A strategy for total recovery of residue from terephthalic acid production process

Article abstract of DOI:10.1134/S1070427215070150

Terephthalic acid is generally produced via oxidation of p-xylene. In this process, a large quantity of residue is produced containing cobalt, manganese, and bromine catalysts as well as by-products such as benzoic acid. Our purpose is to totally recover

Full text of DOI:10.1134/S1070427215070150

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2015, Vol. 88, No. 7, pp. 1201−1206. © Pleiades Publishing, Ltd., 2015.
VARIOUS
TECHNOLOGIES
A Strategy for Total Recovery of Residue
from Terephthalic Acid Production Process1
Vahid Rezaeia and Seyed Ali Akbar Sajadib
a Department of Chemistry, Sharif University of Technology, Azadi Ave, Tehran, 1393 Iran
b Water & Energy Research Center, Sharif University of Technology, Tehran, Iran
e-mail: sajadi@sharif.ac.ir
Received July 15, 2015
Abstract—Terephthalic acid is generally produced via oxidation of p-xylene. In this process, a large quantity of
residue is produced containing cobalt, manganese, and bromine catalysts as well as by-products such as benzoic
acid. Our purpose is to totally recover this residue. Extraction by water has been used as an efficient method to
recover catalyst from this residue. In this study, the best condition for extraction was obtained at water/residue
dilution ratio of 6 : 1, extraction temperature of 45°C and extraction time of 40 min. Efficiency of catalyst recov-
ery was 92 wt % at these conditions. The catalyst containing extracted solution can be reused in the oxidation
process after being concentrated by an evaporator. The benzoic acid content of the residue is also recovered by
recrystallization. The best condition for recrystallization was achieved at water/residue dilution ratio of 4 : 1
and dissolution and crystallization temperatures of 95 and 25°C, respectively. Recovery efficiency and purity of
benzoic acid were 85 and 97 wt %, respectively, at these conditions. The recovered benzoic acid can be used in
many applications. Once catalysts and benzoic acid were recovered from residue, whatever remained was tere-
phthalic acid which was recovered by up to 98 wt % with a purity of 95% and can be recycled to the purity unit.
DOI: 10.1134/S1070427215070150
Several by-products are formed in the
Amoco oxidation process, such as p-toluic acid,
4-carboxybenzaldehyde (4-CBA), and benzoic acid
(Fig. 1). The former two are manufactured by the step
oxidation of p-xylene [10], while the latter is formed
by decarboxylation of terephthalic acid under low pH
value and high temperature condition in the oxidation
reactor [11]. 4-CBA is the major impurity crystallized
together with the terephthalic acid due to their similar
structures. Therefore, the produced terephthalic acid
is an impure compound known as raw terephthalic
acid. The presence of 4-CBA, decreases the average
molecular weight of polyesterand making it to appear
in a yellow color. This impurity reduces polyester
quality necessitating a final purification process [12].
For this purpose, a purification process was developed
by Amoco Company as the provider of up to 60% of
the terephthalate demand in the world [13, 14]. In this
process, the aldehyde group of 4-CBA is hydrogenated
INTRODUCTION
Terephthalic acid (TA) is one of the raw materials
used in the synthesis of polyester. Considering the
increasing demand for polyester, production of TA has
been studied in recent years [1]. Basically, terephthalic
acid is used as a raw material for the manufacturing
of polyethylene terephthalate (PET) which is used in
the course of production of textiles, industrial fibers,
bottles, etc. [2, 3]. Each year, several million tons of
terephthalic acid are produced world wide, 70% of
which is produced by a catalyst system developed by
the Science Design and Amoco Company [4, 9]. In this
process, terephthalic acid is produced by oxidation of
p-xylene in the presence of acetic acid as the solvent
as well as homogeneous catalysts of cobalt(II) acetate,
manganese(II) acetate, and hydrogen bromide (Fig. 1).
1 The text was submitted by the authors in English.
1201

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VAHID REZAEI et al.
СOOH
СOOH
disadvantages such as bromide loss and column fouling,
CH3
CH3
while they required too expensive equipment. Tond-
gouyan Petrochemical Company burns its residue in a
special furnace and extracts catalysts components from
ash [18]. The associated problems are loss of valuable
compounds such as benzoic acid while polluting the en-
vironment via organic compounds combustion.
175–225°C
15–30 bar
+ O2
H2O
Acetic acid
Co/Mn/Br
terephthalic
T
p-Xylene
p-xylene
acid
Several methods are proposed to recover benzoic
and terephthalic acids from the residue. Some examples
include distillation and solvent extraction procedures
[19, 20]. These methods have several disadvantages:
equipment’s fouling due to benzoic acid crystallization,
their need for an expensive distillation column as well
as environmental pollution by organic solvents.
BYPRODUCTS
СOOH
СOOH
СOOH
This study provides an economic and environment-
friendly method for total recovery of residue produced
in Tondgouyan Petrochemical Company. Catalysts com-
ponents are recovered from residue by water extraction
process. In addition, this method makes possible the re-
covery of valuable compounds such as benzoic and tere-
phthalic acid. Recovered catalysts can be reused in the
oxidation process. Recrystallization in water is used to
recover benzoic acid from the solid residue. The recov-
ered benzoic acid has various applications and can be
used in different industries. Once the recovery of cata-
lysts and benzoic acid from residue was accomplished,
whatever remains is raw terephthalic acid that can be
sent to a purification unit.
HСO
CH3
4-Carboxy
benzaldehyde
p-Toluic
acid
Benzoic
acid
Fig. 1. Overall reaction of Amoco oxidation process and
chemical structure of by-products.
in the presence of palladium catalysts and water. Under
these conditions, 4-carboxybenzaldehyde converts to
p-toluic acid which is more soluble in water and can be
easily separated from the raw terephthalic acid.
Due to operating conditions in the oxidation step of
the Amoco process, a large amount of residue is pro-
duced which contains cobalt, manganese, bromine, and
organic compounds such as benzoic acid and terephthal-
ic acid. These valuable compounds should be recovered
by convenient methods because disposing them into the
environment is dangerous. Although almost all existing
methods for catalyst recovery incorporate a solvent ex-
traction step, but purification methods are different. In
the method described in [15, 16], the catalyst content of
extract solution is precipitated by oxalate and carbonate.
Another procedure passed the extract solution through
anion-exchange resin [17]. These methods have several
EXPERIMENTAL
Analysis
Amounts of cobalt and manganese were measured
according to ASTM D-3558 and ASTM D-858
standards, respectively. For this purpose anatomic
adsorption spectrophotometer was used (AA-220,
made by Varian Inc. Co.). The amount of bromine
was measured by a UV spectrophotometer (DR 4000,
made by Hach Company) according to ASTM D-1246
standard. Organic compounds of the solid residue
(benzoic acid and terephthalic acid) were analyzed by
Table 1. HPLC chromatography analysis conditions
Sample
volume
Column
temperature
Column
type
Wavelength
230 nm
Detector
UV-VIS
Pressure
84 bar
Mobile phase
Column dimension
150 mm ×4.6 mm
ACN/HOAC
(1 : 1)
Agilent
C18
0.7 mL
25°C
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A STRATEGY FOR TOTAL RECOVERY
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Table 2. Chemical analysis of residue
a HPLC (Agilent Technologies, 1200 series made by
Santa Clara Company) [21]. Analysis conditions of
HPLC are mentioned in Table 1. It should be noted that
quantity analysis was done based on the area under the
peaks.
Weight percent, wt %
Components
4-CBA
0.64
27.42
Benzoic Acid
1.00
0.82
0.81
1.00
Bromine
Residue Sample
Cobalt
Manganese
p-Toluic Acid
After separating terephthalic acid, solvent and
catalysts from the reactor output in Tondgouyan
Petrochemical Company, a large amount of residue is
remained. This residue is a wet yellow solid. As the
analysis results show, residue sample contains various
organic and inorganic compounds (Table 2).
11.10
57.21
Terephthalic Acid
Water
solid residue into three categories of catalysts, benzoic
acid and raw terephthalic acid, without any loss of
original materials or any secondary pollution.
Schematic design. The residue produced in
the oxidation process of p-xylene contains cobalt,
manganese, bromine and organic compounds such
as benzoic acid and terephthalic acid. Our proposal
for the total recovery of these compounds from the
residue is shown in Fig. 1. According to this technique,
the catalysts were separated through water extraction.
Organic compounds, mainly containing benzoic acid
and terephthalic acid, were separated from the residue
via recrystallization in water due to their different
solubility in water at high temperatures [10, 22]. For
this purpose, the solid residue was filtered after being
completely mixed with water. The catalyst containing
filtrate can be reused in the oxidation process once it gets
concentrated. Although the concentration process can
be accomplished via physical methods such as reverse
osmosis (RO) and ion-exchange resins, however, these
are expensive methods after which there may be an
occurrence of membrane fouling. In addition, chemical
methods such as precipitation can contribute to cobalt
and manganese concentration only while bromine may
still be wasted.
Solid-liquid extraction. Solid-liquid extraction
method was used to recover catalysts containing cobalt,
manganese, and bromine. For this purpose, solid residue
was completely mixed with water at different weight
ratios. The obtained slurry was filtered to separate
dissolved catalyst from insoluble organic compounds.
Then, catalyst containing solution was analyzed to
measure the recovery efficiency of cobalt, manganese,
and bromine. The insoluble organic compounds were
sent to recrystallization process to recover benzoic acid.
Recrystallization. Recrystallization was used to
separate benzoic acid from the solid obtained in the
Water
3
1
2
Residue
The remaining solid on the filter was mixed with
hot water to separate benzoic acid and terephthalic acid
from each other. Under these conditions, benzoic acid
was dissolved in water while terephthalic one remained
insoluble. The obtained slurry was filtered at the same
temperature with the solid remaining on the filter to be
terephthalic acid which can be recycledtothe purification
unit. The temperature of filtrate was then reduced to
crystallize the benzoic acid. Benzoic acid crystals could
be separated by a filter before being used in various
industries such as synthesis of phenol, Caprolactam,
colors, drugs, etc. [22]. In fact, this technique divided
Concentrated
catalyst
4
2
2
Water
5
Recovered
Recovered
benzoic acid
terephthalic acid
Fig. 2. The schematic of total recovery of residue. (1) Mixing
tank, (2) 4 filter, (3) evaporator, (4) condensor, (5) dissolution.
.
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VAHID REZAEI et al.
Table 3. Effect of dilution ratio on catalysts recovery
efficiency
Table 4. Effect of extraction temperature on catalysts recovery
efficiency
Extraction efficiency, %
Extraction efficiency, %
Temperature,
Dilution ratio
°C
bromine
manganese
cobalt
35.0
bromine
manganese
cobalt
67.5
75.2
80.9
86.2
90.2
51.0
56.7
63.1
65.9
67.2
2
80.8
81.7
83.5
85.8
87.2
62.8
63.3
64.1
65.6
68.0
64.0
66.5
69.5
74.6
83.6
25
35
50
65
80
50.7
65.3
71.1
73.3
4
6
8
10
liquid-liquid extraction process. For this purpose, the
solid was completely mixed with hot water at different
weight ratios before the obtained slurry was filtered at
high temperature. After drying, the remaining solid on
the filter was analyzed by HPLC. The filtrate temperature
was reduced so as benzoic acid crystals were appeared.
Once these crystals were dried, they were analyzed by
HPLC to measure the recovery efficiency and purity of
the obtained benzoic acid.
acid, and recycling this compound to the oxidation
reactor leads to a decrease in the oxidation efficiency
of p-xylene. According to these results, the temperature
of 45°C was determined as the optimum temperature at
which the concentration of organic compounds is kept at
lower values at an acceptable extraction efficiency.
Effect of extraction time. To study the effect of
extraction time on recovery efficiency, the extraction
was done at a dilution ratio of 6 and a temperature of
45°C for various extraction times: 10, 20, 30, 40, and
50 min. As shown in Table 5, longer extraction times led
to higher efficiency in catalyst recovery. However, the
long extraction times increased the total cost. Therefore,
an extraction time of 40 min was determined as the
optimum one considering the slight increase in the
recovery efficiency for longer times.
RESULTS AND DISCUSSIONS
Catalysts Recovery
Effect of dilution ratio. Water/residue dilution ratio
is an essential factor contributing to catalyst extraction.
In the present study, extraction efficiency of catalysts
containing cobalt, manganese, and bromine was studied
at water/residue dilution ratios of 2, 4, 6, 8, and 10.
Extraction temperature and mixing time were 25°C and
10 min, respectively. As shown in Table 3, by increasing
dilution ratio, the extraction efficiency exhibits an
early quick raise, while it increases slightly at higher
dilution ratios. In addition, increasing dilution ratio
led to the need for larger equipment and higher energy
consumption in the evaporator. Therefore, the dilution
ratio of 6 was determined as the optimum value.
Benzoic Acid Recovery
Effect of solubility temperature. An essential
parameter in benzoic acid recrystallization is the
solubility temperature. Recovery efficiency and purity
of benzoic acid were studied in solubility temperatures
of 70, 80, 90, and 95°C with a dilution ratio of 3 and a
crystallization temperature of 20°C. Recovery efficiency
of benzoic acid was increased at higher temperatures,
as shown in Table 6. However, benzoic acid purity
decreased at higher temperatures due to an increase in
organic compounds solubility. On the other hand, the
benzoic acid purity was high at lower temperatures,
although low recovery efficiency was obtained.
However, high temperature is desired where the
recovery efficiency change is more significant than that
of the purity. Since the benzoic acid is volatile in vapor
state, pressure equipment should be used to prevent
the water from boiling. The use of this equipment led
Effect of extraction temperature. In this section,
recovery efficiency is studied at extraction temperatures
of 25, 35, 50, 65, and 80°C with dilution ratio and
extraction time of 6 and 10 min, respectively. As shown
in Table 4, higher temperatures are associated with an
increase in the extraction efficiency which is probably
due to the increased solubility of organic compounds
and release of trapped catalysts. However, increasing
the solubility of organic compounds, especially benzoic
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A STRATEGY FOR TOTAL RECOVERY
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Table 5. Effect of extraction time on catalystsrecovery
efficiency
Table 7. Effect of crystallization temperature on benzoic acid
recovery efficiency and purit
Extraction efficiency, %
Recovery
efficiency, %
Purity, %
Temperature, °C
Time, min
bromine
manganese
cobalt
98.6
97.7
72.3
40
30
87.2
89.3
90.9
92.5
74.3
81.0
86.8
91.9
76.3
81.8
86.9
91.2
10
20
30
40
76.7
97.1
96.8
96.5
81.3
82.2
82.9
20
10
0
93.4
92.8
92.1
50
Table 6. Effect of solubility temperature on benzoic acid
recovery efficiency and purity
Table 8. Effect of dilution ratio on benzoic acidrecovery
efficiency and purity
Recovery
efficiency, %
Temperature,
°C
Recovery
efficiency, %
Purity, %
Purity, %
Dilution ratio
98.6
97.5
97.1
96.8
94.7
16.3
28.8
51.9
81.3
65.5
70
80
90
95
100
94.1
95.2
97.1
97.4
97.2
34.2
1
2
3
4
5
59.5
81.3
85.3
86.0
to an increase in investment costs, so that the desired
solubility temperature was determined at 95°C.
sociated with a need for larger equipment, so as a dilu-
tion ratio of 4 was determined as the best dilution ratio.
Effectofcrystallizationtemperature.Inthissection,
the effect of crystallization temperature on benzoic acid
efficiency and purity was studied at 40, 30, 20, 10, and
0°C. The dilution ratio and solubility temperature were
set at 3 and 95°C, respectively. As shown in Table 7,
lower crystallization temperatures led to an increase
in the recovery efficiency while they decreased the
purity slightly. On the other hand, higher crystallization
temperatures caused lower recovery efficiencies due to
loss of benzoic acid. Although lower temperatures were
desired, but supplying such temperatures may lead to
increase in total costs. Therefore, the temperature of
25°C was chosen as the best crystallization temperature.
Terephthalic Acid Recovery
In the recrystallization process, the solid residue
was mixed with water at the dilution ratio of 4 and
temperature of 95°C. Then, the obtained slurry was
filtered and the remained solid on the filter was analysed
by HPLC. The results showed that this solid was raw
terephthalic acid with 95% purity which was recovered
by up to 98%.
CONCLUSIONS
All components of the residue can be recovered
economically without any secondary pollution. The
extracted solution containing catalysts components
can be reused in the oxidation reactor after it gets
concentrated. In this way, catalyst consumption as well
as environmental pollution by heavy metals is reduced.
The present work not only prevented organic compounds
from being burnt, but also recovered them efficiently.
Effect of dilution ratio. Recovery efficiency and pu-
rity of benzoic acid were studied at water/residue dilu-
tion ratios of 1, 2, 3, 4, and 5. Solubility and crystalliza-
tion temperatures were set at 95 and 25°C, respectively.
Dilution ratios of less than 4 lead to lower recovery ef-
ficiencies and purity values (Table 8). However, dilution
ratios of higher than 4 led to a small increase in benzoic
acid recovery efficiency and purity while they were as-
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VAHID REZAEI et al.
vol. A26, pp. 193–197.
The recovered benzoic acid can be used in various
applications such as producing phenol, Caprolactam,
color intermediates, conservators, insecticides,
medications, softeners, etc. The recovered terephthalic
acid can be recycled to purification unit increasing the
terephthalic acid production capacity.
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