Method and apparatus for preparing purified terephthalic acid

A method and apparatus for purifying crude terephthalic acid from a liquid dispersion thereof also containing impurities selected from unreacted starting materials, solvents, products of side reactions and/or other undesired materials is provided. The method comprises the steps of filtering the dispersion to form a crude terephthalic acid filter cake, dissolving the filter cake in a selective crystallization solvent at an elevated temperature to form a solution, crystallizing purified terephthalic acid from the solution in the crystallization solvent by reducing the pressure and temperature of the solution, and separating the crystallized purified terephthalic acid from the solution. According to the invention, the selective crystallization solvent is non-aqueous, non-corrosive and essentially non-reactive with terephthalic acid. Preferably, the selective crystallization solvent is N-methyl pyrrolidone. The method and apparatus produces purified terephthalic acid having a purity desired for use in forming polyester resin and other products at an economically attractive rate and at operating conditions of reduced severity which require a lower capital investment and simplified processing.

The present invention relates to a method and apparatus for preparing 
purified terephthalic acid. It also relates to methods and apparatuses for 
purifying crude terephthalic acid to produce a purified terephthalic acid 
product which is a useful staring material for producing polyester resin, 
which is in turn useful for the production of fibers, film, plastic 
bottles, and polyester resin structures, often reinforced by other 
materials such as glass fiber. 
BACKGROUND OF THIS INVENTION 
Purified terephthalic acid (PTA) is a starting material for the formation 
of polyester resin, which is, in turn, used to make many materials of 
commerce having a variety of utilities. Purified terephthalic acid is 
formed from "crude" terephthalic acid conventionally by a number of 
purification methods, often with the aid of catalysts. The methods for 
purifying crude terephthalic acid heretofore available are not completely 
satisfactory either from an engineering standpoint, or from an economic 
standpoint, yet the purity of the purified terephthalic acid is an 
important determinant of the satisfactoriness of the processes by which 
the polyester resin is formed. 
A number of reaction systems are known for forming crude terephthalic acid 
from a variety of starting materials. The purification aspects of the 
present invention may be used with substantially any of these reaction 
systems, but in accordance with the invention it is preferred that a 
reaction system involving the oxidation of paraxylene (p-xylene) be 
employed, and the use of such a synthesis system forms a part of the 
present invention. 
The problems of existing and prior systems for producing purified 
terephthalic acid center around the difficulties in running the reaction 
systems to produce good yields of crude terephthalic acid economically, 
compounded by the difficulties of refining the crude terephthalic acid to 
eliminate impurities and unwanted components to produce purified 
terephthalic acid of a quality suitable as a starting material for 
producing polyester. Concomitant problems in prior systems include the 
high capital investment required for PTA plants, the severity of operating 
conditions of prior processes, both for the production of crude 
terephthalic acid, and for its purification, and the need for handling 
catalyst systems and reaction solvents, as well as reaction byproducts in 
a way such that environmental problems are minimized, and loss of material 
is also controlled. 
One factor of importance in the production of purified terephthalic acid is 
the formation of crystals having a size and shape giving them good 
handling characteristics, washability and filterability. 
SUMMARY OF THE INVENTION 
In accordance with the present invention there is provided a method and 
apparatus for producing purified terephthalic acid. In one aspect, the 
method includes the production of crude terephthalic acid by the oxidation 
of p-xylene. The oxidation step produces not only terephthalic acid, but 
by side reactions p-toluic acid and 4-caboxybenzaldehyde (4-CBA). The 
product produced in the oxidation step is a liquid dispersion containing 
unreacted starting materials, solvents, if any have been used, the 
products of side reactions, particularly those just mentioned, and other 
materials which are not desired in the sought-for purified terephthalic 
acid. The oxidation step of the present invention is so conducted that the 
conversion to crude terephthalic acid should be at least about 30% by 
weight per pass of p-xylene. 
In further accordance with the invention, the crude terephthalic acid from 
the oxidizer is first grossly separated from the other materials from the 
oxidizer and then it is re-dissolved in a selective crystallization 
solvent and, optionally, one or more additional solvents of the invention 
discussed below. The re-dissolved crude terephthalic acid is then 
crystallized out of the selective crystallization solvent and additional 
solvents of the invention in one or, preferably, two crystallization 
stages. Provision is made to separate out the crystallized and 
progressively purified terephthalic acid from the solvents of the 
invention, and the filter cake of purified terephthalic acid ultimately 
obtained is washed with other solvents of the invention and dried or, 
alternatively, dried (e.g., using a vacuum dryer), sent to a soaker to 
remove residual solvent, and ultimately filtered and dried for storage or 
for further processing. 
Also in accordance with the present invention, improvements in the 
crystallization processes just outlined are provided which produce larger 
more globular crystals that are thought to contain little or no salt of 
the kind which may tend to form when the selective crystallization 
solvent(s) is an organic base. The larger non-salt crystals have the 
advantage that they resist destruction by water rinsing and are otherwise 
easier to recover solvent from, as well as being easier to rinse for 
removal of residual impurities. 
The improvements in the crystallization processes comprise flashing solvent 
from the crystallizing acid by reducing the pressure on it, preferably 
both prior to and during the cooling of the saturated acid solution. It is 
further preferred to reduce the pressure progressively to lower levels, 
either in a batch or continuous flow crystallizer, and this may be 
arranged to be performed stepwise or continuously. Still further, heat may 
be added to the crystallizing acid during the application of reduced 
pressure to increase the rate and quantity of solvent removal, care being 
taken, however, to avoid materially increasing the temperature of the 
crystallizing acid to cause redissolution of the acid and consequent waste 
of energy. 
As was mentioned above, in accordance with the invention, crystallization 
may be performed in multiple stages; when this form of the invention is 
used, it is preferred that some or all of the crystallization improvement 
techniques just discussed be utilized in the second or last stage, 
although the techniques may also be used to advantage in the first stage. 
Further in accordance with the invention, co-solvents may be used for 
purifying terephthalic acid by flash crystallization. A co-solvent having 
a lower boiling point than the solvent can be used to reduce the flashing 
temperature for crystallization and hence the dissolution temperature. 
With a lower flashing temperature, crystallization can be carried out 
under a lower degree of vacuum. 
The co-solvents include water, C.sub.1 to C.sub.5 alcohols, such as 
methanol or ethanol, C.sub.5 to C.sub.10 hydrocarbons, such as pxylene, 
and C.sub.1 to C.sub.10 organic acids, such as formic acid or acetic acid, 
etc. It is thus possible to include about 1 to about 50% inert solvents 
having boiling points ranging from 25 to 200.degree. C. as the 
co-solvents. 
The invention also contemplates that steps are included to reclaim and 
recycle the solvents of the invention at each stage of crystallization and 
washing, including recycle of some of the recovered materials to the 
oxidizer. Steps are also taken to closely control the delivery of any 
objectionable materials to the environment. 
In an important aspect, the present invention is based on several 
discoveries relating to solvents which are effective to bring about the 
purification of crude terephthalic acid through crystallization and 
separation steps. These discoveries may be summarized in several ways as 
follows. 
The selective crystallization solvents useful in the practice of the 
present invention include those in which (a) the impurities desired to be 
separated from terephthalic acid to purify it are relatively more soluble 
in the solvent than is the terephthalic acid at substantially every 
temperature within the desired range of temperatures at which the solvent 
containing terephthalic acid is to be handled, and (b) the terephthalic 
acid is more soluble at an elevated temperature and less soluble at a 
lower or reduced temperature. It is to be understood that the term 
"selective crystallization solvent" is intended to mean solvents useful in 
the selective crystallization of terephthalic acid as described above and 
as described in greater detail below and as shown in FIGS. 1 and 2. 
In this connection it should be noted that U.S. Pat. No. 3,465,035 mentions 
that certain organic solvents (pyridine, dimethyl sulfoxides, dimethyl 
foramide, and the like) have been used to purify terephthalic acid, but 
that they suffer from being unstable in air and easily form addition 
products with terephthalic acid. This same patent, along with several 
others, also teaches the use of acetic acid and water as purification 
solvents for terephthalic acid. By contrast, the selective crystallization 
solvents according to the present invention are (a) non-aqueous, (b) 
non-corrosive, and (c) essentially non-reactive with terephthalic acid and 
do not include those prior practices just described. Specifically, water, 
acetic (and other alkyl) acid, and the above-mentioned organic solvents 
are excluded from the selective crystallization solvents which are 
contemplated by the present invention. 
In accordance with the invention, the primary preferred selective 
crystallization solvents are N-methyl pyrrolidone (NMP) and N,N-dimethyl 
acetamide (DMAC), for the several reasons discussed below, and for their 
superior performance. U.S. Pat. No. 2,949,483, dated Aug. 16, 1960 to Ham, 
discloses NMP used to crystallize terephthalic acid, but does not use the 
same dissolution temperature range as in the present invention. Nor does 
it suggest flash crystallization or its advantageous results. Tr. Vses. 
Nauch.-Issled. Proekt. Inst. Monomerov (1970), 2(2), 26-32; From: Ref. 
Zh., Khim. 1971, Abstr. No. 1N166; V. N. Kulakov, et al.; entitled 
"Purification of Aromatic Dicarboxylic Acids Obtained by Liquid-Phase 
Oxidation of Dialkyl Derivatives of Aromatic Hydrocarbons," very briefly 
mentions NMP as a solvent, but says nothing about dissolution temperatures 
or flash crystallization. 
N-methyl pyrrolidone (NMP) and N,N-imethyl acetamide (DMAC) are the 
preferred selective crystallization solvents for the practice of the 
invention. These solvents are non-aqueous, thermally stable, non-toxic 
(environmentally safe), non-corrosive, and commercially available. NMP is 
the most preferred selective crystallization solvent for the practice of 
the present invention, because its solubility versus temperature curve for 
terephthalic acid slopes upwardly and to the right, which means that 
terephthalic acid can be dissolved in it at elevated temperatures, and 
precipitated or crystallized from it at lower temperatures. 
Although NMP is the most preferred selective crystallization solvent, it is 
to be understood that DMAC exhibits similar desirable characteristics and 
that, in accordance with the present invention, other preferred selective 
crystallization solvents for purification of crude terephthalic acid can 
be selected from various polar organic solvents including, but not 
intended to be limited to, N-alkyl-2-pyrrolidone (such as Nethyl 
pyrrolidone), N-mercaptoalkyl-2-pyrrolidone (such as 
N-mercaptoethyl-2-pyrrolidone), N-alkyl-2-thiopyrrolidone (such as 
N-methyl-2-thiopyrrolidone), and N-hydroxyalkyl-2-pyrrolidone (such as 
N-hydroxyethyl-2-pyrrolidone), N-ethyl pyrrolidone, N-mercaptoethyl 
pyrrolidone, N-methyl thiopyrrolidone, N-hydroxyethyl pyrrolidone, 
1,5-dimethyl pyrrolidone, N-methyl piperidone, N-methyl caprolactam, 
N-formyl morpholine, morpholine, N,N-dimethyl formamide, N,N-dimethyl 
acetamide, and N-formyl piperidine, and the like, and mixtures thereof. 
Still other selective crystallization solvents contemplated by the present 
invention include, but are not intended to be limited to, sulfolane, 
methyl sulfolane, the sulfones, the morpholines (such as, morpholine and 
N-formyl morpholine), the carbitols, C.sup.1 to C.sup.12 alcohols, the 
ethers, the amines, the amides, and the esters, and the like, and mixtures 
thereof. 
It is preferred that the desired selective crystallization solvent be used 
in a multi-stage crystallization process in combination with one or more 
additional solvents, preferably two such additional solvents, particularly 
where the crude terephthalic acid is less than about 98% pure. Preferably, 
a wash solvent, such as, but not intended to be limited to, water, 
p-xylene, acetone, methyl ethyl ketone (MEK) or methanol, and the like, is 
used in the washing of the initial filter cake obtained from the first 
separation of crude terephthalic acid from other materials issuing from 
the oxidizer. In addition, a displacement solvent having a low boiling 
point, such as, but not intended to be limited to, water, methanol, 
acetone, MEK, and the like, may be used. Preferably, water is used as the 
displacement solvent in association with the third filter following the 
second crystallization stage in the preferred process. The desired 
displacement solvent displaces the selective crystallization solvent from 
the resulting filter cake, whereby substantially only the displacement 
solvent is present during the soaking process. The soaking process is 
preferred to eliminate any possible residual solvent trapped in the TA 
crystals before the product is subjected to the final filtration and 
drying steps. 
As described above, NMP and DMAC are the preferred selective 
crystallization solvents for the practice of the invention. They are 
non-aqueous, thermally stable, non-toxic (environmentally safe), 
non-corrosive, and commercially available. No is the preferred selective 
crystallization solvent for the practice of the present invention, 
because, among other things, its solubility versus temperature curve for 
terephthalic acid slopes upwardly and to the right, which means that 
terephthalic acid can be dissolved in it at elevated temperatures, and 
precipitated or crystallized from it at lower temperatures. However, the 
solubility versus temperature curve for terephtbalic acid is of a much 
milder slope than the solubility curves in NMP for other materials sought 
to be separated from crude terephthalic acid, such as benzoic acid, 
4-carboxybenzaldehyde (4-CBA), and p-toluic acid. As a consequence, when 
crude terephthalic acid, containing or associated with unreacted starting 
materials, solvents (if any), and products of side reactions, such as 
those mentioned above, or other undesired materials, is dissolved in NMP 
or DMAC at an elevated temperature, substantially all the materials are 
dissolved or at least highly dispersed. Then upon removal of heat and 
pressure and subsequent cooling of the NMP or DMAC solution of such 
dissolved materials, the pure terephthalic acid preferentially 
crystallizes out, while the other more soluble materials which may be 
regarded as impurities for the present purposes remain in solution in NMP 
or DMAC. A separation is thus effected between purified terephthalic acid 
and its associated impurities. NMP or DMAC may be stripped of the 
impurities in a reclaiming column and recycled into the process, while the 
impurities may be recycled to the oxidizer step or otherwise disposed of. 
From the foregoing, it can be seen that in accordance with one aspect of 
the present invention, a method is provided for producing purified 
terephthalic acid from crude terephthalic acid in which the crude 
terephthalic acid is dissolved in a desired crystallization solvent at an 
elevated temperature to form a solution and further, in which a purified 
terephthalic acid is crystallized from that solution at a reduced pressure 
and temperature. 
In accordance with another aspect of the invention, a method and apparatus 
are provided for purifying crude terephthalic acid from a liquid 
dispersion thereof also containing unreacted starting materials, solvents, 
products of side reactions, and/or other undesired materials in which the 
crude terephtalic acid is filtered from that dispersion to partially 
separate it from the other materials contained therein by filtration to 
produce a crude terephthalic acid filter cake, and then dissolving that 
filter cake in a desired selective crystallization solvent at an elevated 
temperature to form a solution. Purified terephthalic acid is crystallized 
from that solution by reducing the pressure and temperature thereof and is 
separated from the solvent following crystallization. 
In accordance with still another aspect of the invention, a method and 
apparatus are provided for producing purified terephthalic acid from crude 
terephthalic acid by dissolving the crude terephthalic acid in a desired 
selective crystallization solvent at an elevated temperature to form a 
first solution. First stage purified terephthalic acid is crystallized 
from that first solution at a reduced temperature, and preferably at a 
reduced pressure also. The first stage purified terephthalic acid is 
separated from the solvent solution of other impurities and redissolved in 
the desired selective crystallization solvent at an elevated temperature 
to form a second solution. This second solution is crystallized at a 
reduced pressure and temperature to form a second stage purified 
terephthalic acid and the second stage purified terephthalic acid is 
separated from the second solution. 
In accordance with yet another aspect of the invention, crude terephthalic 
acid is synthesized by contacting paraxylene with oxygen in an oxidizer 
reaction. The crude terephthalic acid is withdrawn from the oxidizer and 
separated grossly from the side products of the reaction, and unreacted 
starting materials. The separated crude terephthalic acid is then 
dissolved in a desired selective crystallization solvent at an elevated 
temperature and crystallized from it as purified terephthalic acid at a 
reduced pressure and temperature. More than one stage of dissolving in a 
desired selective crystallization solvent at an elevated temperature 
followed by crystallization at a reduced pressure and temperature, with 
accompanying separation and washing of the crystallized purified 
terephthalic acid, may be performed.

DETAILED DESCRIPTION OF EMBODIMENTS 
I. Process Description 
The present invention relates to the development of a new PTA manufacturing 
technology. Compared to the current widely used PTA technology, this 
technology provides a substantially lower capital investment in new PTA 
plant construction, as well as lower costs of plant operation. It also 
provides means for current DMT plants to co-produce PTA, to strengthen 
their competitiveness against newer PTA plants. 
Process Summary 
The success of this process is based on the development of a low pressure, 
low temperature, non-aqueous, highly selective crystallization technology. 
The crystallization technology can purify the crude terephthalic acid (TA) 
with purity as low as from between about 70% (from the oxidizer) and about 
98+% in the first-stage crystallizer, and about 99.99+% in the 
second-stage crystallizer. This allows the TA oxidizer to be operated at 
much lower severity than those of widely used prior art processes. No 
acetic acid (as solvent/diluent) or bromine-catalyst initiator is needed 
in the oxidizer in accordance with the present invention. The selective 
crystallization solvent used in the crystallization process is 
non-aqueous, thermally stable, non-toxic (environmentally safe), 
non-corrosive, and commercially available. 
When carrying out the method according to the present invention, employing 
NMP or DMAC as the selective crystallization solvent, the present 
inventors have demonstrated TA purity levels of up to 99.9+wt % after a 
first crystallization process, and up to 99.99+wt % after a second 
crystallization process. In particular, Table 1 illustrates the recovery 
of 99.95 wt % pure TA after the first crystallization process and 99.997 
wt % pure TA after the second crystallization process, from crude TA 
(89.89 wt % TA). 
TABLE 1 
______________________________________ 
1st 2nd 
Crystallization Crystallization 
______________________________________ 
(a) Weight of TA: 56.34 grams 
31.81 grams 
(b) Weight of Crystallization 400.02 grams 248.38 grams 
Solvent: 
(c) Saturation Temperature: 60.degree. C. 
(d) Crystallization Temperature: 15.degree. C. 
(one hour) 
______________________________________ 
Benzoic p-Toluic 4-CBA TA Others 
______________________________________ 
(1) Crude TA Product Composition: 
0.39 wt % 4.49 wt % 2.49 WT % 89.89 WT % 274 
WT % 
(2) First Crystallization Product 
35 ppm 143 ppm 359 ppm 99.95 wt % Not 
Detec- 
ted 
(3) Second Crystallization Product 
&lt;20 ppm &lt;20 ppm &lt;10 ppm 99.997+ 
wt % 
______________________________________ 
Table 2 illustrates the recovery of 99.90 wt % pure TA after the first 
crystallization process and 99.9933 wt % pure TA after the second 
crystallization process from crude TA (89.89 wt % TA) by increasing both 
the saturation temperature and the crystallization temperature. 
TABLE 2 
______________________________________ 
1st 2nd 
Crystallization Crystallization 
______________________________________ 
(a) Weight of TA: 138.08 grams 
70.15 grams 
(b) Weight of Crystallization 685.30 grams 247.46 grams 
Solvent: 
(c) Saturation Temperature: 110.degree. C. 105.degree. C. 
(d) Crystallization Temperature: 40.degree. C. 40.degree. C. 
______________________________________ 
Benzoic p-Toluic 4-CBA TA Others 
______________________________________ 
(1) Crude TA Product Composition: 
0.39 wt % 4.49 wt % 2.49 wt % 89.89 wt % 
2.74 wt % 
(2) First Crystallization Product (Recovery: 56.5 wt %) 
28 ppm 367 ppm 390 ppm 99.90 wt % 
229 ppm 
(3) Second Crystallization Product (Recovery: 47.5 wt %) 
&lt;10 ppm &lt;19 ppm 25 ppm 99.9933 wt % 
13 ppm 
______________________________________ 
Table 3 illustrates the recovery of 99.9960 wt % pure TA (single 
crystallization process) from crude TA (98.99 wt % TA). In addition, each 
of benzoic, p-Toluic, 4-CBA, MMI and other impurities were at less than 10 
ppm. 
TABLE 3 
______________________________________ 
(a) Weight of TA: 152.67 grams 
(b) Weight of Crystallization 786.19 grams 
Solvent: 
(c) Saturation Temperature: 100.degree. C. 
(d) Crystallization Temperature: 40.degree. C. 
______________________________________ 
Benzoic 
p-Toluic 4-CBA TA MMT Others 
______________________________________ 
(1) Crude TA Product Composition: 
&lt;10 &lt;10 18 98.99 303 0.98 
ppm ppm ppm wt % ppm wt % 
(2) Crystallization Product (Recovery: 50.2 wt %) 
&lt;10 &lt;10 &lt;10 &gt;99.9960 
&lt;10 &lt;10 
ppm ppm ppm wt % ppm ppm 
______________________________________ 
Table 4 illustrates the recovery of 99.63 wt % pure TA (single 
crystallization process) from crude TA (83.91 wt % TA) on a large scale 
basis. 
TABLE 4 
______________________________________ 
(a) Weight of TA: 1760 grams 
(b) Weight of Crystallization 6162 grams 
Solvent: 
(c) Saturation Temperature: 160.degree. C. 
(d) Crystallization Temperature: 50.degree. C. 
Benzoic p-Toluic 4-CBA TA Others 
______________________________________ 
(1) Crude TA Feed Product Composition: 
1.03 wt % 4.79 wt % 5.03 wt % 83.91 
wt % 5.24 wt % 
(2) Crystallization Product (Recovery: 24.3 wt %) 
38 ppm 852 ppm 0.23 wt % 99.63 
wt % 500 ppm 
______________________________________ 
Table 5 illustrates the recovery of 99.92 wt % pure TA (single 
crystallization process) from crude TA (79.79 wt % TA) on a large scale 
basis. 
TABLE 5 
______________________________________ 
(a) Weight of TA: 1700 grams 
(b) Weight of Crystallization 5928 grams 
Solvent: 
(c) Saturation Temperature: 160.degree. C. 
(d) Crystallization Temperature: 45.degree. C. 
______________________________________ 
Benzoic p-Toluic 4-CBA TA Others 
______________________________________ 
(1) Crude TA Feed Product Composition: 
1.59 wt % 5.19 wt % 7.61 wt % 79.79 
wt % 5.81 wt % 
(2) Crystallization Product (Recovery: 31.5 wt %) 
10 ppm 203 ppm 446 ppm 99.92 
wt % 184 ppm 
______________________________________ 
Table 6 illustrates the recovery of 99.1 5 wt % pure TA (single 
crystallization process) from crude TA (83.90 wt % TA) on a large scale 
basis at a higher saturation temperature of 190.degree. C. 
TABLE 6 
______________________________________ 
(a) Weight of TA: 1965 grams 
(b) Weight of Crystallization 5684 grams 
Solvent: 
(c) Saturation Temperature: 190.degree. C. 
(d) Crystallization Temperature: 40.degree. C. 
______________________________________ 
Benzoic p-Toluic 4-CBA TA Others 
______________________________________ 
(1) Crude TA Feed Product Composition: 
1.23 wt % 5.25 wt % 6.34 wt % 
83.90 wt % 
3.28 wt % 
(2) Crystallization Product (Recovery: 48.9 wt %) 
-- 0.14 wt % 0.61 wt % 
99.15 wt % 
0.1 wt % 
______________________________________ 
Table 7 illustrates the recovery of 99.9915 wt % pure TA from crude TA 
(98.50 wt % TA) on a large scale basis. The supersaturation of the 
crystallization mixture resulted in the formation of substantially larger 
TA crystals than those crystals resulting from the processes summarized 
above. As would be understood by one skilled in the art, the sizes of TA 
crystals are an important consideration with respect to separation thereof 
from solvents and impurities. 
TABLE 7 
______________________________________ 
(a) Weight of TA: 2333 grams 
(b) Weight of Crystallization 5698 grams 
Solvent: 
(c) Saturation Temperature: 160.degree. C. 
(d) Crystallization Temperature: 45.degree. C. 
Benzoic p-Toluic 4-CBA TA Others 
______________________________________ 
(1) Crude TA Feed Product Composition: 
198 ppm 0.15 wt % 1.23 wt % 98.50 wt % 
989 ppm 
(2) Crystallization Product (Recovery: 69.7 wt %) 
&lt;10 ppm 26 ppm 38 ppm 99.9915 wt % 
11 ppm 
______________________________________ 
Table 8 demonstrates the recovery of 99.45 wt % pure TPA (single 
crystallization process) from crude TPA (82.92 wt % TPA), using 
N,N-dimethyl acetamide (DMAC) as the crystallization solvent. The 4-CBA 
content was reduced from 6 wt % to 3276 ppm. The range of the operating 
temperature was very moderate (from 45 to 120.degree. C.). 
TABLE 8 
______________________________________ 
Purifying "TPA" with N,N-Dimethyl Acetamide by Crystallization 
______________________________________ 
1. N,N-dimethylacetamide used: 
1,000.0 grams 
Crude TPA used: 291.5 grams 
N,N-dimethylacetamide for cake wash: 800 ml 
Purified TPA recovered: 135.6 grams 
(not including losses 
due to solids handling 
and sampling) 
______________________________________ 
Sample Benzoic PTA 4-CBA TPA Unknown 
______________________________________ 
Crude TPA 5.25 6.01 4.59 82.92 1.23 
(wt %) 
Purified TPA 689 3276 1519 99.45* 13 
(ppm) 
______________________________________ 
2. Method: 
(a) The mixture was heated to 120.degree. C. in an agitated 
and 
jacketed flask to dissolve the solids, and the mixture was 
held at the temperature for one hour. 
(b) The mixture was then cooled to 45.degree. C. in one hour. 
(c) The cooled slurry was then filtered in a separatory funnel 
under vacuum to separate the mother liquor from the cake. 
(d) The cake was washed once in the separatory funnel with the 
solvent to remove the residual mother liquor in the cake. The 
wash was carried out at room temperature. 
(e) The wet solid was soaked over night with D.I. water at room 
temperature and then washed three times with D.I. water 
in a 
separatory funnel. 
(f) The solids were dried over night at 180.degree. C. 
______________________________________ 
*weight percent 
As has been discussed, important aspects of this invention are related to 
the discovery of methods to crystallize terephthalic acid MTA) from 
organic solution where the solvent tends to form an organic salt with TA. 
The salt is normally formed from cooling the solution of an organic 
solvent or a mixture of organic solvents, which solution is saturated with 
TA at higher temperatures. However, crystal structure of the salt is 
destroyed when it is washed with water or other solvents to remove the 
solvent in the crystal. The washed crystals become very fine powders which 
are very difficult to filter and wash in order to remove the impurities in 
the trapped mother liquor and the residual solvent. 
According to this invention, the solution of an organic solvent (or mixture 
of organic solvents) saturated with TA and impurities such as 
4-carboxybenzaldehyde (4-CBA), p-toluic acid, etc., is fed to a 
crystallizer maintained at a lower pressure (or under vacuum) to allow the 
solvent (or solvent mixture) to flash instantaneously in a continuous or 
batch manner. Then, the solids (nuclei) generated from solvent flashing 
are allowed to grow for a certain period of time at the reduced pressure 
and temperature. It is desirable to subject the saturated solution to a 
number of solvent flash operations in the same crystallizer or in several 
crystallizers connected in series, each at a different reduced pressure 
(or vacuum), to generate higher TA recovery and larger TA crystals. It has 
been found, surprisingly, that the structure of the crystals produced from 
this method is not adversely affected by washing with water or other 
solvents which have significant solubility of the crystallization solvent 
(or mixture of solvents) or by vacuum drying the crystals to remove 
solvent. Consequently, it appears that there was no salt formation or at 
least the salt formation was minimized so that washing with water or other 
solvent which can dissolve the crystallization solvent or vaccuum drying 
did not change the size and shape of the TA crystals. 
As previously mentioned, organic solvents useful in this invention include, 
but are not limited to, N-methyl pyrrolidone (NMP), N-ethyl pyrrolidone, 
N-mercaptoethyl pyrrolidone, N-methyl thiopyrrolidone, N-hydroxyethyl 
pyrrolidone, 1,5-dimethyl pyrrolidone, N-methyl piperidone, N-methyl 
caprolactam, N-formyl morpholine, morpholine, N,N-dimethyl formamide, 
N,N-dimethyl acetamide, N-formyl piperidine, N-alkyl-2-pyrrolidone (such 
as N-ethyl pyrrolidone), N-mercaptoalkyl-2-pyrrolidone (such as 
N-mercaptoethyl-2-pyrrolidone), N-alkyl-2-thiopyrrolidone (such as 
N-methyl-2-thiopyrrolidone), and N-hydroxyalkyl-2-pyrrolidone (such as 
N-hydroxyethyl-2-pyrrolidone). 
In order to remove the residual solvent trapped in the crystals from the 
final TA product, the washed TA crystals are preferably fed to a high 
temperature soaker where water is used to partially or completely dissolve 
the TA crystals. 
The following examples illustrate the principles and features of the 
invention. 
EXAMPLE 1 
Cooling Crystallization 
9761 g of NMP was added to a jacketed crystallizer provided with agitation 
together with 3028 g of TA. This mixture was heated to 180.degree. C. 
under atmospheric pressure until all of the TA was dissolved. 
The mixture was then subjected to surface cooling by circulating a cooling 
medium through the jacket until a temperature of 45.degree. C. was 
reached. Then after 15 minutes, the slurry was filtered to separate the 
solids from the mother liquor, and the cake was washed with room 
temperature pure NMP to displace all the mother liquor from the cake. 
A sample was taken from the cake for observation under a microscope. The 
crystals had a bar-like shape and a size in the range of 120-150 microns. 
In order to remove the solvent from the cake, the cake had to be washed 
with water or other suitable solvents which have high solubility of the 
solvent. Hot water at 80.degree. C. was used to wash the cake. However, 
the bar-like crystals in the cake were completely destroyed by water and 
changed into fine powders which looked more like precipitates than 
crystals produced by a crystallization process. These fine precipitates 
are extremely difficult to wash and handle and the residual solvent 
removal is complicated. 
EXAMPLE 2 
Flashing Crystallization 
The same sample preparation of NMP and TA as in the previous example was 
used, except that the mixture was also, prior to the cooling step, subject 
to a flashing removal of solvent by reducing the pressure from atmospheric 
to 125 mmHg of vacuum. In this way, some solvent was vaporized out and 
condensed through a cooler so the temperature of the mixture dropped from 
180.degree. C. to 147.degree. C. The amount of solvent flashed out created 
a super-saturation condition so the TA dissolved in NMP crystallizes into 
the solid phase. 
Although the flashing step is done instantaneously, crystallization of TA 
requires some time to take place, so the mixture was kept agitated for 30 
minutes to form the nuclei and permit them to grow, thus forming a slurry. 
The slurry was filtered to separate the solids from the liquid phase, 
washed with pure NMP at room temperature and observed under a microscope. 
The crystal shape was globular instead of bar-like, as it was when using 
the previous cooling crystallization method, and very uniform in size but 
smaller--about 40-60 microns. 
Then the cake was washed with hot water at 80.degree. C. and, surprisingly, 
the globular-like crystals were not affected by water washing (their shape 
and size were not changed). These globular-like crystals have a very high 
filtration rate and effectively rinsing them is much easier. 
EXAMPLE 3a 
Crystal Growth 
To promote crystal growth, the experiment, as in the preceding example, was 
repeated except that 6174 g of NMP and 1952 g of terephthalic acid were 
used. 
Also, the flash pressure was 120 mmHg instead of 125 mmHg and the 
temperature was 145.degree. C. Then, the mixture was flashed a second time 
at 40 mmHg, as described in the preceding example, and the temperature 
dropped to 110.degree. C. Thus, more terephthalic acid crystallized. The 
crystal shape was globular-like and the size was increased to 60-80 
microns. 
EXAMPLE 3b 
The experiment as in Example 3a was repeated except that 7490 g of NMP and 
2325 g of terephthalic acid were used. Also, a different pressure profile 
was followed and two more flashes were added: 
first flash: 150 mmHg @ 154.degree. C. 
second flash: 80 mmHg @ 135.degree. C. 
third flash: 40 mmHg @ 117.degree. C. 
fourth flash: 20 mmHg @ 101.degree. C. 
Observation under a microscope showed that the crystal shape was globular 
and the size improved significantly. The final sample contained crystals 
in the range of 120-150 microns. 
EXAMPLE 4a and 4b 
Flash/Vaporizing Crystallization 
The experiment as in Example 3b was repeated except that the temperature of 
the hot oil circulation through the jacket was kept 5-10.degree. C. above 
the crystallizer temperature in a way that some vaporization of the 
solvent occurred at the same time of the flashing. This procedure resulted 
in more solvent flashed/vaporized and a lower temperature profile which 
increases the recovery of the crystals: 
______________________________________ 
FLASH 
No. EXAMPLE 3b EXAMPLE 4a EXAMPLE 4b 
______________________________________ 
First 154.degree. C. 
155.degree. C. 
145.degree. C. 
150 mmHg 150 mmHg 150 mmHg 
755 ml of solvent 1328 ml of solvent 1660 ml of solvent 
removed by flashing removed by flashing removed by flashing 
Second 135.degree. C. 135.degree. C. 130.degree. C. 
80 mmHg 80 mmHg 80 mmHg 
696 ml of solvent 473 ml of solvent 580 ml of solvent 
removed by flashing removed by flashing removed by flashing 
Third 117.degree. C. 110.degree. C. 115.degree. C. 
40 mmHg 40 mmHg 40 mmHg 
248 ml of solvent 110 ml of solvent 340 ml of solvent 
removed by flashing removed by flashing removed by flashing 
Fourth 101.degree. C. 90.degree. C. 95.degree. C. 
20 mmHg 20 mmHg 20 mmHg 
135 ml of solvent 155 ml of solvent 430 ml of solvent 
removed by flashing removed by flashing removed by flashing 
______________________________________ 
When observed under a microscope, the crystals looked globular-like in 
shape as described for Example 2 above. 
EXAMPLE 5 
In this example, the 4-CBA rejection characteristics of the flash 
crystallization method was compared with that of crystallization by 
cooling alone. 
Flash Crystallization 
The crystallizer was charged with 31 g TA/100 g solvent. 4-CBA was added to 
start with a concentration based on solids of around 2%. The mixture was 
heated to 185.degree. C. and agitated until most of the crystals 
dissolved. Some crystals may not have dissolved and these became seeds for 
crystal growth. The oil bath was set to 155.degree. C. The first vacuum 
(150 mmHHg) was pulled to remove around 15-20% of the liquid in about 15 
minutes. Next, the flash vacuum was pulled to 80 mmHg and 6-8% of the 
remaining liquid was removed within 5 minutes. In the third flash, 6-8% of 
the solvent was removed with a vacuum of 40 mmHg requiring about 6-7 
minutes. In the fourth flash, 12% of the solvent was removed with a vacuum 
of 20 mmHg requiring about 10-15 minutes. Then the mother liquor was 
cooled to 50.degree. C. as quickly as possible, taking about 30 minutes. 
The crystals were then removed from the flask and filtered using a Buchner 
funnel and side arm flask. About 200 g of 50.degree. C. solvent was then 
poured over to wash the crystals. The crystals were then put in a pressure 
filter and dried by passing nitrogen for 30 minutes at 40 psi. The final 
crystals were analyzed for 4-CBA content, giving a result of 500 ppm. 
Cooling Crystallization 
The crystalizer was charged with 31 g TA/100 g solvent. 4-CBA was added to 
start with a concentration based on solids of 2%. The mixture was heated 
to 185.degree. C. and agitated until most of the crystals dissolved. Some 
crystals may not have dissolved and these became seeds for crystal growth. 
Cooling of the mix was started to crystallize the TA from the solution. 
The cooling rate was 2.degree. C./min to a final temperature of 50.degree. 
C. The crystals were then removed from the flask and filtered using a 
Buchner funnel and side arm flask. About 200 g of 50.degree. C. solvent 
was then poured over to wash the crystals. The crystals were then put in a 
pressure filter and dried by passing nitrogen for 30 minutes at 40 psi. 
These final crystals were analyzed for 4-CBA content, giving a result of 
about 500 ppm. 
The experiments show that the flash and cooling crystallization processes 
have substantially the same rejection capability for 4-CBA. 
According to the invention, a preferred embodiment of the process is 
divided into five sections: 
(1) Oxidation Section: 
In this section, p-xylene is oxidized according to the following main 
reactions: 
______________________________________ 
(a) p-xylene + oxygen .fwdarw. 
terephthalic acid + water 
(b) p-xylene + oxygen .fwdarw. p-toluic acid + water 
(c) p-xylene + oxygen .fwdarw. 4-carboxybenzaldehyde (4-CBA) + water 
______________________________________ 
The oxidizer residence time is approximately five hours. Since the oxidizer 
effluent will contain up to about 30% TA, mixing in the oxidizer is very 
important in order to maintain the yield and selectivity, and to prevent 
fouling and blockages. The initial mixing of the feed streams may be 
achieved in a static mixer (outside of the oxidizer). Further mixing may 
be provided by an air sparger and by external circulation. Depending on 
the thoroughness of the p-xylene washing step at the filter (discussed 
below), the terephthalic acid (TA) in the solid can vary from between 
about 55% and about 90+%. 
(2) Crystallization Section: 
(A) First Crystallization 
After filtration, the solids from the oxidizer effluent are mixed with the 
mother liquor and the solvent wash liquid from the second-stage 
crystallizer and with additional crystallization solvent. The mixed slurry 
is dissolved in a slurry tank at a predetermined temperature, preferably 
at from between about 140.degree. C. and about 200.degree. C. The 
saturated solution is transferred to a holding tank to remove p-xylene 
through evaporation. The saturated solution is then fed to a first-stage 
batch crystallizer to recover purified TA by flash evaporation of solvent 
at reduced pressure and/or cooling. After the crystallization step, the 
crystallizer content is then dropped to a product holding tank and is 
pumped continuously to a filter (or centrifuge) to collect the solids to 
be recrystallized in the second-stage crystallizer for further 
purification. 
(B) Second Crystallization 
The solids generated from the first crystallizer filter are redissolved in 
a feed dissolver with the crystallization solvent for the second-stage 
crystallizer at a predetermined condition, such as at a temperature of 
from between about 140.degree. C. and about 200.degree. C. The saturated 
solution is pumped to the second-stage crystallizer for crystal growth and 
recovery, again, by flash evaporation of solvent at reduced pressure 
and/or cooling. Then, the crystalizer content is dropped to a holding tank 
for filtration before being sent to the soaker. In the filtration step, 
the solid (cake) is first washed by the crystallization solvent to 
displace mother liquor remaining in the cake. The solid is then washed by 
a low-boiling solvent to displace the crystalation solvent in the cake and 
subsequently dried to remove the final liquid from the PTA product. The 
crystallization solvent alternatively can be displaced by drying the solid 
using a vacuum dryer and subjecting the cake to a soaking process. The 
soaking process comprises partially or completely dissolving the TA in a 
solvent, crystallizing the product in water at a high temperature and high 
pressure to remove residual solvent trapped in the crystals, and 
recrystallizing, filtering and drying the TA cake. 
(3) Mother Liquor/Solvent Recovery Section: 
The mother liquor from the first crystallizer filter is transferred to a 
solvent recovery column to recover the crystallization solvent from the 
column overhead. The impurities, such as, but not intended to be limited 
to, p-toluic acid, benzoic acid, 4carboxybenzaldehyde (4-CBA), and the 
like, are recovered from the bottom of the column. In order to make sure 
the column bottom slurry can be transferred back to the oxidizer, a 
high-boiling diluent is preferably added to the reboiler. 
II. Detailed Process Description and Example 
The present inventions will be described in terms of the production and 
recovery of terephthalic acid (TA) from the air oxidation of p-xylene in 
the presence of a solution of components of catalysis in dimethyl 
terephthalate (DMT) or in a benzoic acid-water solvent system. The 
oxidizer temperature is preferably between about from 150.degree. C. and 
about 250.degree. C. and the pressure is from between about 5 and about 30 
kg per cm.sup.2. Since the oxidizer effluent will contain up to 30% TA, 
mixing in the oxidizer is very important in order to maintain the yield 
and selectivity, and to prevent fouling and blockages. The initial mixing 
of the feed streams may be achieved in a static mixer (outside of the 
oxidizer). Further mixing may be provided by air sparging and external 
circulation. In the preferred form of the process manganese acetate and 
cobalt acetate in aqueous solution are fed to the oxidizer to catalyze the 
oxidation reactions. 
The effluent from the oxidizer at about 160.degree. C. is transferred and 
filtered via a first filter to separate the solid from mother liquor 
(filtrate). During filtering, the solid cake is washed with mxylene which 
is heated from 30.degree. C. to 100-150.degree. C. The mother liquor is 
transferred to a first holding tank. The cake washing liquid is removed 
separately from the first filter to a second holding tank. 
The washed cake is dropped into a first slurry tank to mix with the 
following streams: (1) NMP or DMAC (selective crystallization solvent) 
wash liquor (heated from 45 to 100-150.degree. C.); (2) mother liquor 
(heated from 50.degree. C. to 100-150.degree. C.); and (3) NMP or DMAC 
(heated from 45.degree. C. to 100-150.degree. C.). 
The above mixture is then transferred from the bottom of the first slurry 
tank to a first dissolver tank. The content in the first dissolver tank is 
then heated indirectly from 100-150.degree. C. to 140-200.degree. C. by a 
hot oil heating coil in the first dissolver tank. About 75% of the 
p-xylene and 100% of the sparging nitrogen in the mixture is vaporized 
from the first dissolver tank and removed. Sparging nitrogen is added to 
the first dissolver tank to assist the removal of p-xylene. Vapor streams 
from the first dissolver tank and a crude crystallizer are combined into a 
stream, condensed by a cooler, and sent to a first storage tank. The 
bottom effluent from the first dissolver tank is transferred to the crude 
crystallizer batchwise. 
The batch content in the crude crystallizer is reduced in pressure in the 
manner described above with concurrent removal of flashed solvent and 
cooled from 140-200.degree. C. to 10-20.degree. C. by an external cooler, 
to generate the desired super-saturation for TA crystals to grow. During 
pressure reduction, heat may be added to the batch to effect further 
solvent removal. To improve the crystal size distribution and solid 
recovery, crystal seeding may be helpful. At the conclusion of a batch 
crystallization cycle, the slurry is dropped into a third holding tank and 
transferred to a second filter where it is filtered at a continuous rate. 
During filtering at the second filter, NMP or DMAC is used to wash the cake 
in the second filter. The mother liquor plus NMP or DMAC wash are combined 
to be fed to a crystallization solvent recovery column. The washed cake is 
dropped into a second dissolver tank where it is mixed with NMP or DMAC to 
form the super-saturated feed for a pure crystallizer. NMP or DMAC is 
heated from 45.degree. C. to 140-200.degree. C. and is fed to the second 
dissolver tank. 
The content of the second dissolver tank is transferred batchwise to the 
pure crystallizer where the pressure is reduced in the manner described 
above and the temperature is cooled from 140-200.degree. C. to 
30-60.degree. C. to induce TA crystal growth. The cooling is provided by 
circulating the crystallizer content through an external cooler. Again, to 
improve the crystal size distribution and crystal recovery, crystal 
seeding may be helpful. At the end of the batch cycle, the slurry is 
dropped from the pure crystallizer into a feed tank for the third filter. 
The slurry is continuously fed to the third filter. The mother liquor from 
the first filter is transferred to a fourth holding tank. The cake is 
initially washed with NMP or DMAC at 45.degree. C. to displace the 
remaining mother liquor from the cake, and then the cake is washed with 
the low-boiling displacement solvent, such as water, to displace NMP or 
DMAC from the cake or, alternatively, sent to a vacuum dryer. The NMP or 
DMAC wash (from a crystallization solvent storage tank) and the 
displacement solvent are then added to the third filter. The NMP or DMAC 
wash liquid is sent to the first slurry tank, while the displacement 
solvent is transferred to a fifth holding tank. 
The washed cake from the third filter is passed through a wash column or 
multistage contactor and counter-current water is added to remove the 
crystallization solvent. The slurry from the wash column or contactor is 
then fed to the soaker where the temperature is raised to from between 
about 150-250.degree. C. to remove trapped solvent from the crystals. The 
slurry is finally filtered and dropped to a product dryer where water 
(moisture) in the cake is removed by heating and purging with a 
counter-current flow of heated nitrogen. The dried PTA product is removed 
from the dryer and is stored in the product bin. 
The bottom stream from the fifth holding tank (mixture of NMP and 
displacement solvent), together with the liquid from the wash column or 
multi-stage contactor, is transferred through a heater (to heat the stream 
from 25.degree. C. to 80-120.degree. C.) to a displacement solvent 
evaporator. The displacement solvent vapor from the overhead of the 
displacement solvent evaporator is condensed and sent to the displacement 
solvent tank. The bottom stream from the displacement solvent evaporator 
is split into two streams: one stream to the vent pot and a second stream 
to the crystallization solvent tank. 
The mother liquor and NMP or DMAC wash from the second filter are 
transferred to the crystallization solvent tank and then are fed to the 
NMP or DMAC recovery column. This stream is heated from 15-25.degree. C. 
to 130-170.degree. C. before entering the recovery column. The overhead 
vapor is condensed and sent to a condensate pot. A part of the condensate 
at 160-220.degree. C. is returned to the recovery column as the reflux. 
The rest of the overhead product from recovery column is sent to a 
crystallization solvent check tank. From the crystallization solvent check 
tank, the regenerated NMP or DMAC is pumped to a NMP or DMAC storage tank. 
In order to make sure the slurry in the recovery column reboiler can be 
transferred back to the oxidizer, high-boiling diluent, such as benzoic 
acid or DMT, is added to the reboiler. The slurry plus the high-boiling 
diluent is withdrawn from the bottom of the recovery column and is sent 
back to the oxidizer. 
In FIG. 3, there is shown an arrangement of a crystallizer S-2 useful for 
the practice of the embodiment of the invention in which heat is added to 
the crystallizing acid mixture during the times when the pressure is being 
reduced to flash solvent. As shown in FIG. 3, crystallizer S-2 is there 
provided with both a cooling recirculation circuit with exchanger E-8, and 
a heating recirculation circuit with heater E-8a. Heat is applied to the 
mixture by heater E-8a during flashing, and cooling is applied to the 
mixture at other times by exchanger E-8. Flashed solvent (e.g. NMP or 
DMAC) is removed through line 50 for recycling to the recovery column, and 
the pressure reduction vacuum is also applied to the crystallizer through 
line 50. 
Although a preferred embodiment of the method and apparatus of the present 
invention has been illustrated in the accompanying Drawings and described 
in the foregoing Detailed Description, it will be understood that the 
invention is not limited to the embodiment disclosed, but is capable of 
numerous rearrangements, modifications and substitutions without departing 
from the spirit of the invention as set forth and defined by the following 
claims.