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Recovery of lactic acid from sodium lactate by ion substitution using ion-exchange membrane
Separation and Purification Technology 28 (2002) 69 – 79 www.elsevier.com/locate/seppur
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,
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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
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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
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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
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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:
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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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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.
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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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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.
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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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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.
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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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[8] V.A. Shaposhnik, K. Kesore, J. Membr. Sci. 136 (1997) 35. [9] R. Audinos, Chem. Eng. Technol. 20 (1997) 247. [10] P.J. Moon, S.J. Parulekar, S.P. Tsai, J. Membr. Sci. 141 (1998) 75. [11] S. Resbeut, G. Pourcelly, R. Sandeaux, C. Gavach, Desalination 120 (1998) 235. [12] H. Voss, J. Membr. Sci. 65 (1986) 27. [13] K. Sato, T. Sakairi, T. Yonemoto, T. Tadaki, J. Membr. Sci. 100 (1995) 209. [14] V. Montiel, V. Garcia-Garcia, J. Gonalez-Garcia, F. Carmona, A. Aldaz, J. Membr. Sci. 140 (1998) 243. [15] T. Yamabe, M. Seno, N. Takai, J. Chem. Soc. Jpn. Ind. Chem. Sect. 64 (1961) 556. [16] H. Strathmann, in: A.J.B. Kemperman (Ed.), Handbook on Bipolar Membrane Technology, Twente University Press, Netherlands, 2000. [17] Tokuyama Soda Corp, Bulletin-NEOSEPTA ion exchange membrane, Japan, 1998. [18] Merck & Co. Inc., The Merck Index, New Jersey, 1989. [19] F. Helfferich, Ion Exchange, Dover Publications, New York, 1962. [20] Y.H. Kim, S.H. Moon, J. Chem. Technol. Biotecnol. 176 (2001) 1. [21] W.J. Moore, Physical Chemistry, Prentice-Hall, New Jersey, 1972. [22] T. Sata, M. Tanimoto, K. Kawamura, K. Matsusaki, J. Colloid Interface Sci. 219 (1999) 310. [23] T. Sata, J. Membr. Sci. 167 (2000) 1.
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,
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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
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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
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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
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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:
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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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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.
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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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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.
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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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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.
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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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