742 Indones. J. Chem.

, 2023, 23 (3), 742 - 753

Investigation of Crystal Size Distribution in Purification of Terephthalic Acid

from Polyester Textile Industry Waste by Reactive Crystallization

Bekti Marlena1,2*, Hary Sulistyo1, and Rochmadi Rochmadi1

1
Department of Chemical Engineering, Universitas Gadjah Mada, Jl. Grafika 2, Yogyakarta 55284, Indonesia
2
Center for Standardization and Industrial Pollution Prevention Services, Jl. Kimangunsarkoro No. 6, Semarang 5013, Indonesia

* Corresponding author: Abstract: The purification of terephthalic acid recovered from an alkali-reduction
wastewater by reactive crystallization was investigated. The crude terephthalic acid was
email: bektimarlena@mail.ugm.ac.id
reacted with sodium hydroxide solution to form a salt of disodium terephthalate, then
Received: December 31, 2022 acidified with sulfuric acid to get the terephthalic acid with higher purity. The effects of
Accepted: March 29, 2023 time, pH, concentration, and flow rate of secondary feed solutions, temperature, and
DOI: 10.22146/ijc.80820 stirring rate on crystal size distribution (CSD) of terephthalic acid precipitate were
investigated. The results showed that CSD was influenced by the concentration of
reactants and the pH solution. On the other hand, time, temperature, flow rate of
secondary solution, and stirring rate had no significant effects on the CSD, which the
mean size of crystals ±3 μm. The mean size of crystals at solution pH 5, 4, and 3 were
6.03, 9.42, and 10.34 μm, respectively; meanwhile, at concentrations of 0.5, 0.3, and
0.1 M, were 7.57, 3.24, and 3.09 μm, respectively. The semi-batch reactive crystallization
with double-feeding at constant pH and temperature resulted in monodispersed crystals.
However, this method must be carried out more than once for terephthalic acid
purification, intended for polyethylene terephthalate (PET) polymerization.

Keywords: crystal size distribution; purification; reactive crystallization; terephthalic acid

■ INTRODUCTION The wastewater from this process has extreme

characteristics such as a very alkaline pH (> 12.8) and a
Polyester is the most widely used synthetic fiber due
high organic concentration (COD of 20,000–
to its low production cost and good fiber properties. Still,
100,000 mg/L) [4]. Therefore, some researchers have
it has poor wear comfort because of its low moisture
studied the recovery of terephthalic acid as one of the
absorption and stain removal ability. Therefore, some
attractive treatments for this wastewater [5].
approaches have been developed to improve the
Terephthalic acid recovery is carried out by acidifying
hydrophilicity of polyester fiber [1]. The strong alkaline
wastewater to precipitate the terephthalic acid. This
treatment of polyester leads to the rupture of ester bonds.
method is known as precipitation or reactive
It increases the number of polar functional groups of
crystallization. In this process, the formation of a crystal
alcohol and carboxylic acid on the fiber surface [2].
or solid product is initiated with high supersaturation
On the other hand, alkali treatment causes
conditions which are obtained from the chemical
hydrolysis of the ester bond. The chain hydrolysis of
reaction between the soluble reactants [6].
polyethylene terephthalate (PET) with sodium hydroxide
Supersaturation is a driving force for nucleation which
produces disodium terephthalate, which is highly soluble
is the initial formation of a solid phase or crystal. The
in water. This alkali process also causes changes in fabric
growth of the crystal increases the crystal size.
weight, strength, wettability, and aesthetics [3]. The large
Industrially, crystalline product quality and
amount of weight loss in this hydrolysis process is known
properties are determined by the crystal size distribution
as the weight reduction process.
(CSD), morphology, and purity [7-8]. Chemical

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 Indones. J. Chem., 2023, 23 (3), 742 - 753 743

composition, as represented by the chemical purity and conditions, such as concentration and temperature,
impurity levels, can change the crystal properties, such as using a power law model, as shown in Eq. (3) and (4).
the mechanical, electrical, thermal, and optical properties. B  k N c b (3)
Control of impurities is also essential in many industries, G  k G c g
(4)
especially in the food and pharmaceutical industries,
where B is nucleation rate, kN is nucleation rate
where product purity should reach the strict
constant, Δc is supersaturation, exponent b is an order
specifications required for human use [9]. The crystal size
of nucleation, G is crystal growth, kG is growth rate
distribution (CSD) and crystal habit or morphology can
constant, and exponent g is an order of growth. The
change product and bulk properties such as dusting,
value of the primary nucleation kinetic constant b varies
dissolution rate, compressibility, and flowability [10].
in the range of 1–10 [14]. Meanwhile, the value of
They also influence the efficiency of downstream processes
growth kinetic constant g, in general, is 1 ≤ g ≤ 2, and g
such as filtration, centrifugation, and drying [11].
> 2 only for sparingly soluble compounds [6]. Studies of
The crystallization process control is important to
CSD on semi-batch reactive crystallization were
acquire products with desired and reproducible
conducted by single feed [15-17] or double feeds [18], as
properties. Poor product properties can cause extra
well as double feeds at constant pH by adding external
processing steps, which will increase manufacturing costs
acid or basic [19-20].
and be time-consuming [12]. Obtaining the desired
Studies on the purification and recovery of
particle size can often be challenging due to the
terephthalic acid from alkali weight reduction
interaction among various process parameters. A series of
wastewater using reactive crystallization [5] and cooling
techniques, including mathematical modeling tools, have
crystallization [21] focused on terephthalic acid's purity.
been applied to predict and control particle size and
However, research on CSD of terephthalic acid using
distribution [13].
reactive crystallization, especially from recovery weight
The CSD on crystallization can be predicted using
reduction wastewater, is still scarce.
the Population Balance Equation (PBE). PBE is an
It is assumed that at a constant pH, disodium
equation that represents the balance of the number of
terephthalate will react with sulfuric acid to form
particles in a specific state. For a batch crystallizer or a
terephthalic acid, which increases the concentration of
semi-batch crystallizer with assumptions that the system
terephthalic acid in the solution. It is expected to
is perfectly mixed and there is no net inflow or outflow of
increase the crystal growth rate and size. Thus, the work
crystals. The PBE can be written in Eq. (1).
presented in this paper aimed to study the effect of the
n n
G (1) crystallization processes (time, pH, temperature,
t L
concentration, flow rate of secondary solutions, and
Eq. (1) requires an initial condition and a boundary
stirring speed) on the crystal size distribution. The
condition. The initial condition of n(L, 0) for unseeded
crystallization process was conducted in conditions of
batch suspension crystallizer uses the size distribution of
semi-batch, constant solution pH, and constant solution
crystal at the time of the first appearance crystals. The
temperature (isothermal) by adjusting the feeding of the
boundary condition n(0, t) is the nuclei population
secondary solution of reactants.
density (n0) and is related to the nucleation rate (B), as
shown in Eq. (2). ■ EXPERIMENTAL SECTION
B t  Materials
n  0,t   n0  t   (2)
G  0,t 
The weight-reduced wastewater was collected from
Nucleation kinetics and crystal growth rate cannot a textile industry in Central Java Province, Indonesia.
be predicted theoretically, and in practice, they must be The weight-reduced wastewater was added with
measured and correlated empirically with environmental activated carbon and then acidified to pH 2 [5]. The

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744 Indones. J. Chem., 2023, 23 (3), 742 - 753

terephthalic acid precipitate was then filtered and dried to equipped with four baffles (0.1 of beaker diameter) and
produce crude terephthalic acid, having the four blades of paddle impeller stirrer (0.3 of beaker
characteristics shown in Table 1. diameter) was used as a reactor. The stirrer was driven
Analytical grade chemicals used in reactive by IKA RW 20 digital overhead stirrer. HI9890 pH meter
crystallization were sulfuric acid (Merck, 95–97%), charcoal was connected to a computer to monitor and save the
activated (Merck), and sodium hydroxide (Merck, 99%). data of pH and temperature during the process. A Cole
Instrumentation Palmer water bath was used to maintain and control the
process temperature. Masterflex C/L peristaltic pumps
The semi-batch crystallization system with double-
were employed to feed the secondary disodium
feeding reactants for the study of reactive crystallization
terephthalate and sulfuric acid solutions to the reactor.
of terephthalic acid is illustrated in Fig. 1. Experimental
The crystal size distribution was determined using a
setup consisted of a reactor, a mechanical stirrer, two
Particle Size Analyzer (PSA) Horiba Partica LA 960 V2.
peristaltic pumps, and a pH meter. A glass beaker of 1 L

Table 1. Characteristics of crude terephthalic acid

No Parameter Crude terephthalic acid Unit Method
1 Acid number 572.500 ± 7.500 mg KOH/g Titration
2 Ash 229.900 ± 5.200 ppm (w/w) Gravimetry
3 Moisture 6.160 ± 0.220 % (w/w) Gravimetry
4 Alkali transparency T-400 73.350 % Spectrophotometry
5 Mean size 21.335 ± 0.065 μm Laser diffraction
6 Metal contents
Mn 2.890 ± 0.119 ppm (w/w) Atomic absorption
Ni 0.279 ± 0.219 ppm (w/w) spectroscopy (AAS)
Co 0.020 ± 0.001 ppm (w/w)
Cr 1.219 ± 0.462 ppm (w/w)
7 Iron (Fe) 17.556 ± 4.239 ppm (w/w) AAS
8 Color in 5% dimethylformamide 51.650 - spectrophotometry

Fig 1. Experimental setup

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 Indones. J. Chem., 2023, 23 (3), 742 - 753 745

Procedure terephthalic acid formation by reacting disodium

The amount of 400 mL of 0.06 M primary disodium terephthalate with sulfuric acid. The solution dissolves
terephthalate solution was put into a glass beaker with a the terephthalic acid into ionic terephthalates (Eq. 5-6)
volume of 1 L as a reactor. It was acidified with 0.5 M [23].
sulfuric acid solutions (approximately 50 mL) until the H2 TPAq  H   HTP _ ,k1  103.54 (5)
targeted pH was reached. At this point, initial time HTP   TP2   H  ,k 2  104.46 (6)
sampling was carried out by taking 25 mL of solution. − 2−
where H2TP, HTP and TP indicate the neutral,
Secondary solutions of 0.5 M disodium terephthalate dissociated, and twice dissociated forms of terephthalic
and 0.5 M sulfuric acid solutions were added acid, respectively. The neutral form is called free acid.
continuously to the reactor (primary solution) using two The relative amounts of the three ions depend on the pH
peristaltic pumps in an equal amount between solution. The concentration of each species at
terephthalate and sulfuric acid. The addition of this equilibrium can be calculated by the fraction or alpha (α)
second solution did not affect the pH of the solution due by comparing its concentration with the total
to an equimolar amount of terephthalate and sulfuric concentration of phthalate ions with the help of the
acid. The initial volume of the primary solution at the dissociation constant [24].
targeted pH was approximately 450 mL. The volume of Fig. 2 shows the speciation of terephthalic acid as a
sulfuric acid added at a feeding rate of 1 mL/min was 30 mL, function of the pH solution. The two intersection points
and the volume of disodium terephthalate added was also of these curves were pk1 and pk2. The free acid of
30 mL. The final total volume was 385 mL after subtracting terephthalic acid forms a precipitate when its
the entire sample volume taken (125 mL). Samples were concentration exceeds the solubility value at equilibrium
taken at 5, 10, 20, and 30 min after secondary solutions were conditions (Eq. 7).
charged into the primary solution. The pH and temperature H2 TP s  H2 TPAq ,c*  0.000102 mol / L (7)
of the reaction were monitored and manually controlled
The mass balance of total terephthalates in the
by manipulating the flow rate of the H2SO4 solution.
solution was calculated from concentrations of
Each sample was filtered, and the precipitated
terephthalate ions (HTP− and TP2−) and free acid H2TP
terephthalic acid was oven-dried at 70 °C and then
using Eq. (8).
weighed (until constant). The dried sample was analyzed
using a PSA, which shows the characteristic dimension of
the crystals in the volume equivalent size.
■ RESULTS AND DISCUSSION
The Chemical Reaction of Disodium Terephthalate
with Sulfuric Acid
The reaction of disodium terephthalate with sulfuric
acid is shown in Scheme 1. Disodium terephthalate and
sulfuric acid are ionic reactants in an aqueous solution,
which essentially reacts very fast, producing a terephthalic
acid precipitate [22]. The reactive crystallization Fig 2. Mole fractions of ionic species of terephthalate as
mechanism consists of several steps. The first step is a function of solution pH

O O O O
Na O C C ONa + H2SO 4 HO C C OH + Na2 SO4

Scheme 1. The reaction of disodium terephthalate with sulfuric acid

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746 Indones. J. Chem., 2023, 23 (3), 742 - 753

 2    crude terephthalic acid had a very broad crystal size with

 TPTOTAL    TP    HTP   H2 TP  (8)
a mean size of 26.83 μm.
The electroneutrality balance of all species in the
solution was calculated using Eq. (9). CSD of Terephthalic Acid from Double Feed Semi-
H   2 TP2   HTP   HSO   2 SO2   OH  (9) batch Reactive Crystallization at Constant pH and
       4  4    Temperature
Eqs. (5-9), together with measured pH data and total
The primary disodium terephthalate solution was
sodium concentration, were used to calculate the
acidified with sulfuric acid solutions until the targeted
concentrations of terephthalate ions (TP2− and HTP−),
pH was reached. At this point, initial time sampling was
free terephthalic acid (H2TP) in the solution, and solid,
carried out. Secondary disodium terephthalate and
with the assumption that the solution was an ideal
sulfuric acid solution were added equimolar by keeping
solution at equilibrium and sulfuric acid was a strong acid.
the pH solution constant.
The pH profiles, concentrations of terephthalic acid in
Adding an equimolar secondary solution of
solution, and terephthalic acid solid at the addition of
disodium terephthalate and sulfuric acid at constant pH
sulfuric acid into 100 mL disodium terephthalate solution
was expected to increase the concentration of
with the same concentration (0.45 M) are shown in Fig. 3.
terephthalic acid in the reactor for crystal growth. The
Adding sulfuric acid decreased the solution pH and
feeding time of secondary solutions at constant pH was
the concentration of disodium terephthalate. The gentle
conducted for 30 min.
slope of the pH curve (red line) indicated the occurrence
of spontaneous nucleation. Then, it was followed by the
steep slope in which the solution was sensitive to pH
(around pH = 4), where disodium terephthalate was
equimolar to sulfuric acid. At this stage, adding a small
amount of sulfuric acid (± 3 mL) drastically changed the
pH from 4.64 to 2.85.
The terephthalic acid is sparingly soluble in water
with a solubility of 0.000102 M. If the concentration of
terephthalic acid exceeds the solubility value at equilibrium
conditions, then the solid terephthalic acid forms. The
solubility of terephthalic acid or concentration of Fig 3. Equilibrium concentrations and pH of 0.45 M
terephthalic acid in the solution (H2TP) curve (green line) disodium terephthalate 100 mL with 0.45 M sulfuric acid
was assumed constant regardless of the pH of the solution. additions
The supersaturation condition of terephthalic acid
was the driving force of crystallization which consisted
mainly of nucleation and crystal growth. The relationship
between nucleation and growth and agglomeration or
breakage of crystals certainly affected the final product of
CSD [8].
CSD of Crude Terephthalic Acid
The CSD of crude terephthalic acid used in this
experiment is shown in Fig. 4. The graph shows that CSD
stretched from 0.5–700 μm, with two peaks located at
5.5 μm by 4.1% and 29.9 μm by 3.9 %. It indicated that the Fig 4. CSD of crude terephthalic acid

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 Indones. J. Chem., 2023, 23 (3), 742 - 753 747

Effect of feeding time Effect of solution pH

The experiments were conducted at a solution pH of The solution pH was varied to 3, 4, and 5, as shown
5, at a concentration of a secondary solution of 0.5 M, and in Fig. 7. During the experiment, the feed rate was
at a temperature of 30 °C at various feeding times. controlled manually at a fixed pH. Controlling the
The addition of secondary solutions (in equimolar secondary solution feed rate was relatively easy at
reactants) increased the amount and number of crystals. solution pH 5, compared to solution pH 4 and 3, which
Still, the size distribution was similar at various feeding were sensitive to small fluctuations in the acid feed rate.
times, from 0 to 30 min, as shown in Fig. 5. This means It is illustrated in Fig. 6.
that feeding time did not influence the size of crystals. It The fluctuations in the concentration of H+ ions at
was apparent that the crystallization process (nucleation pH 3, 4, and 5 were between 3.05–2.96, which equaled
and crystal growth) occurred very fast in the time scale of CH+ = 9.10−5 (g ion/L); between 4.12–3.88, which
seconds; meanwhile, samples taken from the solution equaled CH+ = 2.4.10−5 (g ion/L), and between 5.02–
were within 5–10 min intervals. The short time scale of 4.99, which equaled CH+ = 3.10−7 (g ion/L).
the crystallization process was probably due to the high As mentioned above, the terephthalic acid was
supersaturation of terephthalic acid due to the low dissociated in water, and the species of ion
solubility of the terephthalic acid in water [25]. The high terephthalates was a function of the solution pH. Fig. 2
supersaturation caused the spontaneous primary shows that the free acid from terephthalic acid began to
nucleation to be more dominant than the growth of increase at pH 5.5, increasing with the decrease in the
crystals. solution pH. Therefore, decreasing the pH can increase

Fig 5. CSD of terephthalic acid at conditions of pH 5 and

Fig 6. pH fluctuations during experiments
T 30 °C

Fig 7. CSD of terephthalic acid at pH variations

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748 Indones. J. Chem., 2023, 23 (3), 742 - 753

the concentration of terephthalic acid as well as

supersaturation. When the supersaturation was high, and
the solubility was low, the crystallization process was
dominated by nucleation, which produced fine crystals.
On the other hand, a decrease in the pH of the solution
can increase the accumulation of fine crystals [26-27].
Aggregation occurred because there were terephthalate
ions (TP2− and HTP−) and undissociated terephthalate
(H2TP) when lattice crystals were formed so that the
surface of the crystal was easily charged. Fig 8. The mean size of terephthalic acid at variations of
The logarithmic scale has a broader span of crystal pH
size, as shown in Fig. 7(a). Because of this wide span, CSD
in the logarithmic scale showed the presence of two peaks single-feed semi-batch reactive crystallization [26]. In
(bimodal) at pH solution of 4 and 3 in the early minutes general, the acid solution produced fine crystals because
of the experiments. The primary peak dominated the of rapid nucleation and increased agglomeration.
crystal distribution in a smaller size (±10 μm), while the Meanwhile, the double-feed semi-batch crystallization
second peak appeared in a larger size (±130 μm), but its at constant pH showed that pH variation contributed to
frequency was relatively small. Meanwhile, the normal aggregation [20] and morphology [19].
scale (Fig. 7(b)) showed a tendency to increase the Effect of secondary solutions concentration
number of crystals, indicated by an increase in the peak Higher secondary solution concentrations
distribution concerning time. increased the crystal size of terephthalic acid, as
Large-sized crystals at acidic pH probably came presented in Fig. 9. A lower concentration of secondary
from the aggregation of terephthalic acid. Adding a solution produced a narrower distribution of
secondary solution at a constant pH can reduce the terephthalic crystals; meanwhile, a higher concentration
accumulation towards a more uniform crystal made a broadened distribution. The secondary solution
distribution. It was indicated by the disappearance of the concentrations affected mean crystal sizes. The
secondary peak (which is essentially small), leading to an concentrations of 0.5, 0.3, and 0.1 M resulted in a mean
increase in the height of the primary peak. Similar studies size of 7.13–8.75, 2.93–4.20, and 3.02–3.20 μm,
with double feeds at constant pH showed that changes in respectively. A lower concentration led to a lower mean
CSD indicated the occurrence of aggregation [20]. crystal size and variance coefficient, indicating a more
Fig. 8 shows that the mean crystal size of the uniform size.
terephthalic was increased as the pH solution decreased A higher concentration of reactants (in secondary
from 5 to 3. In this case, the pH solution = 5 was chosen solutions) produced a higher terephthalic acid
for further experiments because it was more stable and supersaturation condition, which was a driving force for
easier to control the pH solution. A decrease in pH means nucleation followed by consecutive and rapid growth.
that the rise of H+ concentration (more acidic solution) After nucleation, followed by the growth of the crystal,
essentially affected the saturated concentration of the concentration of the solution decreased gradually,
terephthalic acid and consequently influenced the size of leading to saturated concentration, in which the growth
the terephthalic crystal. Eqs. (3) and (4) can be used to rate stopped.
describe this phenomenon. Similar results were obtained by Tai and Chen [20],
The reactive crystallization by changing the solution reporting that under high supersaturation conditions,
pH (pH swing) was widely applied to produce a less the crystal size increased due to the high growth rate.
soluble acid or base from a salt [8], usually applied for a The experiment was conducted for the precipitation of

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 Indones. J. Chem., 2023, 23 (3), 742 - 753 749

Fig 9. (a) Distribution and (b) size of crystals of terephthalic acid at the variation of secondary solution concentrations

calcium sulfite hemihydrate by using double feeds and terephthalic acid. It is shown in Fig. 10(a), where
operating at a constant pH. These conditions resulted in the CSDs overlapped. The feeding rate of 1, 2, and
nucleation rates that were not too high, and 3 mL/min resulted in a mean size of 2.92–3.76, 3.01–
supersaturation conditions controlled the crystal growth 3.87, and 3.12–4.07 μm, respectively.
rate. The same result was observed by Han and Louhi-
Different results were shown by Rewatkar et al. [15] Kultanen [17] and Tai and Chen [20], stating that
regarding the precipitation of calcium oxalate and Caro et increasing feed speed can increase supersaturation and
al. [16] regarding the precipitation of salicylic acid. These nucleation. Still, if the supersaturation was high, then the
previous studies showed that the higher the reactant effect of feed speed was small enough, so it was not
concentration was used, the finer the crystal size was significant to be analyzed further. To some extent,
obtained. However, these studies were carried out on nucleation was challenging to be controlled by controlling
single-feed reactive crystallization systems. Furthermore, the feeding rate, especially in unseeded operations [29].
a study on reactive crystallization conducted by Utomo et
Effect of temperature
al. [28] comparing single feed and double feeds showed The experiments were conducted under isothermal
that the single feed system produced a smaller and wider conditions at three temperature variations of 30, 50, and
crystal size distribution than the double feed systems. 70 °C.
Effect of feeding rate The tendency of crystal distributions varies with
The feeding rate affects the supersaturation. An time; at 30 °C, the peak tended to shift to the right; at
increase in the feeding rate increases local 50 °C, it tended to increase, while at 70 °C, it had a very
supersaturation, increasing the nucleation rate. close value. Temperature affected the nucleation rate. The
The results showed that a feeding rate of 1 to nucleation rate increased with increasing the temperature
3 mL/min did not significantly affect the CSD of and the degree of supersaturation. In addition, the

Fig 10. (a) Distribution and (b) mean size of crystals of terephthalic acid at the variation of feeding rates

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750 Indones. J. Chem., 2023, 23 (3), 742 - 753

Fig 11. (a) Distribution and (b) mean size of crystals of terephthalic acid at variations of temperatures

solubility of terephthalic acid increased with increasing the solution properties.

the solution temperature. The solubility of terephthalic This study conducted experiments at 120, 300, and
acid in water from experiments at various temperatures 420 rpm stirring rates. Fig. 13(b) shows that the mean
compared to a previous study by Park and Sheehan [30] is crystal size slightly decreased with the increase of
presented in Fig. 12. stirring rate from 120 to 420 rpm, although it was not
A higher temperature caused a higher terephthalic very significant. This result agreed with the research on
acid (saturated) solubility, consequently lowering the the precipitation of hydroxyapatite by Tourbin et al.
supersaturation conditions (Δc = initial concentration – [31], which showed that crystal size increased at a
saturated concentration). Finally, it decreased the stirring rate of 120–600 rpm. However, the results of
nucleation and increased the crystal growth, producing a Caro et al. [16] showed that crystal size increased at a
larger crystal size. However, Fig. 11(b) indicates that stirring rate of 100–400 rpm and decreased at a stirring
terephthalic acid precipitation temperature only
significantly affected the crystal size. The competition
between the increasing reaction rate and the solubility
with the solution temperature was specific for each system
[22]. The competition seems negligible for the reactive
crystallization of terephthalic acid; therefore, the
temperature significantly affected CSD.
Effect of the stirring rate
Theoretically, crystal growth is influenced by mass
transfer rate, as confirmed by Eq. (4). One factor that Fig 12. Terephthalic acid solubility at various
affects the mass transfer rate is the stirring rate, besides temperatures

Fig 13. (a) Distribution and (b) mean size of crystals of terephthalic acid at the variation of stirrer stirring rates

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 Indones. J. Chem., 2023, 23 (3), 742 - 753 751

Table 2. Characteristics of terephthalic acid by reactive crystallization

No Parameter Terephthalic acid Standard Unit Method
1 Acid value 675.338 675 ± 2 mg KOH/g Titrimetry
2 Ash content 31.047 max 15 ppm (w/w) Gravimetry
3 Water content 0.028 max 0.15 % (w/w) Gravimetry
4 Metals 1.115 10 ppm
5 Fe 0.472 2 ppm Atomic absorption
6 Co, Mo, Ni, Ti, Mg 0.643 max 1 ppm spectroscopy
7 4-CarboxyBenzaldehide (4-CBA) 73.141 max 25 ppm Chromatography
8 p-Toluic acid 112.379 max 150 ppm Chromatography
9 Color in 5% dimethylformamide 8.580 max 10 APHA Spectro

speed of 800–1600 rpm. crystallization method needs to be carried out more than
Essentially, the rise of the stirring rate enhances the once to meet the terephthalic acid requirements for PET
degree of turbulence, which increases the mass transfer polymerization.
rate and finally increases the crystal growth rate.
■ CONCLUSION
However, the results showed that the stirring rate only
slightly affected the crystal size distribution, where the The semi-batch reactive crystallization of
mean crystal size remained as the stirring rate increased. terephthalic acid at constant pH and isothermal was
It means that the crystal growth step did not control the conducted to study the effect of the crystallization
overall crystallization process. processes (time, pH, temperature, concentration of
The semi-batch reactive crystallization with double- secondary solutions, flow rate of secondary solutions,
feeding reactants at constant pH and temperature forms and stirring rate) on the CSD of terephthalic acid. The
nearly the same size crystals. It shows that this method can experimental results showed that the pH and
be used to obtain monodispersed crystals. concentration of reactants influenced CSD. The
operational parameters of crystallization of time,
Purity of Terephthalic Acid
temperature, flow rate, secondary solution, and stirring
Terephthalic acid obtained from this experiment rate were found to have no significant effect on the mean
was characterized and compared to the standard of crystal size of terephthalic acid. Crystallization of
commercial terephthalic acid [32] to demonstrate its terephthalic acid was dominated by nucleation, which
potential industrial application. was reflected in the fine terephthalic acid crystal size.
According to ASTM D7976 Standard for purified The semi-batch reactive crystallization with double-
terephthalic acid, the terephthalic acid purity requirement feeding reactants at constant pH and temperature forms
for PET polymerization is a 4-CBA content of 25 ppm nearly the same size crystals. However, purification by
max [21]. The presence of 4-CBA impurities reduces the the reactive crystallization method needs to be carried
rate of polymerization in polyester production because out more than once to meet the TA requirements for
the aldehyde functional group in 4-carboxy benzaldehyde PET polymerization.
cannot react with ethylene glycol in the polymerization
■ ACKNOWLEDGMENTS
process, which limits the polyester chain so that the
molecular weight becomes low [33-34]. The authors thank Kementerian Riset, Teknologi
The results show that reactive purification still dan Pendidikan Tinggi via Saintek Scholarship No. 3535
contains 4-CBA, which is still relatively high at 73 ppm, in 2018 for financial support. The authors also thank the
and ash content of 31.05 ppm (Table 2). Therefore, the Center for Standardization and Industrial Pollution
purification of terephthalic acid using the reactive Prevention Services and the Ministry of Industry for

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752 Indones. J. Chem., 2023, 23 (3), 742 - 753

providing research facilities. Erdemir, D., and Lee, A.Y., Cambridge University
Press, Cambridge, UK, 32–75.
■ AUTHOR CONTRIBUTIONS
[8] Nagy, Z.K., Fujiwara, M., and Braatz, R.D., 2019,
Bekti Marlena was involved in conceptualization, "Monitoring and Advanced Control of
methodology, formal analysis, data curation, writing of Crystallization Processes" in Handbook of
the draft, and visualization. Rochmadi contributed to Industrial Crystallization, Eds. Myerson, A.S.,
conceptualization, methodology, resource acquisition, Erdemir, D., and Lee, A.Y., Cambridge University
writing review, and editing and provided supervision. Press, Cambridge, UK, 313–345.
Hary Sulistyo participated in methodology, validation, [9] Nicoud, L.H., and Myerson, A.S., 2019, The
writing review, and editing and provided supervision. All "Influence of Impurities and Additives on
authors have carefully read and agreed to the final version Crystallization" in Handbook of Industrial
of the manuscript. Crystallization, Eds. Myerson, A.S., Erdemir, D.,
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