Detailed investigation of an electrodialytic process during the separation of lactic acid from a...

Journal of Membrane Science 249 (2005) 173–182

Detailed investigation of an electrodialytic process during theseparation of lactic acid from a complex mixture

Vu Hong Thang, Werner Koschuh, Klaus D. Kulbe, Senad Novalin∗

Department of Food Science and Technology, University of Natural Resources andApplied Life Sciences (BOKU), Muthgasse 18, A-1190 Vienna, Austria

Received 23 January 2004; received in revised form 3 February 2004; accepted 23 August 2004Available online 23 November 2004

Abstract

The electrodialytic separation of lactic acid from grass silage juice, which is a complex mixture, containing inorganic salts, sugars andamino acids in high concentrations, was investigated in detail. The separation was carried out in two steps. In the first step, lactate togetherwith other charged compounds were separated from the neutral ones, such as sugars and non-ionic amino acids, at pH 6.7. In the second step,l phase, thep ms of masst ) step at pH6 ll as longero ionic aminoa©

K

1

pecpsaso

dAfia

-raw

s inem-nce

, EDativerinci-here

lysisnta-

lactica highveryor

0d

actic acid was recovered from the inorganic salts at pH 2.0. Each step was divided into different phases (time periods). For eacherformance in electrodialytic separation of lactic acid was investigated and discussed. This performance is characterized in ter

ransfers, current efficiencies, energy consumptions, and water transfers. The obtained data show that the first electrodialysis (ED.7 is significantly less effective than the second ED step at pH 2.0. Higher energy consumption and lower current efficiency as weperation time were observed during the first ED step. High concentration of uncharged compounds, such as sugars and non-cids likely caused the negative effects during the separation of salts as well as lactate.2004 Elsevier B.V. All rights reserved.

eywords:Grass silage; Desalination; Lactic acid; Electrodialysis; Current efficiency; Energy consumption

. Introduction

In the near future, the use of renewable resources couldlay an important role for the production of chemicals. How-ver, many interesting substances often occur in very compli-ated mixtures, therefore, the methods used for downstreamrocessing will play a very important role. These methodshould meet requirements in terms of technical, economicalnd environmental aspects. Even though there are many pos-ibilities for separating a substance, only few limited meth-ds, which are practicable, can be found.

Recently, grass silage has been considered a good can-idate for the production of fine chemicals[1]. Within “theustrian Programme for the Development of Green Biore-nery”, it was shown that during the ensiling process, lacticcid and amino acids produced make the grasses a potential

∗ Corresponding author. Tel.: +43 1 36006 6288; fax: +43 1 36006 6251.E-mail address:[email protected] (S. Novalin).

source for the production of these substances[2]. This technology is interesting due to the prospect of using cheapmaterial, which, in addition, is a renewable resource.

Electrodialysis (ED) is an electromembrane proceswhich ions are transported through ion permeable mbranes from one solution to another under the influeof a voltage between two end electrodes. Thereforecan separate selectively ions having positive or negcharge and reject ions of the opposite charge. The pples and major applications of ED can be found elsew[3].

Since most recent possible applications of electrodiais separation or production of organic acids from fermetion broths or enzymatically produced solutions[4–7], thistechnology also seems to be applicable for separatingacid from grass silage. However, since grass silage hascontent of mineral ions as well as other components, it isdifficult to produce lactic acid from grass silage with foodpharmaceutical grade[8].

376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
oi:10.1016/j.memsci.2004.08.033

174 V.H. Thang et al. / Journal of Membrane Science 249 (2005) 173–182

Table 1Physico-chemical characterization of grass silage juice

Parameter Value

Conductivity (20.5◦C) (mS cm−1) 35.8pH (20.5◦C) 4.04Colour Dark brownDry matter (g L−1) 136.76Cations (K+, Na+, NH4

+, Ca2+, Mg2+) (g L−1) 19.28Anions (Cl−, NO3

−, PO43−, SO4

2−) (g L−1) 15.47Lactate (g L−1) 37.54Free amino acids (g L−1) 26.13Sugars (glucose, fructose, sucrose, arabinose, xylose,

galactose, mannitol) (g L−1)38.34

Regarding the separation of lactic acid using ED, manyworks have been published. Several works relate to concen-trating lactate by conventional ED, while the others deal withbipolar ED, in which lactate separated by conventional EDwas converted to lactic acid[9–16]. However, most of worksdeal with low salt content mixtures, such as fermentationbroth or model solutions. The performance behavior duringthe electrodialytic process of mixtures, containing high saltcontent as well as other compounds is not clearly studied yet.Therefore, beside the investigation of using silage juice as apotential resource for sustainable production, electrodialysisof this juice should be considered as a good example in orderto get some knowledge about the process performances un-der such condition of high concentration of inorganic salts,sugars as well as amino acids. In a previous work[17], thetwo-stage electrodialysis has shown that it is possible to sep-arate valuable substances, such as lactic acid as well as aminoacids from a renewable resource. This paper investigated indetail an ED process consisting of two ED steps used for sep-arating lactic acid from a complex mixture like grass silagejuice, in connection with the energy consumption and thebehaviors of high content of impurities.

2. Materials and methods

2

inceo wasu micm oreda f theg

2

dia-l siss thed entsh em-b

The membranes used in these experiments were suppliedby Tokuyama Soda Inc., Japan. The specifications of themembranes are shown inTable 2. The flow velocities wereabout 0.08 ms−1. The temperatures of the solutions werebetween 25 and 30◦C. The electrode solution was 0.5 NNa2SO4. In all experiments the voltage was kept constant.

The current efficiency for a single phase is calculated bythe formula:

η = IT

IP(1)

where IT and IP are the theoretical and practical currents,respectively.

The theoretical current is calculated by the for-mula IT =FJAeff, in which F is the Faraday constant(96485.3 A s mol−1), Aeff the effective membrane area andJ the density of charge flux.J is calculated by the formula:

J = MT

Aefft, (mol m−2 s−1) (2)

whereMT is the total of transported molar equivalent chargescorresponding to the amount of cations, which were removedfrom diluate to concentrate andt is the operating time.

The practical current was calculated by measuring thetransported ions, divided by the running time.

The total current efficiency for the whole two-stage elec-t

η

w ur-r EDs so

R

w e.Cd

2

rade.W highq red att

2

LC( ol-u asewt

.1. Grass silage juice

Grass silages were collected from farms in the provf Styria, Austria, and then pressed. The pressed juiceltrafiltered by cross-flow module using 1 kD cut-off ceraembrane (TAMI Industries, France). The filtrate was stt−30◦C for further uses. Properties and composition orass silage juice was presented inTable 1.

.2. Electrodialysis

In this study, a conventional two-compartment electroyzer was used.Fig. 1shows the picture of an electrodialyystem, where 0.54 L of grass silage juice circulates iniluate compartment. The stack used during the experimas eight cell pairs and two additional cation exchange mranes next to the anode.

rodialysis process is calculated by following formula:

T = IT1t1 + IT2t2

IP1t1 + IP2t2(3)

hereIT1, IT2, IP1, IP2 are the theoretical and practical cents in the first ED step (at pH 6.7) and in the secondtep (at pH 2.0), respectively.t1 andt2 are the operating timef the first and the second ED steps, respectively.

The removalRof componenti is defined as follows:

=(

1 − Cf1Vf1

Ci1Vi1

)× 100% (4)

hereVf1 andVi1 are the final and initial volume of diluatf1 andCi1 are the final and initial concentration (g L−1) ofiluate.

.3. Chemicals

The reagents used in all experiments are laboratory gater used for preparation of concentrate solution was

uality reverse osmosis-treated water, which was prepahe in-house water treatment plant.

.4. Analysis

The concentration of lactate was analyzed by HPMerck-Hitachi) equipped with an Aminex HPX-87H cmn (Bio-Rad Co., CA) and RI detector. The mobile phas 10 mM H2SO4 at flow-rate of 0.6 mL min−1. The column

emperature was maintained at 65◦C.

V.H. Thang et al. / Journal of Membrane Science 249 (2005) 173–182 175

Fig. 1. Picture of ED system.

The concentration of inorganic anions was analyzed usingion chromatography (Dionex DX-120, USA) equipped witha DS4 Detection Stabilizer/Electrochemical Detector, an Ion-Pac AS 14 column using NaHCO3/Na2CO3 3,5 mM/1 mM at1.2 mL min−1, 25◦C as mobile phase.

The concentration of inorganic cations was analyzed in thesame ion chromatography (Dionex DX-120, USA) equippedwith a DS4 Detection Stabilizer/Electrochemical Detector, anIonPac CS 12A column using methanesulfonic acid 20 mMat 1.2 mL min−1, 25◦C as mobile phase.

Sugars were analyzed by HPLC (Dionex DX-500, USA)equipped with a CarboPac PA1 column and ED40 IntegratedAmperometric Detector using 25 mM NaOH as an eluent atflow-rate of 1 mL min−1.

Amino acids were analyzed by using HPLC (SummitHPLC Analytical System, Dionex, USA) with RF 2000 Flu-orescence Detector. The samples were analyzed after precol-umn derivatisation with OPA (ortho-phthaldialdehyde) using

a GROM-SIL OPA-1 column purchased from GROM An-alytik + HPLC GmbH, Germany. The column temperaturewas maintained at 25◦C. Eluent A is 25 mM sodium phos-phate, pH 7.2/tetrahydrofuran (995/5); eluent B is 25 mMsodium phosphate, pH 7.2/MeOH/acetonitrile (50/35/15).The analysis was carried out by gradient elution: 100% A(0–2 min), 100–50% A and 0–50% B (2–12 min), 50–40%A and 50–60% B (12–17 min), 40–0% A and 60–100% B(17–23 min), 100% B (23–28 min), 100% A (28–40 min).The flow-rate was 1 mL min−1.

3. Results and discussion

3.1. Limiting current density

When substances are separated from such a solution bymeans of ED, an important question arises, namely, which

Table 2Characteristics of AMX and CMX ion-exchange membranes[18]

Parameter CMX AMX

Permeability Strongly acidic cation permeable Strongly basic anion permeableForm Na-form Cl-formWater content (%) 0.25–0.30 0.25–0.30M trengthEITT .19EB

echanical properties High slectric resistance (� cm2) 2.5–3.5

on exchange capacity (meq. g−1) 1.5–1.8ransport number (total cation or anion) 0.98hickness (mm) 0.17–0ffective area (cm2) 37urst strength (kg cm−2) 5–6

High strength2.5–3.51.4–1.7

0.980.16–0.18

374.5–5.5


current density should be or can be applied during the pro-cess operation[4]. The question is not easily answered, sincethe current in many ways affects the process. Influenced pa-rameters, for example, are electric power requirement, life-time of the membranes, product quality, etc. In any case, theso-called limiting current density (LCD) is an important pa-rameter. However, it should be noted that the LCD is not acertain value; it will change with the working conditions andoperating time of the electrodialysis system.

In this study, current–voltage method (Cowan and Brownmethod) was used[19]. The determination of LCD is shownin Fig. 2. In order to maintain process parameters constant,the same solution (the juice) was used both in diluate and con-centrate tanks. Using this method, the initial limiting currentdensity were determined as 290 and 540 A m−2 correspond-ing to pH 2.0 and 6.7, respectively.

3.2. Performance of the first ED step

In the first ED step, ionic compounds are separated fromnon-ionic ones.Fig. 3presents the effect of time on the con-ductivity and the transported lactate. The removal of lactate,which was calculated by(4), can be reached to 100% if theoperation time is prolonged. In ED process, there is often aregion where the conductivity of the diluate decreases quitef tice,i or

Fig. 3. Behavior of conductivity and transported lactate during the first EDstep at pH 6.7.

optimize the ED process. In the present work, it can be seenthat after 80 min, the conductivity of the diluate decreased50%, while during the last 160 min, it decreased only about5%. In order to investigate the process in detail, the wholeprocess was divided into three different phases as shown inFig. 3. The lactate transport in phase 1 was highest, whilethat in phase 3, as expected, was lowest. In phase 1, it took80 min for transferring about 40% of lactate in comparisonwith 160 min for transferring about another 40% of lactatein phase 2. During the phase 3, the transport rate was signif-icantly declined; only about 10% of lactate was transferredwithin 160 min. One interesting aspect is that within the phase1, the lactate transport is constant although the current is de-creasing. This can be explained by the fact that within thephase 1, the concentration of inorganic ions decreases.

One of the most important parameters concerning to ap-plication of ED is the current efficiency. Current efficiencyas well as other key parameters in ED process is summarizedin Table 3. During the first phase, the average current densitywas not so far from the limiting current density and this re-sulted a high current efficiency. The current efficiency was ata level of 90%. In the second phase, the current efficiency was62%, while in the third phase the current efficiency was only59%. The decline of the current efficiency from 90 to 59% isexplainable by the constant voltage and decreasing concen-t ncyb e alsos sferd e andi

s thes ought pump

ast, while in other regions it decreases slowly. In pract is important to know these regions in order to improve

Fig. 2. Limiting current of grass silage juice at two different pH.

v lec-t anesw sentst sired

ration of ions. However, the difference in current efficieetween phases 2 and 3 was unexpectedly low. It can been fromTable 3that, in the second phase, lactate tranecreased about 34%, in comparison with the first phas

n the third phase, dropped significantly, more 90%.The energy consumption required in an ED process i

um of the electrical energy to transfer ionic species thrhe ion-selective membranes and the energy required toarious solutions through the ED stack. Here, only the erical energy to transfer ionic species through the membras calculated. The specific energy consumption repre

he energy consumption relating to the amount of the de


Table 3Key parameters relating to the first separation step

Parameter Phases Whole step (420 min)

1 (0–80 min) 2 (80–240 min) 3 (240–420 min)

Lactate transfer (kg m−2 h−1) 0.223 0.147 0.018 0.074Practical specific energy consumption for lactatea (kW h kg−1) 2.23 1.36 3.51 2.11Estimated specific energy consumption for lactateb (kW h kg−1) 0.89 1.14 2.49 1.55Current efficiency (%) 90 62 59 67

a The calculation bases on energy consumption for transport of both lactate and inorganic salts.b The calculation bases on energy consumption for transport of lactate only.

compound transferred. In this case, this parameter is calcu-lated for lactate transferred to concentrate. The total energyconsumption is calculated according to the potential readingstaken at the cell electrodes. The practical specific energy con-sumption of lactate for overall operation is 2.11 kW h kg−1.Interestingly, the practical specific energy required for phase2 was smallest, whereas for phase 3 was the biggest. However,regardless of the transport of inorganic salts, the estimatedspecific energy consumption for lactate increases throughoutthe three phases, as expected.

Fig. 4presents the changes in composition of transportedanions during ED1 step at pH 6.7. It can be seen that inorganicsalts were mainly transported in phase 1, while the main re-maining anion in the diluate was lactate. If it were supposedthat each transferred ion in phase 1 requires an equal value ofenergy consumption, the energy used for transporting lactatesalt would be about 40% of energy consumption for transport-ing both organic and inorganic salts during this phase. Thisenergy consumption corresponds to 0.89 kW h kg−1. This re-sult shows that a considerable part of energy consumption wasspent for transporting inorganic salts. With similar calcula-tion, lactate salt transferred in phase 2 needs 1.14 kW h kg−1,which took a part of 84% of energy consumption for trans-porting both organic and inorganic salts during this phase. Inthe phase 3, transferred lactate needed 2.49 kW h kg−1, whichr ans-p over,

F t EDs

Fig. 5. Contribution of energy consumption in different phases during thefirst ED step at pH 6.7.

data inFig. 5show that the greatest part of consumed energywas spent for phase 1. It was also estimated that about 27%of total energy consumption for the first ED step at pH 6.7was spent for transporting inorganic salts.

The behaviors of current density and lactate as well aswater transfer are presented inFig. 6. It can be seen the sim-ilarity in the change of current density and water transportduring the separation. The ionic species passing through themembrane carry certain amount of water. This molar fluxof water results from transport of water of hydration bound

F g thefi

epresented a part of 71% of energy consumption for trorting both organic and inorganic salts in phase 3. More

ig. 4. Changes in composition of transported anions during the firstep at pH 6.7.

ig. 6. Behaviors of current density, lactate and water transport durinrst separation step at pH 6.7.


to the ions and a smaller part from transport of water byelectroosmosis.

One great advantage of ED is that the product can be con-centrated up to a certain amount together with the separationprocess. The maximum concentration depends upon the ratiobetween lactate transport to water transport. The average lac-tate transport and water transport in this study were 0.074 and0.434 kg m−2 h−1, respectively. Therefore, the transport ratioof water/lactate was calculated to be 5.9 g g−1, which cor-responds to a maximum achievable lactate concentration of14.5%. In literature, the maximal lactate concentration, whichcan be reached by conventional ED, is about 220–280 g L−1

[20–22]. The lower result in our case is due to the additionaltransport of water by the inorganic salts. In other works, casestudies were fermentation broth or model solutions, whichhave much less salt contents in comparison with our case.

Fig. 7shows the transport behavior of ions during the elec-trodialytic process at pH 6.7. The average mass transfer ofinorganic salts was 0.091 kg m−2 h−1. As expected, monova-lent ions could be transferred as much as 100%, while divalentions were more difficultly transferred. In the early stage, thetransfer rates of monovalent ions are much higher than thoseof divalent ions. Chloride and nitrate are all removed from thediluate. On the contrary, sulfate appears to be difficult to re-move. It is mainly transferred during the last stage, when mostm ctateh anici intot lfate,

F6

with only 52%. However, the difficulty in transferring sul-fate offers an advantage: this can be considered as a partialremoval of sulfate from lactate. This removal could be im-proved further by using monovalent selective anion exchangemembranes.

The obtained data indicate how the process operation canbe optimized. A reduction of voltage during the second andthe third phase should improve the current efficiency. How-ever, as a result, the lactate transfer as well as the transfer ofinorganic salts will be somewhat reduced. Alternatively, theionic concentration in the diluate in the second or the thirdphase can be increased. This has to be investigated more indetail.

3.3. Performance of the second ED step

The pH of the concentrate obtained from the first ED stepwas adjusted to pH 2.0 by addition of concentrated hydrochlo-ric acid. In principle, it can be used a strong inorganic acid,such as hydrochloric, nitric, sulfuric or phosphoric acid toadjust pH. However, since the main inorganic anion in thejuice is Cl−, hydrochloric acid was chosen. Moreover, Cl−is more easily transported than SO4

2− or PO43−. During the

second ED process, inorganic ions are separated from lacticacid. Fig. 8 shows that the loss of lactic acid increases byt ase1 losso e endo ctated ssesm t wasf sedi s thatd onlyi ved.S e pHo

onovalent inorganic anions as well as about 80% of laave been removed from the diluate. At the end, the inorg

ons are almost completely transferred from the diluatehe concentrate with the level more than 95%, except su

ig. 7. Effect of time on the removal of ions during the first ED step at pH.7.

F d EDs

ime. However, the loss was slightly increasing during ph, but significantly in phase 2. At the end of phase 1, thef lactic acid was 1.1% but this loss reached to 6.6% at thf phase 2. It is clear that the knowledge of the loss of lauring the separation is very important. In practice, the loust be kept as low as possible. On the other hand, i

ound that the conductivity of the diluate strongly decrean phase 1, but barely changed in phase 2. This proveuring phase 1, lactate was kept in non-ionic state and

norganic ions as well as free hydrogen ions were remoince hydrogen ions were removed from the diluate, thf the diluate increased. It can be seen fromFig. 9 that the

ig. 8. Behavior of conductivity and lactic acid loss during the secontep at pH 2.0.


Table 4Key parameters relating to the second separation step

Parameter Phases Whole step (105 min)

1 (0–80 min) 2 (80–105 min)

Lactic acid loss (%) 1.10 5.52 6.62Practical desalting energy consumption of salts (kW h kg−1) 0.74 2.83 0.77Current efficiency (%) 98 57 88

pH of the diluate in the second ED step (diluate 2) increasedfrom 2.0 to 2.9 after 80 min. The equilibrium between lac-tic acid and lactate changed with the increase of lactate ions,since the previously undissociated lactic acid started to disso-ciate due to the change of pH. Thus, lactate ions were furthertransported into the concentrate. It was shown that the undis-sociated lactic acid in the solution decreased from 99% (atpH 2.0) to 85% (at pH 2.9)[23]. Besides, a part of lactic acidwas probably transported due to the diffusion.

Key parameters in the second separation step are summa-rized inTable 4. Regarding to the current efficiency, we cansee that this parameter decreased with the operation time andthere was a great change of the current efficiency betweenthe two phases. A very high current efficiency of 98% duringphase 1 dropped to 57% in phase 2. Moreover, the energyconsumption for desalting in phase 2 was especially high,increasing approximately four times in comparison with thatin phase 1. One reason is due to the low concentration of ionsin phase 2.Fig. 10shows that inorganic anions were alreadyremoved from the diluate during phase 1. FromFig. 11, itcan be seen that 95% of energy consumption for the secondED step (at pH 2.0) was spent in phase 1. After analyzing thedata inFig. 10, it can be found that, the energy consumptionin phase 1 was mainly used for the desalting but in phase 2,a great part of energy consumption was used for transferringd more9 rredi entedo ns-f

stepw -

Fig. 10. Changes in composition of transported anions during the secondED step at pH 2.0.

parison with the energy consumption of 1.07 kW h kg−1 fortransferring the inorganic salts in the first ED step (at pH 6.7).It can be seen how the high concentration of uncharged com-pounds, such as sugars and amino acids in this case, effectthe transport of charged compounds. When lactate is sepa-rated from a fermentation broth, concentrations of sugars andamino acids are considerably lower. We assume that a barrierlayer onto the membranes was created by sugars and aminoacids, which reduced the transport of salts and lactate throughthe membrane.

The behaviors of current density and water transfer dur-ing the second ED are shown inFig. 12. Like in the previous

F g thes

issociated lactate. In phase 1, inorganic salts occupied9% of total molar anion equivalent, which were transfe

nto concentrate. Contrary, in phase 2 these salts represnly 58% of total molar anion equivalent, which were tra

erred.The desalting energy consumption for whole ED 2

as 0.77 kW h kg−1, which is significantly lower in com

Fig. 9. Changes of pH during the second ED step at pH 2.0.
ig. 11. Contribution of energy consumption in different phases durinecond ED step at pH 2.0.


Fig. 12. Behavior of current efficiency and water transfer during the secondED step at pH 2.0.

step, these behaviors are similar, since the water transport isproportional to the current density. The average water trans-port was 1.062 kg m−2 h−1. This results in the decrease ofthe diluate volume (about 10%). Since water is transportedinto concentrate and lactic acid remains in the diluate, thesecond ED step can be considered as another small concen-tration step for lactic acid. However, the starting solution wasdiluted (factor 1.05) by using concentrated hydrochloric acidto adjust pH. Therefore, lactic acid is not concentrated inpractice.

In the second ED step, mainly inorganic salts but not lac-tate were transported. Therefore, the transport ratio of wa-ter/salts can be calculated to be 2.2 g g−1. If this result isconsidered with regard to the first ED step, the transport ratioof water/lactate could be calculated as 4.2 g g−1. Therefore,the maximum theoretically achievable lactate concentrationafter the first ED step would be expected to be 238 g L−1 ifthe solution contains no salts. Based on the fact that differentmembranes and process conditions were applied, it can beseen that this result fits quite well to the result of many otherworks[20–22]. Bailly et al. also studied about concentrationof lactate by conventional ED, in which the maximum the-oretically achievable concentration of lactate was 280 g L−1

[21,22].Fig. 13shows the separation behaviors during the desali-

n tep( ans-f lenti than9 masstvfi e isi EDs f thes nd 2,t ively.S illh

Fig. 13. Effect of time on the removal of inorganic ions during the secondED step at pH 2.0.

3.4. Performance of whole two-stage electrodialysis

The summary of separation performance in two steps isshown inTable 5. The obtained data show that a great part oftotal energy consumption lays on the first ED separation step,in which both the general energy consumption and specificenergy consumption for lactic acid are approximately twotimes higher than the second step. The total specific energyconsumption for lactic acid production in the two-stage elec-trodialysis process is about 3.19 kW h kg−1, regardless of theenergy used for pumping.

The current efficiency during the first step is also lowerthan that of the second step. However, the current efficiencyof the whole two-stage process was relatively high, sincethe current efficiency in the second ED step (at pH 2.0) wassignificantly higher than that in the first ED step (at pH 6.7).

The recovery of lactic acid for whole two-stage processwas in the range of 87–91%. At the end of the second EDstep, the desalination level can reach to 100% but the lossof lactic acid will rapidly increase too. Therefore, the stop-ping moment is of great importance. A desalination level of97.5% with 1.1% loss of lactic acid the end of phase 1 (at80 min) can be acceptable. The total inorganic ions in thelactic acid solution were 0.88 g L−1. In addition, amino acidsand sugars remained in the lactic acid solution was 1.5 and0 −1 pu-r tions

ation step at pH 2.0. A behavior similar to the first ED sat pH 6.7) can be found. Monovalent ions were easily trerred, as much as 100% in a very short time, while divaons were more difficultly transferred, as expected. More5% of these ions were removed in phase 1. The average

ransfer of inorganic salts was 0.490 kg m−2 h−1, which wasery high in comparison with that of 0.091 kg m−2 h−1 in therst ED step (at pH 6.7). Even if the transport of lactatncluded, the specific mass transfer of ions in the firsttep is, as mentioned, still 2.7 times smaller than that oecond ED step (at pH 2.0). At the end of phases 1 ahe desalination levels were 97.5% and 98.8%, respectince it is difficult to remove all salts, the final product wave some impurities, such as Na2SO4 and K2SO4.

.92 g L , respectively. These compounds resulted theity of lactic acid as 90%. Therefore, the further purificatep is required.


Table 5Summarized comparison of lactic acid separation from silage juice using two-stage electrodialysis

Parameter ED steps Whole two-stage process

First Second

Current efficiency (%) 67 88 75a

Specific energy consumption for lactic acid (kW h kg−1) 2.11 1.08 3.19General energy consumption for the juice (kW h L−1) 0.074 0.037 0.111Lactic acid recovered (%) 93 94–98 87–91Average mass transferb (kg m−2 h−1) 0.165 0.490 –Sugar separation (%) 97 – 97Amino acid separation (%) 90.1 33.7 93.5Desalination level (%) – 98 98

a Calculation bases on the formula(2).b Calculation bases on the transport of inorganic ions and lactate together.

One of the important advantages of ED is that the sepa-ration and concentration is carried out simultaneously. Themaximum final concentration of lactic acid, which can beachieved on the basis of the above data, is 14.5%. High inor-ganic salt content prevent the achievement of a higher finalconcentration in concentrate.

4. Conclusions

As can be seen from the present application (lactic acidseparation from silage juice), the feed solutions can possessa relatively high concentration of salts. The question arises,namely, which method can be used for desalting? Taking intoconsideration three key criteria: technique, economy and en-vironmental impact, it seems that only ED and possibly, pro-cess chromatography are methods that can be used in prac-tice. Especially, looking into sugar industry, process chro-matography has been successfully applied. In this applica-tion, molasses—a complex feed—is used too. Therefore, itcan be expected that process chromatography could be ap-plied for silage juice.

Besides, it is difficult to find a technology in which lacticacid can be separated from silage juice. Most reported worksdeal with fermentation broths (low sugar and salt contents)a ction,e ata nge,e alts isd ultt romt

ms.O ferredt ump-t us-i cingp thev aset s thed con-

sidered in connection with the product yield and purity. Inaddition, economical questions have to be answered.

Nevertheless, ED might play an important role for theseparation of valuable substances form complex mixtures.Independent from that, this work shows the usefulness toinvestigate an ED process more in detail. Beside the scientificdata, the economy of the process can be improved.

Acknowledgment

The authors wish to thankOAD (OsterreichischerAustauschsdienst—Austrian Academic Exchange Service)for the main supports during this work.

References

[1] M. Andersen, P. Kiel, Integrated utilization of green biomass in thegreen biorefinery, Ind. Crops Prod. 11 (2000) 129–137.

[2] S. Kromus, Grune BioraffinerieOsterreich—Entwicklung eines inte-grierten Systems zur Nutzung von Grunlandbiomasse, Ph.D. thesis,Graz University of Technology, Austria, 2002.

[3] H. Kawate, K. Tsuzura, H. Shimizu, Ion exchange membranes, in:K. Dorfner (Ed.), Ion Exchangers, Walter de Gruyter, New York,1991, pp. 595–656.

cidsess,

cidalina-

ery

ropi-

un,our.

cidiol.

[ ationdial-

nd the investigated techniques like ion exchange, extratc. can hardly fulfill the above criteria. It is well known thhigh salt content leads to big problems for ion excha

.g. resins are soon exhausted or a great amount of sischarged. In case of extraction, it will be extremely diffic

o find out a high selective solvent to extract lactic acid fhis juice.

Of course, ED also cannot solve completely all problene disadvantage lays on the fact that the salts are trans

wo times. This causes, e.g. an increase of energy consion for the whole process. A pretreatment of silage juiceng nanofiltration can improve the situation, e.g. by reduartially the inorganic salt content. The optimization ofoltage or current applied during the separation will increhe efficiency. The most important parameter, however, iuration of the separation process itself, which has to be

[4] S. Novalic, T. Kongbangkerd, K.D. Kulbe, Recovery of organic awith high molecular weight using a combined electrodialytic procJ. Membr. Sci. 166 (2000) 99–104.

[5] S. Novalic, J. Okwor, K.D. Kulbe, The characteristics of citric aseparation using electrodialysis with bipolar membranes, Destion 105 (1996) 277–282.

[6] M. Moresi, F. Sappino, Economic feasibility study of citrate recovby electrodialysis, J. Food Eng. 35 (1998) 75–90.

[7] S.T. Zhang, H. Matsuoka, K. Toda, Production and recovery of ponic and acetic acids in electrodialysis culture ofPropionibacteriumshermanii, J. Ferment. Bioeng. 75 (1993) 276–282.

[8] H. Danner, L. Madzingaidzo, M. Holzer, L. Mayrhuber, R. BraExtraction and purification of lactic acid from silage, BioresTechnol. 75 (2000) 181–187.

[9] M. Hongo, Y. Nomura, M. Iwahara, Novel method of lactic aproduction by electrodialysis fermentation, Appl. Environ. Microb52 (1986) 314–319.

10] P. Boyaval, C. Corre, S. Terre, Continuous lactic acid fermentwith concentrated product recovery by ultrafiltration and electroysis, Biotech. Lett. 9 (1987) 207–212.


[11] D.A. Glassner, R. Datta, Process for production and purification oflactic acid, EP 0393818 A1, 1990.

[12] Y.H. Yen, M. Cheryan, Electrodialysis of model lactic acid solutions,J. Food Eng. 20 (1993) 267–282.

[13] V. Heriban, J. Skara, E. Sturdik, J. Ilavsky, Isolation of free lacticacid using electrodialysis, Biotech. Technol. 7 (1993) 63–68.

[14] R. Datta, S.P. Tsai, P. Bonsignore, S.H. Moon, J.R. Frank, Techno-logical and economic potential of poly (lactic acid) and lactic acidderivatives, FEMS Microbiol. Rev. 16 (1995) 221–231.

[15] E.G. Lee, S.H. Moon, Y.K. Chang, I.K. Yoo, H.N. Chang, Lacticacid recovery using two-stage electrodialysis and its modelling, J.Membr.Sci. 145 (1998) 53–66.

[16] M.C.M. Cockrem, P.D. Johnson, Recovery of lactate esters and lacticacid from fermentation broth, US Patent 5210296, 1993.

[17] V.H. Thang, W. Koschuh, K.D. Kulbe, S. Kromus, C. Krotscheck,S. Novalin, Desalination of high salt content mixture by two-stageelectrodialysis as the first step of separating valuable substances fromgrass silage, Desalination 162 (2004) 343–353.

[18] http://phychem.kjist.ac.kr/312.pdf.[19] D.A. Cowan, J.H. Brown, Effect of turbulence on limiting current

in electrodialysis cell, Ind. Eng. Chem. 51 (12) (1959) 1445.[20] C. Nolasco-Hipolito, V.H. Thang, G. Kobayashi, K. Sonomoto, A.

Ishizaki, Lactic acid recovery from model solutions and fermenta-tion broth by electrodialysis., J. Fac. Agr., Kyushu Univ. 45 (2001)531–540.

[21] M. Bailly, Strategie de dimensionnement de procedes de pro-duction d’acides organiques integrant des etapes electromem-branaires, Ph.D. thesis, Universite Paul Sabatier, Toulouse, France,2000.

[22] M. Bailly, H. Roux-de Balmann, P. Aimar, F. Lutin, M. Cheryan, Pro-duction processes of fermented organic acids targeted around mem-brane operations: design of the concentration step by conventionalelectrodialysis, J. Membr. Sci. 191 (2001) 129–142.

[23] J.H. Choi, S.H. Kim, S.H. Moon, Recovery of lactic acid fromsodium lactate by ion substitution using ion-exchange membrane,Separ. Purif. Technol. 28 (2002) 69–79.

http://phychem.kjist.ac.kr/312.pdf

Detailed investigation of an electrodialytic process during the separation of lactic acid from a...

Documents

Transcript of Detailed investigation of an electrodialytic process during the separation of lactic acid from a...