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Separation and Purification Technology 28 (2002) 69 – 79
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Recovery of lactic acid from sodium lactate by ion
substitution using ion-exchange membrane
Jae-Hwan Choi, Sung-Hye Kim, Seung-Hyeon Moon *
Department of En6ironmental Science and Engineering, Kwangju Institute of Science and Technology (K-JIST), 1 Oryong-dong,
Buk-gu, Gwangju 500 -712, South Korea
Received 12 May 2001; received in revised form 2 February 2002; accepted 2 February 2002
Abstract
Electrodialysis experiments were carried out to achieve efficient production of lactic acid from sodium lactate.
Conventional electrodialysis (CED) consisting of cation- and anion-exchange membrane and ion substitution
electrodialysis (ISED) consisting of only cation-exchange membrane were performed and the results were compared
in terms of lactic acid loss and the ratio (pI) of sodium ions transported to the current supplied. While both
electrodialysis operations removed over 95% of sodium ions from the feed solution, the CED operation, however,
resulted in a considerable loss of lactic acid, whereas there was no such loss in the ISED operation. Furthermore it
was found that the ratio of sodium ions transported to the current supplied was above 1.5 during the early stage of
operation in the ISED experiment, indicating that the sodium ions are transported not only by electric force but also
by concentration gradient in the ISED operation. By comparing the results of the two electrodialysis operation
modes, it was found that the ISED operation is advantageous for the production of lactic acid from sodium lactate
in terms of preventing lactic acid loss. In the ISED operation, however, process efficiency decreased with the
accumulation of sodium ions in the acid compartment because sodium ions in the acid solution are transported back
to the feed stream. This finding suggests the desirability of using a proton permselective cation-exchange membrane,
which is preferentially permeable to proton ions among the various cations present in the solution. © 2002 Elsevier
Science B.V. All rights reserved.
Keywords: Electrodialysis; Ion substitution reaction; Lactic acid; Donnan dialysis; Proton permselective membrane
1. Introduction
The recovery of organic acids by means of a
series of effective separation steps plays an important role in many biochemical industries since the
* Corresponding author. Tel.: +82-62-970-2435; fax: + 8262-970-2434.
E-mail address: [email protected] (S.-H. Moon).
conventional fermentation processes produce organic acids as calcium, ammonium, or sodium
salts. Thus, purification and acidification steps are
needed to recover free acid from the acid salt
[1,2]. The organic acid recovery processes, such as
crystallization, solvent extraction, and electrodialysis (ED), have been extensively studied, revealing
certain limitations [2–5]. The crystallization
method is disadvantageous in terms of low yield,
1383-5866/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S 1 3 8 3 - 5 8 6 6 ( 0 2 ) 0 0 0 1 4 - X
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J.-H. Choi et al. / Separation and Purification Technology 28 (2002) 69–79
high chemical costs and waste production. Solvent
extraction is handicapped by unfavorable distribution coefficients and environmental problems
associated with hazardous solvent use [6]. However the third process, electrodialysis, is promising
for the downstream processing of organic acid
from fermentation broth. ED is an electrochemical separation process in which electrically
charged membranes and an electrical potential
difference are used to separate ionic species from
an aqueous solution and other uncharged components [7]. It has been widely applied for production of table salt, organic acids, and sugar
demineralization as well as for blood treatment
and wine stabilization and it represents one of the
most important membrane processes for environmentally clean technology in biochemical industries [8– 12].
Electrodialytic desalination of a mixed solution
of an inorganic salt and organic compound (carboxylic acids or amino acids) has been studied
previously [11,13,14], with the observation that
there is considerable loss of the organic compound. This results from the simultaneous transport of organic compound with that of inorganic
ions through the membrane. The permeation occurs because the organic compound have different
ionic forms depending on the pH of the solution,
which is the case for carboxylic acids or amino
acids. Resbeut et al. [11] carried out desalination
experiments for a mixed solution of DL-phenylalanine, Na2SO4 and (NH4)2SO4 at its isoelectric
point. Although the demineralization of the solution is required to be accomplished with a minimum associated loss of phenylalanine, they found
a 14% loss of the initial phenylalanine level from
the diluate for a 97% of demineralization. Yamabe et al. [15] carried out desalination experiments
for an amino acid solution. They reported that
the permeation of the amino acid was minimized
when the pH of the solution was at its isoelectric
point (pI). Some of the amino acid, however, can
permeate the membrane even at the pI, because a
portion of the amino acid exists in charged form,
although the net charge is zero. In order to remove the inorganic salt while maintaining a low
level of amino acid loss, Sato et al. [13] used
electrodialysis with a charge-mosaic membrane
which is a kind of bipolar ion-exchange membrane having a parallel arrangement of cationand anion-exchange domains. They found that
such a membrane arrangement significantly reduced the amino acid loss compared to conventional electrodialysis. However about 8% glutamic
acid was lost for a 97% of salt removal. Also
when the mosaic membrane is applied to desalination the current efficiency decreases because of the
characteristics of the mosaic membrane. The recovery of lactic acid from lactic salts can be
performed also by a bipolar membrane where a
cation- and an anion-exchange membrane sheet
are laminated together. Lee [6] recovered lactic
acid from sodium lactate by conventional electrodialysis using bipolar membranes instead of anion-exchange membranes. Lactate ions associating
with hydrogen ions in the feed compartment form
lactic acid, and at the same time sodium ions
passing through the cation-exchange membrane
form sodium hydroxide with the hydroxyl ions in
the permeate compartment. However, when the
feed solution contains metal ions, such as calcium
and magnesium, water-splitting electrodialysis has
the drawback of membrane fouling resulting from
the precipitation of metal ions in the cation-exchange membrane. Another disadvantage is found
in the membrane price. The lack of large scale
membrane production leads to prices varying
from less than Euro 200 to more than Euro 2000;
very expensive compared to the conventional ionexchange membrane [16]. In this regard the use of
low-cost inorganic acids as a proton source is an
alternative method for acidification of sodium
lactate.
A conventional ED cell arrangement consists of
a series of anion and cation-exchange membranes
arranged in an alternating pattern between anode
and cathode. However, the electro-ion substitution reaction can be carried out in an ED stack in
which membranes are all of the same kind, either
anion or cation-exchange membranes [9]. In the
stack shown in Fig. 1 a unit cell has two compartments consisting of acid and feed streams. When
an electric current is supplied to the stack, protons in the acid stream are transported to the feed
stream and convert the negatively charged organics to neutral. As a result the inorganic cations in
J.-H. Choi et al. / Separation and Purification Technology 28 (2002) 69–79
71
the feed stream, in this case sodium ions, are
transported to the acid compartment to meet the
electroneutrality in the feed stream.
In this study, desalination experiments by ion
substitution electrodialysis (ISED) with cation-exchange membranes were carried out for a sodium
lactate solution. Similar experiments featuring
conventional electrodialysis (CED) were carried
out with cation- and anion-exchange membranes.
The feasibility of ISED for the purification of
lactic acid from sodium lactate was studied by
comparing both experimental results. In addition
the influence of operating conditions on the overall performance are discussed in terms of the
conversion efficiency and the ratio of sodium ions
transported to the current supplied.
2. Experimental
2.1. Preparation of feed solution
In order to obtain a lactic acid from sodium
lactate by electrodialysis operation, two cell
configurations were applied, conventional electrodialysis (Fig. 2(a)) and ion substitution electrodialysis (Fig. 2(b)). The feed solution for the ion
substitution electrodialysis was 0.1 N sodium lactate. Sodium lactate solution (NaL) was prepared
by adding sodium hydroxide to the lactic acid
solution previously prepared by diluting the reac-
Fig. 2. (a) Schematic diagram of the electrodialysis cell used in
CED experiments. (1) Electrode rinse solution (0.5 N
Na2SO4), (2) concentrate compartment solution (0.1 N
Na2SO4), (3) feed solution (0.1 N Na2SO4 +0.1 N lactic acid).
M, multimeter. (b) Schematic diagram of the electrodialysis
cell used in ISED experiments. (1) Electrode rinse solution (0.5
N Na2SO4), (2) acid compartment solution (0.5 N H2SO4 or
0.25 N Na2SO4 +0.25 N H2SO4), (3) feed solution (0.1 N
sodium lactate). M, multimeter.
tants (Acros, lactic acid 90%) with distilled water.
The feed solution used in the CED experiments
was a mixed solution of 0.1 N lactic acid and 0.1
N sodium sulfate, prepared by adding sulfuric
acid into the sodium lactate to convert the negatively charged lactate to an electrically neutral
lactic acid.
Fig. 1. Electrodialysis stack configuration for the production
of organic acid from organic acid salt by ion substitution
reaction.
2.2. Con6entional electrodialysis (CED)
experiments
The main component of the stack is a plexiglas
J.-H. Choi et al. / Separation and Purification Technology 28 (2002) 69–79
72
membrane cell, which consists of five separate
compartments. The two outer compartments contain the working electrodes, i.e. a platinized titanium anode and a stainless steel cathode. The
membranes used in these experiments were supplied by Tokuyama Soda Inc., Japan. A Neosepta
CMX membrane containing the sulfonic acid
group as fixed charges functioned as the cationexchange membrane and a Neosepta AMX membrane containing quaternary ammonium groups
as fixed charges functioned as the anion-exchange
membrane. The main properties of ion-exchange
membranes used in this study are listed in Table 1
[17]. Fig. 2(a) shows the membrane sequence in
cell configuration along with the transport direction of the different ionic species. The effective
area of each membrane was 25 cm2.
The feed solution (250 ml mixed solution of 0.1
N lactic acid and 0.1 N Na2SO4) was circulated
through the central compartment at a flow rate of
150 ml/min. The electrode solution was 1.0 l of
0.5 N Na2SO4 and the initial concentrate compartment solution was 1.0 l of 0.1 N Na2SO4.
Using an Orion pH meter (Model 250A) and
conductivity meter (Cole– Parmer, Model 124) the
pH and conductivity of the feed solution were
measured as a function of time. The concentration
of sodium ions in the feed solution was analyzed
using ion chromatography (Dionex DX-500, ED
40 conductivity detector, CS 12A column), and
the concentration of lactic acid was analyzed by
HPLC (Waters 410) equipped with an Aminex
HPX-87H column (Bio-Rad Co., CA).
Constant current (10 mA/cm2) was applied to
the ED stack during the experiment by a power
supply (HP 34410). The voltage drop across the
unit cell was measured by Ag/AgCl electrodes
connected to a multimeter (HP34401A), and the
data were saved automatically every 30 s by a
computer. All experiments were carried out at
25 °C.
2.3. Ion substitution electrodialysis (ISED)
experiments
The cell configuration of ISED is depicted in
Fig. 2(b). The feed solution (250 ml of 0.1 N
sodium lactate) was circulated in the central compartment at a flow rate of 150 ml/min. The initial
acid compartment solution on both sides of the
feed compartment consisted of 1.0 l of 0.5 N
H2SO4 solution. During the stack operation,
sodium ions are transported from the feed solution to the acid compartment. After a while
sodium ions can be transported back to the feed
compartment resulting in decreased process efficiency. In order to study the effects of the composition change in the acid compartment solution on
the process efficiency, electrodialysis experiments
were carried out for the mixed acid solution of
0.25 N H2SO4 and 0.25 N Na2SO4. The other
experimental conditions were same as the CED
experiments.
Table 1
The major properties of the ion-exchange membranes used in this study
Properties
CMX
CMS
AMX
Electric resistancea (V cm2)
Thickness (mm)
Exchange capacity (meq/g dry
membrane)
Transport numberb
Water contentc
Characteristics
2.5–3.5
0.17–0.19
1.5–1.8
2.0–2.5
0.15–0.20
1.4–2.0
2.5–3.5
0.16–0.18
1.4–1.7
0.98B
0.25–0.30
High mechanical strength cation
membrane
0.98B
0.20–0.30
Mono-cation
permselective
0.98B
0.25–0.30
High mechanical strength anion
membrane
a
Equilibrated with 0.5 N NaCl solution at 25 °C.
Measured by electrophoresis with sea water at a current density of 2 A/dm2.
c
Equilibrated with 0.5 N NaCl solution.
b
J.-H. Choi et al. / Separation and Purification Technology 28 (2002) 69–79
pNa =
Fig. 3. Ionic fraction of lactic acid as a function of pH.
3. Results and discussion
The fraction of the organic acid species that
exist in ionized form depends on the pH of the
solution, which is consequently to strongly influence ED performance. The fraction can be obtained via consideration of appropriate
dissociation reactions. For lactic acid (HL) with
monovalent anionic form, we must consider
HLUH+ +L−,
[H+][L−]UKHL[HL]
(1)
where KHL represents the dissociation constant.
The fractions of each species can be obtained by
using the equilibrium relations in Eq. (1). These
fractions are expressed as
f1 =
[L−]
KHL
=
[HL]T KHL + [H+]
(2)
f0 =
[HL]
= 1−f1
[HL]T
(3)
where, f1 and f0 denote the ionized fraction and
non-ionized fraction of lactic acid, respectively.
The subscript T denotes the total concentration of
lactic acid. The pK value of lactic acid is 3.79 [18].
The ionic fractions of lactic acid as a function of
pH are plotted in Fig. 3. It can be seen that most
lactic acid is negatively charged above pH 5 and
neutral below pH 2.
The sodium removal efficiency (pNa) was calculated by the following equation:
73
C 0Na − C tNa
× 100
C 0Na
where, C 0Na and C tNa are the concentrations of
sodium ion in the feed solution at time zero and
time t, respectively. The time courses of the removal efficiency of sodium ion and the concentration of lactic acid in the feed solution for the CED
and ISED operations are shown in Fig. 4. Both
experiments were carried out at the constant current density of 10 mA/cm2 for 240 min. At 160
min, ISED achieved about 95% sodium ion removal, a removal level attained at about 240 min
by the CED operation. It is noticeable that there
was a significant difference in lactic acid loss
between the CED and ISED operations. In the
ISED experiment there was no change in lactic
acid concentration during the experiment. On the
contrary the concentration of lactic acid in the
feed solution decreased significantly in the CED
operation. The concentration of lactic acid decreased slowly during the early stage of the experiment and showed a rapid decrease at about 120
min. At the end of the experiment, although
about 95% of the sodium ions were removed,
approximately 57% of the lactic acid was found to
be lost. The loss of lactic acid can be explained by
the pH change in the feed solution over the
operation time. Fig. 5 shows the time courses of
the pH change in the feed solution for the CED
and ISED experiments. Initially the feed solution
pH for CED and ISED were 2.42 and 6.54,
respectively. In the ISED experiment the feed
solution pH decreased with time because of the
ion substitution reaction in the feed solution. On
the other hand the pH increased, possibly due to
the transport of protons from the feed solution to
the concentrate solution, showed the peak at 150
min and then decreased in the CED operation.
Although the proton concentration was much
smaller than that of the sodium ions in the initial
feed solution, the much higher mobility of the
proton ions led to their transportation to the
concentrate solution along with the sodium ions,
resulting in a pH increase in the feed solution.
With the elapsed time, the concentration of
sodium ions decreased in the feed solution, while
due to the pH increase the lactic acids were
converted to lactate ions and transported through
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J.-H. Choi et al. / Separation and Purification Technology 28 (2002) 69–79
the anion-exchange membrane. Thus a significant
loss of lactic acid occurred in the CED operation.
Another possible explanation for the loss of lactic
acid in the CED process is the polarization near
the anion-exchange membrane. The concentration
of salt ions decreased with the operation time in
the feed solution, resulting in significant concentration polarization in the boundary layer. In this
case the lactic acid can be dissociated and pass
through the anon-exchange membrane. On the
contrary no lactic acid loss was observed in the
ISED since cation-exchange membranes were
used in the stack.
The current efficiency of an electrodialysis process is defined as the ratio of the number of ions
passed through the membrane to the stack current
[19]. The current efficiency is very useful for the
process evaluation of electrodialysis [20]. In the
cell configuration of ISED, however, ions can be
transported across the membrane due to a concentration difference (Donnan dialysis) as well as
an electric force (electrodialysis). To compare the
performance of the ISED and CED processes at
the same current density, the ratio (pI) of sodium
ions transported to the current passed through the
stack was used in this study. The ratio was
defined as following:
pI =
(C 0Na − C tNa) · V · F
I ·t
(4)
where V is volume of feed solution, F is Faraday’s
constant, I is the current supplied and t is operation time.
Fig. 6 shows the time courses of the ratio of
sodium ions transported to the current supplied
for the CED and ISED operations. In the CED
operation the change of the ratio with operation
time showed a similar pattern to the pH change
observed in Fig. 5. During the early stage of the
experiment the pI value was 0.65, later increased
up to 0.75. After 150 min the ratio decreased due
to transport of lactate through the anion-exchange membrane. On the other hand the pI value
for the ISED operation exceeded over 1.0 during
the initial 140 min, as high as 1.5. It is apparent
that some of the sodium ions were transported by
another driving force, Donnan dialysis. Since the
proton concentration in the acid compartment is
significantly higher than that in the feed solution,
there is a driving force for the transport of protons from the acid solution to feed solution. The
Fig. 4. Time profiles of sodium removal efficiency and the concentration of lactic acid in the feed solution for the CED and ISED
operations at the current density of 10 mA/cm2.
J.-H. Choi et al. / Separation and Purification Technology 28 (2002) 69–79
75
Fig. 5. pH change in the feed solution with operation time for the CED and ISED operations.
CMX membrane is permeable for cations, and
thus the proton ions will diffuse from the acid
compartment to the feed compartment. At the
same time sodium ions will diffuse in the other
direction because electroneutrality must be maintained. As the operation proceeds the concentration difference between acid and feed
compartments decreases resulting in decrease in
ionic flux by Donnan dialysis. Although there was
an additional Donnan dialytic transport in the
ISED operation, the pI value linearly decreased
with time, being 0.62 at the end of the experiment.
The decrease in pI value can be explained by the
pH change of the feed solution (see Fig. 5). When
most lactate is converted to lactic acid, a minor
portion of sodium ions remains in the feed
stream. This implies that the proton concentration
in the feed stream cannot be ignored as a movable
ion. The proton ions transported from the acid
compartment to the feed stream are not entirely
utilized in the ion substitution reaction, and some
of the protons are directly transported to the next
acid compartment again along with the sodium
ions. This phenomena caused to decrease in the pI
value.
Fig. 7 shows the cell potential during the exper-
iments for the CED and ISED operations. In the
case of the ISED experiment, the potential increased with time, and then slowly decreased after
150 min. On the contrary, in the CED operation
the potential increased steeply throughout the experiment. The different trends in potential drop
between the CED and ISED operations are due to
the concentration of transporting ions in the feed
stream. The initial concentration of 0.1 M sodium
ion in the feed solution decreased as the experiment proceeded, resulting in a slow increase in
potential. As the cell potential is reciprocally proportional to electrolyte concentration, the potential increased exponentially with the operation
time in the CED operation. The potential drop at
the end of the ISED operation is much lower than
that of CED.
The time profile of cell potential in the ISED
operation showed a close relationship with the pH
value in the feed solution. The pH of the feed
solution started to decrease slowly from the initial
value of 4.5. The slowness indicates that ion substitution reactions take place effectively in the
feed solution. Above pH 3, proton can be ignored
as a transporting ion in the feed stream because
the proton concentration is very low compared to
that of the sodium ions remained in the feed
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J.-H. Choi et al. / Separation and Purification Technology 28 (2002) 69–79
solution. Therefore most of the proton ions transported from the acid compartment are consumed
in the ion substitution reaction. However below
pH 3, the proton concentration cannot be ignored
as a transporting cation in the feed stream. Thus
proton ions compete with the sodium ions to pass
through the cation-exchange membrane. Moreover, proton may transport more easily than
sodium ions because the equivalent conductivity
of proton is about seven times higher than sodium
ion [21]. As a result cell potential decreases and
the ratio of sodium ions transported to the current supplied also decreases.
By comparing the results of both electrodialysis
experiments, we found that the ISED operation is
more efficient in the production of lactic acid
from sodium lactate in terms of the prevention of
lactate loss. In the ISED operation, however,
sodium ions are transported to the acid compartment and accumulated during the operation. The
sodium ions accumulated in the acid stream can
decrease the process efficiency because sodium
ions may transport to the feed stream. So the
effect of the composition in the acid compartment
solution on the process performance was examined. After replacing 0.5 N H2SO4 solution with
the mixed solution composed of 0.25 N H2SO4
and 0.25 N Na2SO4 as an acid compartment
solution, ISED experiments were carried out at
the current density of 10 mA/cm2.
The sodium removal efficiency and the ratio of
sodium ions transported to the current supplied
are presented in Fig. 8, where they are compared
with the result of pure sulfuric acid. At 240 min
the sodium removal efficiency reached to 83% for
the mixed solution, while nearly all sodium ions
were removed for the pure acid solution. However, it is noticeable that the pI value was relatively high considering the concentration ratio of
acid (H2SO4) to salt (Na2SO4). Also it can be seen
that Donnan dialytic transport occurred significantly at the early stage of operation.
In order to enhance the process efficiency, it is
necessary that only protons in the acid compartment solution transport to the feed stream. To
meet the requirement the cation-exchange membrane should have a high proton permselectivity
under the conditions of the mixed acid solutions
composed of protons and sodium ions. Permselectivity among cations through a cation-exchange
membrane is governed by the difference in affinity
of the cations with the membrane (ion exchange
Fig. 6. Change of the ratio of ions transported to total current supplied (pI) during the CED and ISED operation experiments at
the current density of 10 mA/cm2.
J.-H. Choi et al. / Separation and Purification Technology 28 (2002) 69–79
77
Fig. 7. Time courses of cell potential for the CED and ISED operations.
equilibrium constant) and by the difference in
migration speed of the respective cations (mobility
ratios among the cations) [22,23].
For examining the variation in permselectivity
according to ion-exchange membrane the ISED
experiments were performed using Neosepta CMS
membranes instead of CMX membranes. The acid
compartment solution used in this experiment was
a mixed solution composed of 0.25 N H2SO4 and
0.25 N Na2SO4. The sodium removal efficiency
and the pI values for both the CMX and the CMS
membranes are presented in Fig. 9. At 240 min,
sodium removal efficiencies of 85.1 and 91.7%
were obtained for CMX and CMS, respectively.
The sodium removal efficiency and the pI values
increased for CMS, indicating that CMS has a
higher proton permselectivity than CMX. Thus,
CMS was found to be more efficient in the ISED
operation than CMX. The difference in pI value
may be due to the characteristic of each cation-exchange membrane. As can be seen in Table 1, the
CMX membrane is a standard grade membrane
for general concentration or desalination purposes, while the CMS membrane is a movovalent
selective cation-exchange membrane. This mem-
brane is prepared by forming a thin cationic
charged layer on the membrane surface [7]. Due
to the cationic charge on the surface, it is more
difficult for the sodium ion to transport through
the CMS membrane than the CMX membrane.
On the contrary protons are not affected significantly by the cationic charge of the CMS membrane. Thus the proton permselectivity for the
CMS membrane can be higher than the CMX
membrane. In the ISED operation the driving
forces are sum of concentration gradient and electric potential gradient. The concentration gradient
decreased with time so that the pI value also
decreased. From Fig. 9 it can be seen that the pI
value for the CMX membrane was higher than
that for the CMS membrane during the early
stage of the operation, and the trend was reversed
at 60 min. This indicates that the number of
sodium ions transported by Donnan dialysis
through CMX is larger than that through CMS
during the early stage of the experiment.
The results in this study indicate that to increase the process efficiency of ISED it is desirable to use a high proton permselective
cation-exchange membrane which is preferentially
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J.-H. Choi et al. / Separation and Purification Technology 28 (2002) 69–79
permeable to proton ions among various cations.
Particularly Fig. 9 shows that the CMS membrane
exhibits more proton permselective characteristic
compared to the CMX. It has been reported that
amphoteric-type ion-exchange membranes are
preferentially permeable to proton ions by preparing the membranes coated with a thin cation
charged layer, resulting in a proton selectivity
against sodium of up to 12 [7]. Therefore it is
crucial to choose a highly proton selective membrane for efficient operation of ISED.
4. Conclusion
A study on the production of lactic acid from
sodium lactate was carried out by comparing the
results of CED and ISED in terms of lactic acid
loss and the ratio of sodium ions transported to
the current supplied. The CED operation resulted
in a considerable loss of lactic acid, while no loss
of lactic acid was observed in the ISED operation.
Furthermore, it was found that sodium ions were
transported by both electric force and concentra-
tion gradient in the ISED operation. The ISED
operation is more efficient than the CED in the
production of lactic acid from sodium lactate, due
to the prevention of lactic acid loss. In ISED
operation, however, sodium ions are transported
to the acid compartment and accumulated, causing to decrease in the process efficiency because
the sodium ions can transport back to the feed
stream. Therefore it is desirable to use a proton
permselective cation-exchange membrane in
ISED.
The major finding of this study is the potential
advantage of ISED over CED in a wide range of
industrial application. Even though sodium lactate was used as the feed solution in this study, it
is apparent that when ammonium lactate is used,
ammonium sulfate is obtained as a by-product,
fertilizer. In addition a similar process can be
applied to produce pure organic acid or amino
acid from their salt forms, for example, the production of pure lysine from lysine hydrochloride
using anion-exchange membranes and base solution by chloride substitution reaction.
Fig. 8. Effect of acid compartment solution composition on the sodium removal and the ratio of ions transported to total current
supplied (pI) in the ISED operation.
J.-H. Choi et al. / Separation and Purification Technology 28 (2002) 69–79
79
Fig. 9. Effect of cation-exchange membranes on the sodium removal and the ratio of ions transported to total current supplied (pI)
in the ISED operation.
Acknowledgements
This work was supported by the National Research Laboratory (NRL) Program of Korea Institute of Science and Technology Evaluation and
Planning (Project No. 2000-N-NL-01-C-185).
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