolving aromatic polycarboxylic 
acids and impurities by forming weakly bonded complexes with the acids. 
Two solvent groups are used in this invention as major solvents: 
N,X-monocyclic compounds and alkylamine compounds. A major solvent also 
includes a mixture of N,X-monocyclic compound and alkylamine compound in 
any proportion. 
N,X-Monocyclic Compound 
An N,X-monocyclic compound in this invention is a mono-heterocyclic 
compound containing 3 to 8 atoms in the ring with a nitrogen(N) and an 
hetero-atom(X), such as oxygen, sulfur, or another nitrogen, as the 
hetero-atoms. The nitrogen atom may have three or five valences. The 
compound includes all combinations of hetero-atom and carbon-atom at 
different positions in the ring, and their saturated and unsaturated 
compounds with one or more hydrogen atoms that may be substituted by an 
alkyl, aryl, or acyl group. The N,X-monocyclic compound also includes the 
ammonium salts derived from such compounds. 
The N,X-monocyclic compound includes parent compounds, i.e. a given number 
of atoms in a ring including all of the unsaturated and saturated with 
hetero-atom at all possible ring positions, such as oxazocines, 
oxazepines, oxazines, oxazoles, isoxazoles, oxadiazetes, oxazirines, 
thiozocines, thiazepines, thiazines, thiazoles, isothiazoles, thiazetes, 
thiazirines, diazocines, diazepines, pyrazines, pyridazines, pyrimidines, 
imidazoles, pyrazoles, diazetes, diazirines, etc. If a compound is in 
solid or gaseous form under normal condition, then its aqueous solution 
from 0.0001 wt % to saturation will be used. 
The N,X-monocyclic compound can be used either alone or as a mixture of two 
or more thereof in any proportion. The preferred N,X-monocyclic compound 
for this invention is a saturated oxazine or mixtures of said oxazines. 
The commonly used or adopted name of the saturated oxazine is morpholine, 
CAS Registry Number 110-91-8. A morpholine compound in this invention 
means morpholine, substituted morpholines, morpholine derivatives, and 
mixtures thereof. Typical examples of a morpholine compound are 
morpholine, N-methylmorpholine, N-ethylmorpholine, N-propylmorpholine, 
N-isopropylmorpholine, N-methylmorpholine oxide, N-phenylmorpholine, 
4-morpholinepropionitrile, 1-morpholine-1-cyclohexene, etc. 
Other examples are piperazine, N-methylpiperazine, 2-methylpiperazine, 
N,N-dimethylpiperazine, etc. Two other compounds are also included as 
members of this group: 1,4-diazabicyclo[2.2.2]octane (CAS Registry Number 
280-57-9), 1,8-diazabicyclo [5.4.0]undec-7-ene (CAS Registry Number 
6674-22-2). 1,4-diazabicyclo[2.2.2]octane is also a preferred compound. 
Alkylamine Compound 
An alkylamine compound in this invention includes aliphatic amine; 
alicyclic amines; ammonium salts derived from these aliphatic and 
alicyclic amines. 
Aliphatic amines include methylamine, dimethylaamine, trimethylamine, 
ethylamine, diethylamine, triethylamine, n-propyamine, di-n-propylamine, 
tri-n-propylamine, isopropylamine, diisopropylamine, triisopropylamine, 
cyclohexylamine, and C.sub.4 to C.sub.8 aliphthalic amines. Other examples 
of suitable amines are ethylenediamine, N-methylethyleneamine, 
N,N-dimethylethylenediamine, N,N'-dimethylethylenediamine, 
N,N,N'-trimethylethylenediamine, N,N,N',N'-tetramethylethylenediamine, 
1,2-diaminopropane, 1,3-diaminopropane, N-methyl-1,2-diaminopropane, 
N-methyl-1,3-diaminopropane, N,N-dimethyl-1,2-diaminopropane, 
N,N-dimethyl-1,3-diaminopropane, N,N,N'-trimethyl-1,2-diaminopropane, 
N,N,N'-trimethyl-1,3-diaminopropane, 
N,N,N',N'-tetramethyl-1,2-diaminopropane, 
N,N,N',N'-tetramethyl-1,3-diaminopropane, monoethanolamine, 
diethanolamine, triethanolamine, and glycine. 
Alicyclic amines include pyrrolidine, 1-methyl-pyrrolidine, piperidine, 
N-methylpiperidine, hexamethyleneimine, and N-methyl hexamethyleneimine. 
If a compound is in solid or gaseous form under normal condition, then its 
aqueous solution from 0.000 wt % to saturation will be used. Among these 
alkylamine compounds, aliphatic amines having up to 15 carbon atoms are 
preferred. Triethylamine and triethanolamine are the most preferred 
alkylamine compounds. Again, these alkylamine compounds may be used either 
alone or as a mixture of two or more thereof in any proportion. 
Co-Solvent 
Two classes of co-solvents are used in this invention: water and alcohol. 
The co-solvent also includes mixtures of water and alcohol in any 
proportion. Oxygen-containing solvents may be used together with water 
and/or alcohol to further enhance selectivity for product recovery or 
impurity removal. Major solvents form weakly bonded complexes with crude 
aromatic polycarboxylic acids, and the amount of complex dissolved may 
vary depending on the composition of the co-solvent and the temperature of 
the solution. In this invention, co-solvent composition is one of the 
important controlling factors to enhance the selectivity for product 
recovery and/or impurity removal. The composition is the concentration of 
components in a mixture. Co-solvents may also be used alone or with an 
acid solvent and/or major solvent to wash out residual impurities captured 
in the inclusion of the precipitated crystals. 
Water 
Water is the most inexpensive, abundant, and nontoxic solvent. It normally 
does not react with crude aromatic polycarboxylic acids and their 
impurities under the operating conditions in this invention. Using water 
as co-solvent not only enhances the selectivity for product recovery, 
impurity removal, or both, but also reduces the overall cost of solvent 
used to dissolve crude aromatic polycarboxylic acids. 
Alcohol 
An alcohol, as a co-solvent, is selected from the groups consisting of 
aliphatic monohydric alcohols such as methanol, ethanol, n-propanol, 
isopropanol, n-butyl alcohol, isobutyl alcohol, sec-butyl alcohol, 
tert-butyl alcohol, n-amyl alcohol, isoamyl alcohol, sec-amyl alcohol, 
tert-amly alcohol, neopentyl alcohol, hexyl alcohol, heptyl alcohol, octyl 
alcohol, nonyl alcohol, and decyl alcohol; alicyclic monohydric alcohols 
such as cyclopentyl alcohol and cyclohexyl alcohol; aliphatic 
straight-chain diols such as ethylene glycol, diethylene glycol, glycerol, 
1,2-propylene glycol, and 1,3-propylene glycol; alicyclic diols such as 
1,2-cyclopentanediol, 1,3-cyclopentanediol, 1,2-cyclohexanediol, 1,3 
cyclohexanediol, 1,4-cyclohexanediol; and aliphatic polyols such as 
glycerol and pentaerythritol. Aliphatic monohydric alcohols having 3 or 
less carbon atoms and diols having 3 or less carbon atoms, are preferred. 
Alcohols can be used either alone or as a mixture of two or more thereof 
in any proportion. 
Oxygen-containing Solvents 
In addition to water and alcohol, other oxygen-containing solvents may also 
be used with water, alcohol, or a water and alcohol mixture to further 
enhance the selectivity of product recovery or impurities removal. The 
oxygen-containing solvents include ketones, ethers, aldehydes, glycols, 
and alicyclic compounds such as furan, dihydrofuran, tetrahydrofuran, 
tetrahydropyran, 2-methyl cyclopentanone, cyclopentanone, cyclohexanone, 
cyclohexanol, 2-methyl tetrahydrofuran, 3-methyl tetrahydrofuran, 
gamma-butyrolactone, gamma-valerolactone, etc. Again, these 
oxygen-containing solvents may be used alone or as a mixture of two or 
more thereof in any proportion. Co-solvents also include mixtures of 
water, alcohol, and oxygen-containing solvent in any proportion. 
Acid Solvent 
An acid solvent includes aliphatic carboxylic acids such as formic acid, 
acetic acid, propionic acid, butyric acid, glycolic acid, lactic acid, 
malic acid, tartaric acid, mesotartaric acid, citric acid, 
monochloroacetic acid, monobromoacetic acid, trifluoroacetic acid, and 
trichloroacetic acid; and inorganic acids such as nitric acid, 
hydrochloric acid, hydrogen bromide, hydrogen iodide, sulfuric acid, 
phosphoric acid, and perchloric acid. If an acid is in solid or gaseous 
form under normal operating conditions, its aqueous solution from 0.000 wt 
% to saturation is used in this invention. 
These acids may be used either alone or as a mixture of two or more thereof 
in any proportion provided they are chemically compatible. Those who are 
skilled in the art would recognize and appreciate that, for example, 
concentrated sulfuric acid should not be used with hydrogen iodide. The 
acid may be used as is or in the form of an aqueous solution. An aliphatic 
carboxylic acid having up to 15 carbon atoms, such as acetic acid, is 
preferred. The amount of an acid used is in the amount from about 0.01 to 
100 times, preferably from about 0.2 to 10 times, in molar ratio, to the 
amount of crude acid present in the solution. The acid solvent also 
includes a mixture of the acid with a co-solvent, in a range from about 0 
wt % to about 100 wt %, preferably in a ratio from about 50 wt % to about 
99 wt %. 
Mixed Solvent 
A mixed solvent is a mixture of a major solvent and a co-solvent, a 
co-solvent and an acid solvent, a major solvent and an acid solvent, or a 
major solvent, a co-solvent and an acid solvent, and other combinations 
that contain oxygen-containing solvents. The mixed solvent comprises a 
major solvent from 0.1 wt % to 99.9 wt %, a co-solvent from 0.1 wt % to 
99.9 wt %, an acid solvent from 0 wt % to 99.9 wt %, and an 
oxygen-containing compound from 0 wt % to about 85 wt %. 
The proportion of the N,X-monocyclic compound to water or alcohol is from 
1:1000 to 1000:1 by weight, preferably from about 25:75 to about 95:5 by 
weight. The proportion of the alkylamine compound to water or alcohol in 
the mixed solvent is from 1:1000 to 1000:1 by weight, preferably from 
about 25:75 to 95:5 by weight. 
The exact amount of the major or mixed solvent to be used in this invented 
process is not specifically limited as long as it is sufficient to 
dissolve the crude aromatic polycarboxylic acid to minimize losses of 
CAPA. The amount usually varies depending on the composition and 
concentration of water or alcohol present in the N,X-monocyclic compound 
or alkylamine compound, and the temperature at which the crude aromatic 
polycarboxylic acid is dissolved. The amount of a major solvent or a mixed 
solvent is used in the range of from about 0.1 to 100 times by weight, 
preferably from about 1 to 50 times by weight, to the amount of the crude 
aromatic polycarboxylic acid. If the amount of solvent used is below the 
lower limit specified above, purification effect may be insufficient. If 
the amount exceeds the upper limit specified above, while technically 
feasible and included in this invention, the process becomes less 
economical. 
As a crude aromatic polycarboxylic acid is dissolved in a major solvent, a 
major solvent and a co-solvent, or a mixed solvent, the mixture is called 
a solution. After aromatic polycarboxylic acid is precipitated from the 
solution, the remaining solution is called a mother liquor. 
Most major solvents do not react with oxygen in air under the operating 
conditions of this invention, but some may slightly do and cause gradual 
change of solvent color. Under this circumstance, the process will be 
preferably operated under low oxygen environment or non-oxidizing 
atmosphere in a closed system and/or by purging with a non-oxidizing or 
inert gas or other suitable methods. Examples of such non-oxidizing or 
inert gases are N.sub.2, CO.sub.2, CO, Ar, He, H.sub.2, etc., and N.sub.2 
and CO.sub.2 are preferred. 
In most cases, the co-solvent is used to increase crude aromatic 
polycarboxylic acid solubility. However, in other cases, the co-solvent is 
used to decrease the solubility of the acid to enhance product recovery. 
For instance, pure N-methyl morpholine oxide has about 15% solubility for 
terephthalic acid at room temperature. This solubility is too high for 
product recovery. However, adding suitable amount of water to the solution 
will reduce the solubility and enhance product recovery. 
The mixed solvent may be miscible or immiscible mixture of its components. 
Therefore, the mixed solvent may be in a single phase or multiple phases. 
In the case of multiple liquid phases, the solubilities of crude aromatic 
carboxylic acids and impurities in different liquid phase may be 
different. A proper choice of solvent in different phase allows us to take 
advantage of liquid-liquid extraction method to separate the impurities. 
Solution Pretreatment 
The key to these purification processes is to precipitate pure aromatic 
polycarboxylic acids from solutions while keeping as much impurities in 
the solution as possible. However, crude aromatic polycarboxylic acids may 
contain insolubles, color substances, or impurities that can be easily 
separated by a pretreatment of the solution. The following lists the 
optional pretreatment methods that can be used individually or in any 
combination to remove such substances. 
Crude aromatic polycarboxylic acid may contain insoluble impurities that 
can be separated by using any suitable method, such as filtration, 
centrifugation, sedimentation, magnetic separation, evaporation, and 
others. 
The solution may be treated with an activated carbon or other suitable 
adsorbents by: 
(1) a batch-wise method in which a predetermined amount of activated carbon 
or other adsorbent is added to the solution, and the resulting mixture is 
stirred, with or without heating, and then filtered; Or (2) a counter 
current or concurrent continuous method in which the solution is passed 
through a column packed with activated carbon or other adsorbents. 
Using one or more stages of liquid-liquid extraction to remove impurities 
by choosing solvents that are not immiscible. 
Purification Processes 
A purification process used in this invention means one or a combination of 
the following five processes. The choice depends on the aromatic 
polycarboxylic acid to be purified, the solvent selected, and the 
operating conditions. Details are set forth throughout the specifications. 
The processes can be used for one or more times. First, a crude aromatic 
polycarboxylic acid having one, two, three or more condensed rings is 
dissolved in a selected solvent or solvent mixture. Depending on the crude 
aromatic polycarboxylic acid to be purified, the solvent or solvent 
mixture may contain a particular group of major solvent, co-solvent, or 
mixtures thereof. Whether specifically described or not, this purification 
process can be carried out with or without a purge of a non-oxidizing or 
inert gas in all phases. Typical gases are N.sub.2, CO.sub.2, CO, Ar, He, 
H.sub.2, and others. 
Following this dissolution step is a step of purification process selected 
from the five processed described below. While the basic theories of this 
invention are also described and they are believed to be true, they are 
used only to illustrate and demonstrate the invention. In no way are these 
theories intended to limit the scopes or inventiveness of this invention. 
Process 1. 
(a) Changing the composition of the solvent used to dissolve the crude 
aromatic polycarboxylic acid by removing lower-boiling components in the 
mixed solvent by flashing under reduced process pressure, by evaporating 
at constant or variant temperatures, by distillation, or by adding more 
co-solvent, to precipitate the solid formed between aromatic 
polycarboxylic acid and the major solvent. In the case of evaporation or 
flashing, the solid is precipitated mainly due to the change of solvent 
composition and the reduction of solvent quantity. 
(b) Separating the precipitated solid followed by treating the separated 
solid with an acid solvent to obtain a purified aromatic polycarboxylic 
acid. 
Process 2. 
(a) The same process as 1 (a) is used to precipitate a solid. 
(b) Separating the precipitated solid followed by heating the separated 
solid to a higher temperature, with or without purging with a 
non-oxidizing or inert gas such as N.sub.2, CO.sub.2, CO, He, Ar, H.sub.2, 
and others, to decompose the solid to obtain a purified aromatic 
polycarboxylic acid. 
Process 3. 
(a) A crude aromatic polycarboxylic acid is dissolved in a solvent at 
elevated temperature. The solution is cooled to a lower temperature to 
precipitate the solid formed between aromatic polycarboxylic acid and the 
major solvent. The solid is precipitated mainly due to the change of 
temperature in solution. 
(b) The solid is separated and then treated with an acid solvent to obtain 
a purified aromatic polycarboxylic acid. 
Process 4. 
(a) The same process as 3 (a) is used to precipitate a solid. 
(b) The solid obtained is separated and then heated to a higher 
temperature, with or without purging with a non-oxidizing or inert gas, 
such as N.sub.2, CO.sub.2, CO, He, Ar, H.sub.2, and others, to decompose 
the solid to obtain a purified aromatic polycarboxylic acid. 
Process 5. 
An acid solvent is added to the solution containing the crude aromatic 
polycarboxylic acid, at constant or variant temperature, to precipitate a 
purified aromatic carboxylic acid from the solution. The purified aromatic 
polycarboxylic acid solid is precipitated mainly due to the change of 
complex structure. The acid component substitutes the aromatic 
polycarboxylic acid and form a new complex with the major solvent. 
In Process 1 and Process 2, the control and change of the solvent 
composition depend on the major solvent and co-solvent selected for a 
particular aromatic polycarboxylic acid and the desired process 
conditions. Absolute and relative solubilities of the aromatic 
polycarboxylic acid to be purified and the impurities present are of 
paramount importance. For instance, at lower temperatures such as room 
temperature, the solubilities of aromatic polycarboxylic acids are 
generally low in most major solvents or co-solvents. However, at the lower 
temperatures, it was unexpectedly discovered that the solubilities of 
aromatic polycarboxylic acids vary significantly with the composition of 
the mixture. Most solubilities are negligible in a pure major solvent, 
increase with the addition of a co-solvent, reach to a maximum, and then 
gradually decrease to negligible again in a pure co-solvent composition. 
However, some major solvents were found to have significant solubilities 
for aromatic polycarboxylic acids at room temperature, and gradually 
decrease with increasing amounts of co-solvent in the compositions. This 
invention takes advantage of all these discovered surprising and 
unexpected differences in the purification process. 
Impurity solubilities also vary with co-solvent compositions. Generally, 
their solubility patterns were found to be similar to, but in a 
significantly higher absolute level than, aromatic polycarboxylic acid 
solubilities in a range of compositions. Therefore, there exists an 
optimal way to control the solvent composition in order to achieve best 
product recovery and impurity removal. 
Higher-boiling components may leave together with lower-boiling components 
in Process 1 and Process 2. The total amount of solvent removed from the 
mixed solvent is from about 0.1 to 100 wt % of its original amount 
presented in the mixed solvent, preferably from about 50 to 95 wt %. In 
the case of adding more co-solvent as part of the purification process, 
the added amount may change the final co-solvent composition from 0.01 to 
99.9 wt %, preferably from about 20 to 75 wt %. The temperature of 
solution is from about -100 to 350.degree. C., preferably 30 to 
180.degree. C.; the pressure is in a range of 1 mmHg (absolute) to 100 
atmospheres (absolute), preferably about 20 mmHg (absolute) to 2.0 
atmosphere. 
In Process 3 and Process 4 above, a crude aromatic polycarboxylic acid is 
precipitated from solution by cooling. The dissolving temperature may be 
in the range from about -100 to about 350.degree. C., preferably from 
about 80 to 170.degree. C. The temperature may be cooled to the range from 
about -100 to 150.degree. C., preferably from 25 to 100.degree. C., more 
preferably from about 40 to 60.degree. C. 
In Process 5, an acid solvent is added to the solution to precipitate 
purified aromatic polycarboxylic acid while keeping most of impurities in 
solution. The amount of acid solvent used may be from 0.01 to 100 times, 
preferably, about 0.2 to 30 times, in molar ratio, to the number of moles 
of crude aromatic polycarboxylic acid present. The operating temperature 
may vary from about -100 to 350.degree. C., preferably from about 25 to 
180.degree. C. This process may be used for 1 to 100 times, preferably, 1 
to 3 times. 
The solids precipitated by Process 1 and Process 3 are treated by adding a 
predetermined amount of an acid solvent to the solids. The resulting 
admixture is stirred at a temperature from about -100 to 350.degree. C., 
preferably from about 25 to 180.degree. C., for a period of about 0 to 10 
hours, preferably from 0 to 2 hours. The purified aromatic polycarboxylic 
acid is recovered subsequently from the mixture. The amount of acid 
solvent used may be from about 0.01 to 100 times by mole, preferably, 
about 0.2 to 10 times by mole, to the amount of crude aromatic 
polycarboxylic acid. This process may be used for 1 to 100 times, 
preferably 1 to 3 times. 
In Process 2 and Process 4, the exact temperature used to decompose the 
solids to obtain a purified aromatic polycarboxylic acid is not 
specifically limited as long as it is sufficient for the decomposition. 
Filtration And Product Recovery 
The solids obtained from one of the above five processes are filtered or by 
other suitable methods to remove the mother liquor from the solids. The 
mother liquor from the separation of solid in various stages of the above 
purification process containing impurities. The mother liquor can be 
re-used repeatedly for crystallization, without any particular treatment 
or, if required, after being subjected to purification. The mother liquor 
may be recycled or purified by any suitable method, such as distillation, 
filtration, centrifugation, sedimentation, evaporation, cooling, adding 
more solvent, etc., or any combination of the methods, to separate 
impurities. The recovered impurities can then be recycled totally or 
partially to the oxidation reaction section or removed from the process. 
The filtered solids may be optionally subjected to post-treatment such as 
washing or other methods for further removal of impurities, solvents, or 
acid, as described in the following section, before drying. The solids are 
then dried by any suitable method known to those skilled in the art to 
remove any residual co-solvent or traced acid and major solvents from the 
solids. 
With one or a combination of the above mentioned processes of this 
invention, purified aromatic polycarboxylic acids with high purity can be 
obtained from the crude aromatic polycarboxylic acids. 
Post Treatment 
All filtered solids obtained from the above methods can be optionally 
washed one or more times with a co-solvent, acid solvent, or mixed solvent 
to remove residual impurities from the solids. The temperature of the 
washing solvent can be between about 0.degree. C. to about 150.degree. C., 
preferably from about 25.degree. C. to about 100.degree. C. The amount of 
washing solvent used to the amount of aromatic polycarboxylic acid present 
is in the range from about 0.01 to about 100 times by molar ratio, 
preferably, from about 0.2 to about 10 times. 
The particle sizes obtained from the above methods are generally finer, but 
more uniform, than those from the current PTA processes. As a result, the 
bulk density of the PTA produced by this invention may be different from 
those of current commercial products. If desirable, the bulk density of 
PTA produced by this instant invention can be adjusted by 
re-crystallization. This can be achieved by a number of means known to 
those skilled in the art. Only one example is given here. The PTA solids 
from this invention is dissolved in a co-solvent or acid solvent, such as 
water or acetic acid, at elevated temperature and pressure. The solution 
is then flashed under reduced process pressure or evaporated under 
constant or variant temperature, and with or without cooling, to produce 
PTA crystals that are not only similar to current commercial PTA in bulk 
density but also further purified to contain even less impurities. 
Synergistic Effects 
It was unexpectedly found in this invention that the solubilities of 
impurities remained to be high in many mixed solvents with properly 
selected co-solvent compositions after aromatic polycarboxylic acids were 
precipitated out of the solution. With one or more crystallization stages, 
the present invention is capable to process crude aromatic polycarboxylic 
acids with impurity content in a range of 0.0001 to 98%, preferably from 
about 0.5 to 30 wt %. This capability is considerably higher than the 
capacity of 0.5 to 1.0% of impurities that the current PTA purification 
processes can purify. 
In addition, other impurities present in the crude aromatic polycarboxylic 
acids from processes other than catalytic liquid-phase oxidation process, 
such as DMT or NDC esterification process, can also be easily separated 
from the aromatic polycarboxylic acid. Therefore, this invention can be 
used to co-produce purified aromatic polycarboxylic acids from a new or 
existing aromatic polycarboxylate ester plant. For example, monomethyl 
terephthalate is one of the major impurity in the crude acid from DMT 
process, and its solubility was found to be close to the two major 
impurities of p-toluic acid and benzoic acid. This process can then be 
used to co-produce terephthalic acid from the oxidizer of a DMT plant, 
wherein the oxidizer effluent is filtered or by other means to obtain the 
solids containing crude terephthalic acid. 
Furthermore, acids used as solvent in the oxidation reaction section may be 
entrained with or included in the crude aromatic polycarboxylic acid. In 
most of current processes these residual acids need to be separated prior 
to entering the purification section. Since the process of this instant 
invention has great tolerance of residual acids presented in the crude 
aromatic polycarboxylic acids, residual acid recovery process steps before 
purification section can thus be eliminated. The present invention is 
capable to process crude aromatic polycarboxylic acids having residual 
acid solvent contents ranging from about 0 to about 30.0 wt %, preferably 
from about 0 to about 15 wt %, and most preferably from about 0 to about 5 
wt %, depending on the specific aromatic polycarboxylic acid to be 
purified, the selected mixed solvent, and purification process conditions. 
The aromatic polycarboxylic acid product purity from the present invention 
can be significantly higher than that obtained from the currently existing 
processes. Under properly selected conditions as described hereinwith, the 
process is even capable to remove some impurities from crude aromatic 
polycarboxylic acids to undetectable levels by current HPLC, High Pressure 
Liquid Chromatography, analytical method. As illustrated in Example 1, 
pure terephthalic acid with undetectable level of benzoic acid and 
p-toluic acid and 8 ppm of 4-CBA in a single stage of crystallization had 
been achieved. After optimization of the process or adding one or more 
crystallization stages, it is possible to reach to the purity level where 
all of the three impurities cannot be detected by the current HPLC 
measuring method. Product purity from this invention can reach to about 
99.99999% level. The ultra pure product can be used to develop new 
applications that are impossible with the current available product 
purity. 
To meet polymer-grade PTA specification, current purification methods by 
hydrogenation or oxidation, allow total impurity level in crude 
terephthalic acid only up to 1 wt %, preferably 0.5 wt % to avoid 
overloading the purification section. To reduce impurity concentrations, 
either the oxidation reaction section producing crude aromatic 
polycarboxylic acid from starting materials is modified to run at higher 
severity, or additional process steps are added after the oxidation 
reaction step. Examples of such additional steps are secondary oxidation 
and reslurry in fresh acetic acid. However, since the purification process 
in this invention allows processing up to about 98 wt % total impurity in 
the crude aromatic polycarboxylic acid feed and can still meet the polymer 
grade specification, it relaxes requirements in the oxidation reaction 
section and imparts the following additional synergistic effects. 
To [a] substitute the solvent used in the crude aromatic polycarboxylic 
acid producing oxidation reactor with less corrosive materials, such as 
benzoic acid, methyl benzoate, ethyl benzoate, and phenyl benzoate, [b] 
use less amount of oxidation promoters, [c] use different kind of 
promoters, [d] reduce the severity of operating condition by reducing 
reaction temperature to 100 to 175.degree. C., [e] use a combination of 
the aforementioned [a] through [d] in order to use cheaper construction 
material, such as 316 SS, in the oxidation reactor or other parts of the 
process for lower capital investment. 
To run the oxidation reaction producing the crude aromatic polycarboxylic 
acid at a lower severity to reduce combustion loss of feedstock and acid 
solvent and to recycle the un-reacted feedstock back to oxidation reaction 
vessel to increase the overall yield and production efficiency. 
To process crude aromatic polycarboxylic acids from processes that are less 
efficient in producing high-purity product but cheaper in initial capital 
investment or operation cost. Examples are Henkel processes, HNO.sub.3 
oxidation of PX, the terephthalic acid or NDA from DMT or NDC process 
respectively, etc. Impurity contents obtained from such esterification 
processes may be as high as 30 wt %. 
Further Descriptions 
The purification processes in the instant invention are applicable to crude 
aromatic polycarboxylic acids having one, two, and three or more condensed 
rings with two or more carboxylic acid groups at any position of the ring 
or rings. The processes use two major solvents: N,X-monocyclic compound 
and alkylamine compound, and two co-solvents: water and alcohol. Five 
processes can be used to purified the crude acids. In consideration of 
previously described prior art (U.S. Pat. No. 5,344,969) that used 
alkylamine compound as major solvent; alcohol as co-solvent; processes 
similar to Process 3, 4, and 5 to purify crude aromatic polycarboxylic 
acids having two condensed rings and two carboxylic acid groups, the 
instant invention is applicable in four aspects that are summarized as 
follows: 
The invented processes can be carried out with or without a purge of a 
non-oxidizing or inert gas. Such a gas is selected from the group of 
N.sub.2, CO.sub.2, CO, He, Ar, H.sub.2, and mixtures thereof The crude 
aromatic polycarboxylic acid contains from about 0.000 wt % to about 98.0 
wt % impurities, preferably from about 0.5 wt % to about 30 wt % or from 
about 0.000 wt % to about 0.1 wt %. It may also contain from about 0 wt % 
to about 30.0 wt % residual acid used and produced in producing the crude 
aromatic polycarboxylic acid. At least portions of the impurities present 
can be recycled. 
A mixture of major solvent and co-solvent can be in one or multiple phases. 
All these purification processes can be used for 1 to 100 times, 
preferably for 1 to 3 times. In Process 1, 3, and 5, the amount of acid 
solvent used to precipitate said aromatic polycarboxylic acid, in molar 
ratio, is from about 0.01 to about 100 times of the crude aromatic 
polycarboxylic acid at a temperature range of -100 to 350 .degree. C. and 
the precipitation step is carried out in a period of 0 to 10 hours, more 
preferably from about 0.2 to about 10 times of the crude acid at a 
temperature range of 25 to 180.degree. C. and carried out in a period of 0 
to 2 hours. An aliphatic carboxylic acid having up to 15 carbon atoms, 
such as acetic acid, is preferred. 
When Process 1 or Process 2 is used, the composition of said solvent is 
changed by removing lower-boiling components in the mixed solvent by one 
or a combination of the following methods: flashing under reduced process 
pressure, evaporating at constant or variant temperatures, distillation, 
adding a co-solvent. The preferred amount of solvent removed is from about 
0.1 wt % to about 100 wt % of the original weight of the solvent used. The 
temperature range is between -100 and 350.degree. C. and the pressure is 
in the range of 1 mm Hg to 760,000 mm Hg (1000 atmospheres) absolute. 
Preferred ranges are 25 to 180.degree. C. and 25 mm Hg to 7,600 mm Hg (10 
atmospheres) absolute, respectively. 
When Processes 3, or 4 is used, the preferred temperature for dissolution 
is in the range from about -100 to about 350.degree. C., preferably from 
about 80 to 170.degree. C., and the preferred temperature to cooled to is 
in the range from about -100 to about 150.degree. C., preferably from 
about 25 to about 100.degree. C. When Process 5 is used, the preferred 
temperature to carry out the purification process is in the range from 
about -100 to about 350.degree. C., most preferably from 25 to 180.degree. 
C. Process 1 or 2 is preferred when co-solvent is used together with the 
major solvent. 
The pre-treatment and/or post-treatment described above can be used in 
conjunction with all parts of this aspect of the invented process. 
In the case of purifying terephthalic acid, dimethyl terephthalate (DMT) 
can be a co-product and the crude terephthalic acid is from downstream of 
an oxidizer in a DMT esterification process. In the cases of purifying 
naphthalene dicarboxylic acids, dimethyl naphthalene dicarboxylate is a 
co-product and the crude naphthalene dicarboxylic acid is from downstream 
of an oxidizer in a naphthalene dicarboxylic acid esterification process. 
The crude aromatic polycarboxylic acid can be produced in an oxidation 
process using a less corrosive solvent selected from the group of benzoic 
acid, methyl benzoate, ethyl benzoate, phenyl benzoate and mixtures 
thereof. These processes are capable of producing an aromatic 
polycarboxylic acid of purity of 99.999 wt %, or 99.9999 wt % or as high 
as 99.99999 wt %. The crude aromatic polycarboxylic acid can be produced 
in an oxidation process operating at a temperature from about 100 to about 
175.degree. C., much lower than the prevalent commercial reaction 
temperatures wherein total impurity concentration is in the range from 
about 0.1 wt % to about 30 wt %. The oxidation reactor or vessel can be 
made of stainless steel materials. 
Below are more detailed descriptions of the four aspects of the invented 
processes. 
First Aspect 
In this aspect, it involves a process for purifying a crude aromatic 
polycarboxylic acid having one or more condensed rings, which comprises 
the following steps: a) dissolving said crude aromatic polycarboxylic acid 
in a solvent comprising a major solvent or a mixture of a major solvent 
and one or more co-solvents, and wherein said co-solvent is selected from 
the group consisting of water, an acid solvent, an oxygen-containing 
solvent, and mixtures thereof wherein the proportion of said major solvent 
to said co-solvent is in the range of 0.1:99.9 to 99.9:0.1 by weight, and 
wherein said solvent is used in an amount from 0.1 to 100 times by weight 
the amount of said crude aromatic polycarboxylic acid; and b) conducting 
purification process; and c) filtering; to obtain a high purity aromatic 
polycarboxylic acid product having one or more condensed rings. Drying 
after filtering is a preferred embodiment of this invention. 
Examples of aromatic polycarboxylic acids amenable to this process are 
terephthalic acid, isophthalic acid, orthophthalic acid, trimellitic acid, 
pyromellitic acids, 2,6-naphthalene dicarboxylic acid, 2,7-naphthalene 
dicarboxylic acid, 1,7-naphthalene dicarboxylic acid, 1,8-naphthalene 
dicarboxylic acid, 2,3,6-naphthalene tricarboxylic acid, and others. 
Examples of preferred major solvent and co-solvent combinations are: a 
major solvent comprising an N,X-monocyclic compound, an alkylamine 
compound, or a mixture of the two compounds, a major solvent and a 
co-solvent comprising water, a major solvent and an acid solvent, a 
morpholine compound and water; an N,X-monocyclic compound and water, a 
major solvent and an oxygen-containing compound. Preferred alkylamine 
compound is triethylamine and triethanolamine. The most preferred mixed 
solvent is morpholine and a co-solvent comprising water. 
All of the five purification processes as described above are applicable to 
this aspect of the invented process. 
Second Aspect 
Another aspect of this instant invention involves a process for purifying a 
crude aromatic polycarboxylic acid having one or three condensed rings, 
which comprises the following steps: a) dissolving said crude aromatic 
polycarboxylic acid in a solvent comprising a major solvent or a mixture 
of a major solvent and one or more co-solvents, wherein said co-solvent is 
selected from the group consisting of an alcohol, water, an acid solvent, 
an oxygen-containing solvent, and mixtures thereof, wherein the proportion 
of said major solvent to said co-solvent is in the range of 0.1:99.9 to 
99.9:0.1 by weight, and wherein said solvent is used in an amount from 0.1 
to 100 times by weight the amount of said crude aromatic polycarboxylic 
acid; and b) conducting purification process; and c) filtering; to obtain 
a high purity aromatic polycarboxylic acid product having one or three 
condensed rings. Drying after filtering is a preferred embodiment of this 
invention. 
Examples of aromatic polycarboxylic acids amenable to this process are 
terephthalic acid, isophthalic acid, orthophthalic acid, trimellitic acid, 
pyromellitic acids, and others. 
Some typical preferred major solvent and co-solvent combinations are: a 
major solvent comprising an N,X-monocyclic compound, an alkylamine 
compound, or a mixture of the two compounds, a major solvent and water, a 
major solvent and an alcohol, a major solvent and an acid solvent, a 
morpholine compound and water; an N,X-monocyclic and water, a major 
solvent and an oxygen-containing compound. Preferred alkylamine compound 
is triethylamine and triethanolamine. Co-solvent is preferably water, 
methanol, or ethanol. Water and methanol are the most preferred 
co-solvent. 
All of the five purification processes as described above are applicable to 
this aspect of the invented process. 
Third Aspect 
Yet another aspect of this invention involves a process for purifying a 
crude aromatic polycarboxylic acid having two condensed rings, which 
comprises the following steps: a) dissolving said crude aromatic 
polycarboxylic acid in a solvent comprising an N,X-monocyclic compound or 
a mixture of an N,X-monocyclic compound and one or more co-solvents 
wherein said co-solvent is selected from the group consisting of an 
alcohol, water, an acid solvent, an oxygen-containing solvent, and 
mixtures thereof wherein the proportion of said major solvent to said 
co-solvent is in the range of 0.1:99.9 to 99.9:0.1 by weight, and wherein 
said solvent is used in an amount from 0.1 to 100 times by weight the 
amount of said crude aromatic polycarboxylic acid; and; b) conducting 
purification process to precipitate said aromatic polycarboxylic acid; and 
c) filtering; to obtain a high purity aromatic polycarboxylic acid product 
having two condensed rings. Drying after filtering is a preferred 
embodiment of this invention. 
Examples of aromatic polycarboxylic acids amenable to this process are 
2,6-naphthalene dicarboxylic acid, 2,7-naphthalene dicarboxylic acid, 
1,7-naphthalene dicarboxylic acid, 1,8-naphthalene dicarboxylic acid, 
2,3,6-tricarboxylic acid, and others. 
N,X-monocyclic compound is the major solvent in this aspect. Some preferred 
combinations are: a major solvent and water, a major solvent and an 
alcohol, a major solvent and an acid solvent, a major solvent and an 
oxygen-containing compound. Preferred N,X-monocyclic compound is 
morpholine compound, and the most preferred is morpholine. Preferred 
co-solvent is water, methanol, or ethanol. The most preferred co-solvent 
is water and methanol. 
All of the five purification processes as described above are applicable to 
this aspect of the invented process. 
Fourth Aspect 
A further aspect of this invention involves a process for purifying a crude 
aromatic polycarboxylic acid having two condensed rings, which comprises 
the following steps: a) dissolving said crude aromatic polycarboxylic acid 
in a solvent comprising an alkylamine compound or a mixture of an 
alkylamine compound and one or more co-solvents wherein said co-solvent is 
selected from the group consisting of an alcohol, water, acid solvent, 
oxygen-containing solvent, and mixtures thereof wherein the proportion of 
said major solvent to said co-solvent is in the range of 0.1:99.9 to 
99.9:0.1 by weight, and wherein said solvent is used in an amount from 0.1 
to 100 times by weight the amount of said crude aromatic polycarboxylic 
acid; b) conducting purification process to precipitate said aromatic 
polycarboxylic acid wherein the composition of said solvent is changed by 
removing a portion of said solvent; and c) filtering; to obtain a high 
purity aromatic polycarboxylic acid product having two condensed rings. 
Drying after filtering is a preferred embodiment of this invention. 
Examples of aromatic polycarboxylic acids amenable to this process are 
2,6-naphthalene dicarboxylic acid, 2,7-naphthalene dicarboxylic acid, 
1,7-naphthalene dicarboxylic acid, 1,8-naphthalene dicarboxylic acid, 
2,3,6-tricarboxylic acid, and others. 
Alkylamine compound is the major solvent in this aspect. Some preferred 
combinations are: a major solvent and water, a major solvent and an 
alcohol, a major solvent and an acid solvent, a major solvent and an 
oxygen-containing compound. Preferred alkylamine compound is triethylamine 
and triethanolamine. Preferred co-solvent is water, methanol, or ethanol. 
The most preferred co-solvent is water and methanol. 
Process 1 and Process 2 of the purification processes as described above 
are applicable to this aspect of the invented process.

The following examples are presented hereinafter to facilitate an 
understanding of the process of the present invention. They are presented 
for the purposes of illustration only and are not intended to limit the 
scope of the present invention. 
EXAMPLE A 
(Prior Art, Comparative Example) 
A sample of crude terephthalic acid (CTA) from a PTA manufacturer with the 
following levels of impurities was used in the experiment: 
______________________________________ 
4-CBA Benzoic Acid 
p-Toluic Acid 
______________________________________ 
PTA(ppmw) 2436 1097 515 
______________________________________ 
Where ppmw means parts per million by weight. 
A CTA with similar composition was then subject to a conventional 
hydrogenation purification method as discussed in the prior art to give a 
PTA product with the following impurity level: 
______________________________________ 
4-CBA Benzoic Acid 
p-Toluic Acid 
______________________________________ 
PTA(ppmw) 15 0 141 
______________________________________ 
EXAMPLE B 
(Prior Art, Comparative Example) 
Similar experiment as Example A is carried out with an oxidation 
purification process method as discussed in the prior art. The PTA product 
contained the following levels of impurities: 
______________________________________ 
4-CBA Benzoic Acid 
p-Toluic Acid 
______________________________________ 
PTA(ppmw) 25 52 150 
______________________________________ 
The impurity levels in the purified products represent typical commercially 
available polymer grade terephthalic acid. The following examples 
illustrate this instant invention. 
EXAMPLE 1 
A sample of 15 grams of CTA used in Example A was dissolved at room 
temperature into a solution containing 32 grams of morpholine and 30 grams 
of water. The temperature of this solution was raised to and maintained at 
140.degree. C., under atmospheric pressure, long enough to reduce the 
total solution volume by 19 c.c. The solution was then cooled to allow 
solids to precipitate. These precipitated solids were then filtered to 
separate from the mother liquor. The filter cake was subsequently washed 
with a morpholine and water mixture. The recovered solids were then 
reslurried in 27 grams of acetic acid, followed by filtration, rinse with 
water, and drying. A dried cake, 7.4 grams, of purified terephthalic acid 
was obtained. Analysis with HPLC showed the PTA contained. 
______________________________________ 
4-CBA Benzoic Acid 
p-Toluic Acid 
______________________________________ 
PTA(ppmw) 8 0 0 
______________________________________ 
EXAMPLE 2 
An experiment similar to Example 1 was carried out with a mixed solvent 
containing a mixture of 50% H2O and 50% N-methyl morpholine at room 
temperature. Terephthalic acid has negligible solubility in either pure 
N-methyl morpholine or water alone. Its solubility in the mixed solvent in 
this experiment was found to be about 38 wt % at room temperature. This 
solution was treated with 50 wt % acetic acid in water to precipitate a 
crystalline terephthalic acid product of higher purity. 
EXAMPLE 3 
An experiment similar to Example 2 was carried out with methanol used to 
replace water as the co-solvent for N-methyl morpholine. Terephthalic acid 
solubility was found to be 30 wt % at 60.degree. C. After dissolving CTA 
at 60.degree. C., the solution was allowed to cool to room temperature, at 
which temperature terephthalic acid has a solubility of only 8 wt %. 
Terephthalic acid solids with improved purity were precipitated in the 
process. 
EXAMPLE 4 
An experiment similar to Experiment 2 is carried out with a mixed solvent 
containing 50 wt % triethylamine and 50 wt % water. At room temperature, 
terephthalic acid has negligible solubilities in either pure triethylamine 
or water. However, it was unexpectedly found that terephthalic acid is 
soluble in the mixture of triethylamine and water that contains 
significant portion of water. The solubility was found reaching about 28 
wt % in the 50/50 mixture. The solubilities of benzoic acid, 4-CBA, and 
p-toluic acid were found to be around 155 wt %, 70 wt %, and 90 wt % in 
the 50/50 mixture respectively. From the above solubility data, 
terephthalic acid purity can be improved by first dissolving the crude 
terephthalic acid in a mixed solvent of 50% water and 50% triethylamine at 
room temperature. The water content is then reduced from 50 wt % to 
approximately 10 wt %. Terephthalic acid with improved purity can be 
obtained. 
EXAMPLE 5 
An experiment similar to Example 4 is carried out by replacing water with 
methanol as the co-solvent. The solubility was found reaching to about 48 
wt % at room temperature in the solvent mixture which contains 50 wt % 
methanol. From the above solubility data, terephthalic acid purity can be 
improved by dissolving the crude terephthalic acid in a mixed solvent of 
50 wt % methanol and 50 wt % triethylamine at room temperature. Most of 
the impurities are found to remain in solution when adequate amount of 
acid is added to the solution to precipitate out terephthalic acid solids. 
When a 50 wt % of aqueous acetic acid solution is added to the solution 
containing CTA, terephthalic acid solids with improved purity can be 
obtained by precipitation. 
EXAMPLE 6 
An experiment is carried out by dissolving CTA in pure N-methyl morpholine 
oxide at 80.degree. C. The solubility of terephthalic acid at 80.degree. 
C. was found to be 22 wt %. However, its solubility was found to decrease 
to about 1 wt % in a mixed solvent containing 75 wt % of water and 25 wt % 
N-methyl morpholine oxide at room temperature. Solubilities of other 
impurities were found to be significantly higher than the solubility of 
terephthalic acid under similar conditions. The CTA containing solution is 
cooled down to room temperature and an amount of water equal to three 
times the weight of N-methyl morpholine oxide is added. Terephthalic acid 
of improved purity is precipitated out of the solution. 
EXAMPLE 7 
An experiment similar to Example 2 is carried out to purify 2,6-naphthalene 
dicarboxylic acid (2,6-NDA). At room temperature 2,6-NDA has negligible 
solubilities in either pure morpholine or water. However, it was 
unexpectedly found that the solubility of 2,6-NDC increased to about 12 wt 
% in a solvent mixture that contained 40 wt % water and 60 wt % 
morpholine. The solubility of 1,2,4-benenetricarboxylic acid, a known 
impurity in crude 2,6-NDA, was found to be much higher at about 45 wt % at 
40 wt % of H2O. Solubilities of other impurities in crude 2,6-NDA are 
expected to be significantly higher than the solubility of 2,6-NDA. 
Crude 2,6-NDA is dissolved in a mixed solvent of 40 wt % water and 60 wt % 
morpholine at room temperature. When a 40 wt % acetic acid in water is 
added to the solution containing crude 2,6-NDA, 2,6-naphthalene 
polycarboxylic solids with improved purity are precipitated out of the 
solution. 
EXAMPLE 8 
An experiment similar to Example 2 is carried out to purify isophthalic 
acid. At room temperature a crude isophthalic acid sample is dissolved in 
a mixed solvent containing of 50 wt % water and 50 wt % morpholine. At 
room temperature, isophthalic acid has negligible solubilities in either 
pure morpholine or water. However, in this mixed solvent containing of 50 
wt % water and 50 wt % morpholine, the solubility of isophthalic acid was 
unexpectedly found to be about 53 wt % at room temperature. At the same 
time, the solubilities of m-toluic acid and benzoic acid, known impurities 
in crude isophthalic acid, were found to be 50 to 105 wt % in the same 
mixed solvent. To this solution containing crude isophthalic acid is added 
acetic acid. Solids of isophthalic acid with improved purity are 
precipitated out of the solution.