Composition and method for reducing fouling in process equipment used for manufacturing aromatic materials

Methods for reducing the fouling of process equipment used in the manufacture of aromatic compounds such as dimethyl-2,6-naphthalenedicarboxylate are disclosed. The methods require treating manufacturing process streams with a metal complexing agent. Novel compositions useful in the manufacture of aromatic materials such as dimethyl-2,6-naphthalenedicarboxylate also are disclosed.

FIELD OF THE INVENTION 
This invention relates generally to a method for reducing fouling in 
process equipment used for manufacturing aromatic materials such as 
dimethyl naphthalenedicarboxylates. More specifically, this invention 
relates to a method for reducing fouling in process equipment used for 
manufacturing aromatic materials such as 
dimethyl-2,6-naphthalenedicarboxylate by treating manufacturing process 
streams with one or more metal complexing compounds. 
BACKGROUND OF THE INVENTION 
Dimethyl-2,6-naphthalenedicarboxylate, or "NDC," is representative of a 
monomer that can be used to prepare a variety of polyester materials. For 
example, NDC can be condensed with ethylene glycol to form 
poly(ethylene-2,6-naphthalate), or "PEN," a high performance polyester 
material. 
Fibers and films made from PEN have considerably improved strength and 
superior thermal properties relative to films and fibers made from 
poly(ethyleneterephthalate). PEN therefore is an exceptional material for 
preparing commercial articles such as thin films used for the manufacture 
of magnetic recording tape and electronic components. Additionally, 
because of PEN's superior resistance to the diffusion of gases such as 
carbon dioxide, oxygen and water vapor, films made from PEN particularly 
are useful for manufacturing articles such as "hot fill" food containers. 
PEN also is useful for preparing high strength fibers which can be used to 
manufacture items such as tire cord. 
Processes for manufacturing NDC and other aromatic esters from aromatic 
acids are well known. For example, U.S. Pat. Nos. 5,254,719, 5,262,560 and 
5,350,874 describe processes for manufacturing NDC from 
2,6-naphthalenedicarboxylic acid or "NDA" by reacting NDA with methanol. 
These and other processes for manufacturing NDC and other aromatic esters 
typically involve one or more ester crystallization or recrystallization 
steps, as well as a distillation step where the aromatic ester is 
distilled, typically using a fractional distillation column, to prepare 
high purity esters suitable for preparing PEN and other polyesters. 
Aromatic acids useful for preparing aromatic esters can be prepared in a 
number of ways. For example, NDA advantageously is obtained by oxidizing a 
suitable naphthalenic feedstock such as 2,6-dimethylnaphthalene. Such 
oxidation reactions typically are conducted in a liquid phase mixture 
using one or more heavy metal catalysts to catalyze the oxidation of the 
naphthalenic feedstock to NDA. One preferred method uses a mixture of 
cobalt and manganese catalyst metals in a liquid phase oxidation of 
2,6-dimethylnaphthalene. This method uses a low molecular weight acid such 
as acetic acid as the reaction solvent and air as the source of oxygen for 
oxidizing the methyl groups on 2,6-dimethyinaphthalene to the carboxylic 
acid groups of NDA. One such process is discussed in detail in U.S. Pat. 
No. 5,183,933 to Harper et al., the disclosure of which is hereby 
incorporated by reference. 
We have discovered that when NDA is prepared by such an oxidation process, 
the catalyst metals such as cobalt and manganese can cause severe fouling 
of the process equipment used to manufacture NDC from NDA. Process 
equipment exhibiting such fouling includes heat exchangers used to 
increase the temperature of a mixture of NDA and methanol for a subsequent 
esterification reaction, heat exchangers used to increase the temperature 
of a mixture of NDC, NDA, and methanol for a subsequent recrystallization, 
filter cloths on equipment used to recover NDC particles from methanol, 
heat exchangers used to heat and evaporate filtration mother liquor to 
recover solvent, the internal portions of the reactor and associated 
piping used in the esterification process, and the internals of NDC 
distillation columns. 
Equipment such as that noted above may become encrusted or fouled with 
solid deposits which reduce the efficiency of the equipment. If operations 
continue under conditions that permit fouling, the equipment can become 
completely plugged or otherwise inoperative. Fouled equipment can 
substantially reduce plant production rates. Furthermore, when fouling 
becomes severe, production of NDC must be stopped to clean out the fouled 
equipment. Thus, manufacturers of aromatic materials such as NDC require a 
method to reduce or eliminate fouling of process equipment. Our invention 
provides such a method. 
SUMMARY OF THE INVENTION 
We have found that fouling problems in the production of aromatic materials 
from heavy metal-containing feedstocks can be drastically reduced by using 
a metal complexing agent during the production process. Use of metal 
complexing agents such as phosphorus salts has been found to increase the 
run time of process equipment such as heat exchangers and filters several 
fold, thereby dramatically increasing plant throughput and minimizing the 
need for fouling-related plant maintenance. 
In a first embodiment of the invention, a method for reducing fouling in 
equipment used to process a metal-containing aromatic feedstock mixture 
requires treating a process stream of the aromatic feedstock mixture with 
a metal complexing compound. 
The term "metal-containing aromatic feedstock mixture" refers to a mixture 
containing at least 5 parts by weight of an aromatic compound and between 
about 10-40,000 total parts per million of one or more heavy metals having 
atomic numbers from 21 to 82. 
The term "metal complexing compound" means any compound that remains 
sufficiently stable under processing conditions to complex with the heavy 
metal or metals contained in an aromatic feedstock mixture to prevent 
fouling of the processing equipment. Such metal complexing compounds 
include, for example, phosphorus-containing compounds and other metal 
complexing compounds such as sulfur- and oxygen-containing compounds like 
sulfates, sulfites and oxalates, as well as amine complexing agents and 
materials such as crown ethers. 
In a second embodiment of the invention, fouling in equipment used to 
process a metal-containing naphthalenic feedstock mixture is reduced by 
treating the process stream of the naphthalenic feedstock mixture with a 
phosphorus-containing compound. Suitable phosphorus-containing compounds 
include both inorganic and organic phosphorus-containing materials, with 
inorganic phosphate salts often being preferred. 
The term "metal-containing naphthalenic feedstock mixture" refers to a 
mixture containing at least 5 parts by weight of a naphthalenic compound 
and between about 10-40,000 total parts per million of one or more heavy 
metals having atomic numbers from 21 to 82. 
Still another embodiment of the invention includes novel, low fouling 
compositions useful in the manufacture of aromatic materials compositions. 
In yet another embodiment of the invention, processes for manufacturing 
aromatic carboxylates from alkyl- or acyl-substituted aromatic compounds 
are disclosed. These processes includes the steps of oxidizing an akyl- or 
acyl- substituted aromatic compound in the presence of one or more heavy 
metal catalysts to form aromatic acids of the alkyl- or acyl- substituted 
aromatic compound and then esterifying a reaction mixture containing the 
aromatic acids and heavy metal catalysts in the presence of a 
phosphorus-containing compound in an amount equal to about 0.1 to 2.0 
moles of phosphorus, calculated as elemental phosphorus, per mole of heavy 
metal, calculated as elemental metal. 
The foregoing processes are particularly well suited for minimizing fouling 
during the manufacture of dimethyl naphthalenedicarboxylates, and have 
been found to offer dramatic improvements in process equipment run times 
over processes which do not employs metal complexing compounds. 
DETAILED DESCRIPTION OF THE INVENTION 
We have discovered that treating process streams used to manufacture 
aromatic esters such as NDC with one or more metal complexing compounds 
greatly reduces fouling in process equipment. While our invention will be 
described in detail below in connection with an NDC manufacturing process, 
the invention is believed to be useful in other manufacturing processes 
using aromatic feedstocks, such as in the esterification or purification 
of terephthalic or isophthalic acid. 
NDC manufacturing process streams can contain a wide variety of 
naphthalenic and other compounds. In a typical manufacturing process, NDA 
is mixed with an amount of methanol typically in excess of that which 
would be required to convert all of the carboxylic acid groups of NDA to 
methyl esters. This mixture is heated, with or without a catalyst, to form 
the dimethyl ester of NDA. The temperature used to form the dimethyl ester 
usually is about from 200 to about 700.degree. F., and preferably about 
500 to about 650.degree. F. The product from this esterification reaction 
is purified by one or more crystallization, recrystallization and/or 
distillation steps to form purified NDC. Typically, to form highly pure 
NDC, a distillation step is required. Thus, the process streams used to 
manufacture NDC from NDA can range from a mixture comprising mostly NDA to 
mostly NDC, or mixtures thereof, and may include varying concentrations of 
methanol. The process streams also can comprise the monomethyl ester of 
NDA and other naphthalenic compounds. Several representative processes for 
preparing NDC from NDA are disclosed in U.S. Pat. Nos. 5,254,719; 
5,262,560; and 5,350,874, the specifications of which are incorporated 
herein by reference. 
Processes for preparing NDA feedstocks are described, for example, in U.S. 
Pat. No. 5,183,933, the specification of which also is incorporated herein 
by reference. NDC manufacturing process streams typically contain one or 
more heavy metals with atomic numbers ranging 21 to 82 that were used to 
catalyze the oxidation of a dialkylnaphthalene reactant. In many cases, 
NDA preferably is made by the liquid phase oxidation of 
2,6-dimethylnaphthalene in the presence of cobalt and manganese oxidation 
catalysts. The crude NDA resulting from such an oxidation step may contain 
from about 10 parts per million by weight (ppm) to about 20,000 ppm, more 
typically about 500 ppm to about 10,000 ppm, and most typically from about 
1000 ppm to about 6000 ppm of cobalt and manganese, calculated as 
elemental cobalt and elemental manganese. 
When NDA containing the above-noted concentration of oxidation catalyst 
metals is used in processes for preparing NDC, the equipment used fouls 
rapidly. Typically fouled process equipment includes heat exchangers used 
for increasing the temperature of the process stream comprising NDA and 
methanol prior to the NDA esterification reaction, as well as reactors and 
piping used in connection with the esterification reaction. 
Metal-containing process streams also foul the filter surfaces used to 
filter NDC from crystallization and recrystallization solvents such as 
methanol, and foul internal portions of distillation columns used to 
fractionally distill NDC to form highly pure NDC. The term "foul" as used 
herein means the development or build-up of solids or other material on 
the working surfaces or internal passages of process equipment which 
results in an observable decrease in the capacity of the equipment. This 
development or build-up of material on such surfaces or in such passages 
results in a decrease in the efficiency of the operation of such equipment 
and eventually can result in inoperability of such equipment. As used 
herein, the term "inoperable" refers to a piece of equipment that has been 
fouled to a point that it functions at less than 60 percent of its rated 
capacity. 
We discovered that the addition of one or more phosphorus-containing 
compounds to the aforementioned process streams greatly reduces or 
eliminates fouling of such process equipment. The amount of 
phosphorus-containing compound added is an amount that results in the 
reduction of the fouling of the process equipment. The amount of 
phosphorus-containing compound required to reduce fouling typically is at 
least about 0.1, preferably at least about 0.5, and more preferably at 
least about 0.8 moles of phosphorus, calculated as elemental phosphorus, 
per mole of total heavy metal, calculated as the elemental metal or 
metals, present in the process stream. Most preferably, the amount of 
phosphorus-containing compound added is an amount such that the mole ratio 
of phosphorus, calculated as elemental phosphorus, to the total of the 
heavy metal components, calculated as the elemental metal or metals, is 
about 1:1. Mole ratios which exceed about 1:1, however, have not been 
found to be detrimental to the purpose of this invention. 
Phosphorus-containing compounds useful in the invention are any 
phosphorus-containing compounds that will reduce or prevent the fouling of 
the equipment. The term "phosphorus-containing compounds" as used herein 
includes both inorganic and organic compounds. If organic 
phosphorus-containing compounds are used, the compounds preferably are 
selected so that they have a low volatility. Inorganic 
phosphorus-containing compounds useful in the invention include monomeric 
phosphates, dimeric phosphates, and higher linear and cyclic 
polyphosphates, as well as the alkali metal salts and alkaline earth metal 
salts of these same phosphorus-containing compounds. Such compounds 
include, for example, P.sub.2 O.sub.5, H.sub.3 PO.sub.4, H.sub.4 P.sub.2 
O.sub.7, H.sub.5 P.sub.3 O.sub.10, trimeta phosphoric acid, tetrameta 
phosphoric acid, one or more sodium phosphates such as, for example, 
NaH.sub.2 PO.sub.4, Na.sub.2 HPO.sub.4, Na.sub.3 PO.sub.4, as well as 
hydrated versions thereof, and pyrophosphates such as sodium or potassium 
pyrophosphates. Mixtures of the foregoing are also well suited to use in 
the invention. Inorganic phosphates and salts thereof are a preferred 
source of phosphorus for the method of this invention. Organic 
phosphorus-containing compounds such as alkyl or aryl phosphates and 
phosphites also are believed to be suitable for use in the invention. For 
example, trimethyl phosphate, triphenyl phosphate, trimethyl phosphite, 
and triphenyl phosphite are believed to be useful. 
Phosphorus-containing compounds can be added to one or more NDC 
manufacturing process streams at any suitable point. These streams can 
contain anywhere from about 1 to 99 parts, and more typically 5 to 70 
parts, by weight of naphthalenic compounds. For example, the 
phosphorus-containing compound can be added to the process stream 
containing NDA and methanol, which stream is then reacted in an 
esterification reaction to form crude NDC. The phosphorus-containing 
compound also can be added to the methanol or to the NDA, or to the 
mixture of methanol and NDA. 
The mixture of NDA and methanol so treated typically comprises about 5 to 
about 50 parts by weight NDA and about 50 to about 95 parts by weight 
methanol. The NDA used for such mixture typically contains about 10 ppm to 
about 20,000 ppm of heavy metal, typically cobalt and manganese, based on 
the weight of the metal. The molar ratio of cobalt to manganese therein 
typically is about 30:1 to about 1:30, more typically about 10:1 to 1:10, 
and most typically about 4:1 to 1:1. The amount of phosphorus-containing 
compound added is the amount stated hereinabove, based on the amount of 
heavy metal present in the process stream, i.e., at least about 0.1, 
preferably at least about 0.5, and more preferably at least about 0.8 mole 
of phosphorus, calculated as elemental phosphorus, per mole of heavy 
metal, calculated as the elemental metal. 
While it is advantageous to add the phosphorus-containing compound to the 
process stream prior to the esterification reaction, the 
phosphorus-containing compound can be added at a later stage in the 
process to prevent fouling in downstream equipment such as the filters 
used to filter solutions of NDC or the distillation column used to distill 
NDC. The phosphorus-containing compound can be added in increments to the 
process stream or it can be added continuously. It can be added at more 
than one location in the process and either simultaneously or at different 
times, depending on the existing need to prevent fouling in the process 
equipment.

Addition of phosphorus-containing compound to various NDC process streams 
is illustrated by the following Examples. 
EXAMPLE 1 
A heat exchanger used to elevate the temperature of a mixture of NDC, NDA, 
and methanol to a temperature sufficient to melt the NDC was fouled and 
became inoperable after 72 hours of continuous operation. The mixture 
typically contained 30 wt. % methanol, 0.1 to 2 wt. % NDA, and about 1000 
ppm to about 1300 ppm total cobalt and manganese, based on the weight of 
NDC. 
The addition of from about 100 to about 500 ppm of phosphorus, based on the 
NDC content and added as sodium hexametaphosphate to the same mixture of 
NDC, NDA, and methanol, provided for the operation of the same exchanger 
for more than about 900 hours without fouling. 
EXAMPLE 2 
A filter used to recover crude crystalline NDC from a stream containing 
NDC, NDA, and methanol fouled and became inoperable after about 240 hours 
of continuous operation. This stream was obtained after cooling the total 
reactor effluent from the reactor used to esterify NDA with methanol. This 
mixture typically contained 20-30 wt. % NDC, 80-70 wt. % methanol, 0.1-2 
wt. % NDA and monomethyl NDC, and from about 1000 ppm to about 1300 ppm 
total cobalt and manganese based on the weight of NDC present in the 
stream. 
The addition of from about 100 ppm to about 600 ppm of phosphorus, based on 
the weight of NDC in the stream and added as sodium hexametaphosphate to 
the same mixture of NDC, NDA, and methanol, provided for the operation of 
the same filter for more than about 900 hours without fouling. 
EXAMPLE 3 
A heat exchanger used to elevate the temperature of a mixture of NDC, NDA, 
and methanol to a temperature sufficient to melt the NDC fouled and became 
inoperable after 72 hours of continuous operation. The mixture typically 
contained 30 wt. % methanol, 0.1 to 2 wt. % NDA, and 1700 ppm to about 
2400 ppm total cobalt and manganese, based on the weight of NDC. 
The addition of from about 300 to about 1900 ppm of phosphorus based on the 
NDC content, added as sodium dihydrogenphosphate to the same mixture of 
NDC, NDA, and methanol, provided for the operation of the same exchanger 
for more than about 900 hours without fouling. 
EXAMPLE 4 
A filter used to recover crude crystalline NDC from a stream containing 
NDC, NDA, and methanol was fouled after about 240 hours of continuous 
operation. This stream was obtained after cooling the total reactor 
effluent from the reactor used to esterify NDA with methanol. This mixture 
typically contained 20-30 wt. % NDC, 80-70 wt. % methanol, 0.1-2 wt. % NDA 
and monomethyl NDC, and from about 1700 ppm to about 2400 ppm total cobalt 
and manganese based on NDC. 
The addition of from about 300 ppm to about 1900 ppm of phosphorus based on 
NDC and added as sodium dihydrogenphosphate to the same mixture of NDC, 
NDA, and methanol, provided for the operation of the same filter for more 
than about 900 hours without fouling. 
The following prophetic Example 5 provides further guidance as to the use 
of our invention. 
EXAMPLE 5 
A reboiler is used to boil a mixture of NDC, monoesterified monomethyl 
2,6-NDC, and NDA at approximately 500 degrees .degree.F. fouls after less 
than 1000 hours of continuous operation. The mixture typically contains 
approximately 7% NDA, 20% monomethyl 2,6-NDC, 70% NDC, and about 3 to 4 
total weight percent of cobalt and manganese. 
About 0.7-1.3% of phosphorus is added, as sodium dihydrogen phosphate, to 
the same mixture and provides for operation of the same reboiler for more 
than about 1000 hours without fouling. 
We believe similar results can be expected when using other 
phosphorus-containing compounds such as sodium hexametaphosphate, sodium 
dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, 
phosphoric acid, tripolyphosphates such as sodium tripolyphosphate, 
trimethyl phosphite, trimethyl phosphate, triphenyl phosphite, triphenyl 
phosphate and mixtures thereof. 
While the foregoing Examples illustrate the particular utility of our 
invention with respect to the manufacture of NDC from NDA, we believe that 
the process is useful in connection with a wide variety of manufacturing 
processes in which a metal-containing aromatic feedstock mixture is 
reacted under aromatic feedstock processing conditions. 
Phosphorus-containing or other metal complexing compounds may be added to 
these processes in the same ratios and manners as discussed above in 
connection with the NDC based Examples. 
As used herein, the term "metal-containing aromatic feedstock mixture" 
means a mixture containing at least 5 parts by weight of an aromatic 
compound and between 10-40,000 total parts per million of one or more 
heavy metals having atomic numbers from 21 to 82. The term "aromatic 
feedstock processing conditions" means an operating temperature of between 
about 100 to 750 degrees Fahrenheit and at operating pressures from 
between about 15 mm Hg absolute and about 1500 psia. 
For example, the invention is to believed to be especially useful in 
processes for manufacturing aromatic carboxylates from alkyl- or acyl- 
substituted aromatic compounds which first oxidize an alkyl- or acyl 
substituted aromatic compound in the presence of one or more heavy metal 
catalysts to form aromatic acids of the alkyl- or acyl- substituted 
aromatic compound and which thereafter esterify a reaction mixture 
containing the aromatic acids and heavy metal catalysts. The use of a 
metal complexing compound such as a phosphorus-containing compound in an 
amount equal to about 0.1 to 2.0 moles of phosphorus calculated as 
elemental phosphorus per mole of heavy metal calculated as elemental metal 
should greatly minimize operational difficulties related to fouling 
equipment used in those processes such as filters, heat exchangers, and 
distillation columns. The following prophetic Examples illustrate the 
utility of the invention in some of these applications. 
EXAMPLE 6 
A heat exchanger used to elevate the temperature of a mixture of 
terephthalic acid (TA), dimethyl terephthalate (DMT) and methanol to a 
temperature sufficient to esterify the TA (typically about 500.degree. F.) 
is fouled to the extent where heat transfer is significantly reduced. The 
mixture typically contains about 80 wt. % methanol, 16 wt. % TA, 4% 
recycle DMT, and about 50 ppm to about 200 ppm total cobalt and manganese, 
based on the weight of TA. 
The addition of from about 25 to about 100 ppm of phosphorus based on the 
TA content, added as sodium hexametaphosphate to the same mixture of TA, 
DMT, and methanol greatly extends the operating time before the exchanger 
is significantly fouled. 
EXAMPLE 7 
A filter used to recover crude crystalline DMT from a stream containing 
DMT, TA, and methanol is fouled to the point where the filtration rate is 
seriously reduced. This stream is obtained after cooling the total reactor 
effluent from a reactor used to esterify TA with methanol. This mixture 
typically contains about 23 wt. % DMT, 75 wt. % methanol, 2 wt. % TA and 
monomethyl TA, and from about 50 ppm to about 200 ppm total cobalt and 
manganese based on the weight of DMT present in the stream. 
The addition of from about 25 ppm to about 100 ppm of phosphorus based on 
the weight of DMT in the stream, added as sodium hexametaphosphate to the 
same mixture of DMT, TA and methanol extends the operating time before the 
filter is significantly fouled. 
EXAMPLE 8 
A heat exchanger used to elevate the temperature of a mixture of DMT, TA, 
and methanol to a temperature sufficient to dissolve the DMT is fouled to 
the extent where heat transfer is significantly reduced. The mixture 
typically contains 30 wt. % methanol, 0.1 to 2 wt. % TA, and 25 ppm to 
about 100 ppm total cobalt and manganese, based on the weight of DMT. 
The addition of from about 25 to about 100 ppm of phosphorus based on the 
DMT content, added as sodium dihydrogen phosphate, to the same mixture of 
DMT, TA, and methanol, substantially extends the operating time before the 
filter is significantly fouled. 
Additionally, while phosphorus-based compounds are the preferred compounds 
for treating metal-containing aromatic feedstock mixtures in accordance 
with our invention, we believe that the advantages of the invention may be 
realized using other metal complexing compounds. The term "metal 
complexing compound" means any compound that remains sufficiently stable 
under aromatic feedstock processing conditions to complex with the heavy 
metal or metals contained in an aromatic feedstock mixture to prevent 
fouling of the processing equipment. Such "metal complexing compounds" 
include, for example, phosphorus-containing compounds as well as other 
metal complexing compounds, for example, sulfur- and oxygen-containing 
compounds such as sulfates, sulfites and oxalates, as well as amine 
complexing agents and materials such as crown ethers. 
While our invention has been discussed primarily in connection with the 
manufacture of aromatic esters such as dimethyl-2,6-naphthalene 
dicarboxylate, other applications will be apparent to those skilled in the 
art. Our invention, therefore, is intended to be limited only by the scope 
of the following claims.