Lactic Acid Extraction by Means of Long Chain Tertiary Amines: A

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Lactic Acid Extraction by Means of Long Chain Tertiary Amines: A Comparative Theoretical and Experimental Study G. Kyuchoukov and D. Yankov* Institute of Chemical Engineering, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Building 103, 1113 Sofia, Bulgaria ABSTRACT: The extraction of lactic acid with different long chain (from C8 to C12) tertiary amines was studied, both theoretically and experimentally. In view to determine the concentration of free acid and bounded with extractant acid in the aqueous phase, the experimental results were treated by mass balance equations. The minimal and maximal fractions of the free acid and bounded acid were calculated for different compositions of the organic phase. The obtained results clearly showed that in general the interaction product concentration in the aqueous phase could not be neglected and, in some cases, reached more than 50% from the total acid concentration. The disparate influences of inert and active diluents (dodecane and oleyl alcohol, respectively) were elucidated. It was shown that the overall distribution coefficient depends not only on the extraction product concentration in the aqueous phase but also on the composition of the organic phase.

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efficiencies of the tertiary amines. Han et al.7 and Jung et al.30 studied the extraction of lactic acid with tertiary amines with different chain lengths (from tributylamine to trioctylamine (TOA)) in different diluents. They mentioned the importance of the organic phase composition for successful extraction. While Han et al. found TOA to be the most effective extractant, Jung et al. suggested the use of trihexylamine in isobutyl alcohol. To the best of our knowledge, there are no investigations comparing lactic acid extractions with tertiary alkylamines with chain lengths longer than eight. These alkylamines are known to possess lower water solubility and lower toxicity toward microorganisms than short chain ones. The aim of the present paper is to study both theoretically and experimentally the extraction of lactic acid by means of long chain tertiary amines.

INTRODUCTION In recent years, long chain aliphatic tertiary amines have been widely used for extraction of organic acids such as lactic, citric, propionic, and acetic.1−6 Amine extraction is recognized as a very promising method for separation of organic acids from fermentation broth with low product concentration. The product separation is obligatory because of its inhibitory effect and pH drop of the medium as a result of acid formation. Lactic acid (2-hydroxypropionic acid) fermentation is typical example of such end-product inhibited process. Lactic acid finds wide use in the food industry (as a food additive and preservative), in the pharmaceutical industry, for the production of biodegradable polymers, or for environmentally friendly solvents (lactate esters). Various tertiary amines, dissolved in different diluents, were used for the extraction of lactic acid from aqueous solutions or fermentation broth. Among amines, tri-n-octylamine,1,7−12 tri-n-decylamine,13,14 Alamine 336 (a mixture of tri-n-octylamine and tri-n-decylamine),15−21 and triisooctylamine,12,22 are the most often used. Frequently applied diluents are octanol,1,2,7−9,11,12,23 decanol,2,7,11,12 oleyl alcohol,13,14,17−20,22 methyl isobutyl ketone,1,7−9,21 hexane,7,27 toluene,1,15,16 and paraffin oil.8,9 In order to improve the extraction of lactic acid, attempts have been made by using mixed extractants. Mixtures of two amines,23 trioctylamine, and di(2-ethylhexyl)phosphoric acid (D2EHPA),10 as well as trioctylamine and quaternary ammonium salt,24−26 were tested. Matsumoto et al.27 and Zakhodyayeva et al.28 studied lactic acid extraction with a mixture of alkylamine and tributyl phosphate. Recently Yamamoto et al.29 published a paper in which the authors compared lactic acid extraction with di- and trioctylamines dissolved in nonpolar diluents. Better results are reported with dioctylamine dissolved in decane, and addition of a polar modifier is necessary for improving of the extraction ability of trioctylamine. The majority of the papers were devoted to the influence of diluent type (polar or nonpolar) or some process parameters, and little attention was paid to the comparison of extraction © 2012 American Chemical Society

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THEORY Our experimental results were treated with the presumption of chemical interaction between an amine and lactic acid according to E̅ + HA ↔ E:HA

(for hydrogen‐bond formation) (1)

E̅ + H+ + A− ↔ EH+A−

(for ion‐pair formation) (2)

where E is the extractant, H+ is the hydrogen ion, A− is the anion of monoacid, HA is the monoorganic acid, and E:HA and EH+A− are the interaction products with hydrogen-bond and ion-pair formation, respectively. The overbars denote the species in the organic phase. The apparent equilibrium constants (in terms of species concentrations) for both mechanisms are as follows: Received: Revised: Accepted: Published: 9117

March 1, 2012 May 21, 2012 June 10, 2012 June 10, 2012 dx.doi.org/10.1021/ie3005463 | Ind. Eng. Chem. Res. 2012, 51, 9117−9122

Industrial & Engineering Chemistry Research K(E:HA) =

[E:HA] [E][HA] ̅

Article

If the interaction products cannot be neglected, and in the absence of other cations except H+ and EH+, the material balance for monocarboxylic acid will be

(3)

for eq 1 and + −

[HA] + [A−] + [EH+A−] + [E:HA] = CHA

+ −

[EH A ] [EH A ] K(EH+A−) = + − = [E][H ][A ] [E]̅ K a[HA] ̅

Taking into account that in this case

(4)

[A−] = [H+] + [EH+]

for eq 2, where KE:HA is the extraction constant for hydrogenbond formation, K(EH+A−) is the extraction constant for ion-pair

[HA] =

formation, Ka is the dissociation constant of extracted monocarboxylic acid, and square brackets express molar

m(E:HA) [E:HA] = (1 − α)[CHA] 1−α

m(EH+A−) [EH+A−] = (1 − α)[CHA] 1−α

+ [E:HA] = CHA

(5)

(6)

m = m(E:HA) + m(EH+A−) (7)

where α is the molar fraction of dissociated molecules, CHA is the total equilibrium acid concentration in the aqueous phase, m is the overall distribution coefficient of the acid, given by the ratio between the total acid concentration in the organic phase and its total (analytical) concentration in the aqueous phase, and m(E:HA) and m(EH+A−) are the partial distribution coefficients of the interaction products that represent the ratio between the concentration of the corresponding interaction product in the organic phase and the total acid concentration in the aqueous phase. The concentration of interaction products in the aqueous phase in general is not taken into account, but is this is correct? In reality the molecules of species in one two-phase system are always in equilibrium. This means that all existing species in the organic phase exist also in the aqueous phase and the ratio between them is constant. Consequently, the existing molecules and ions in the aqueous phase in this case are HA, H+, A−, EH+A−, E:HA, EH+, and E, where EH+ is a cation resulting from the interaction between the basic extractant and the hydrogen ion in the aqueous phase: E + H+ ↔ EH+

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MATERIALS AND METHODS The working solutions of lactic acid were prepared from 85% L(+)-lactic acid (Aldrich). Because of the presence of acid dimers in the concentrated lactic acid solutions (about 25% of the total concentration), 10-fold diluted solution was boiled under reflux 8−10 h for dimer hydrolysis. The resulting solution, containing 100−120 g L−1 lactic acid, was used for preparation of aqueous phases, containing about 10 g L−1 lactic acid for the extraction studies. Different long chain tertiary amines were used as extractants. Alamine 304 (tridodecylamine), Alamine 308 (triisooctylamine), Alamine 310 (triisodecylamine), and Alamine 336 (tri-(C8C10)-amine) were from Henkel, while tri-noctylamine (95%) and tri-n-dodecylamine (85%) were from Fluka. Before further use all extractants were washed three times with distilled water. The organic phase was composed of extractant dissolved in dodecane (99%, Aldrich) and/or oleyl alcohol (65%, Aldrich). The experiments were carried out in 125 mL separatory funnels. Equal volumes (25 mL) of aqueous phase containing lactic acid and organic phase were shaken for

(8)

with the equilibrium constant K(EH+) =

[EH+] [E][H+]

(9)

If the interaction products do not exist in the aqueous phase or can be neglected, and in the absence of other cations, the measured pH must be near the calculated one for the given equilibrium total acid concentration (CHA) and its value can be calculated from the measured pH value according the following equation: [H+]2 + K a[H+] − CHA·K a = 0

(14)

Equation 14 clearly shows why the measured pH does not correspond to the calculated (theoretical) one (see eq 10) according to the measured total acid concentration in the aqueous phase when basic extractant is used. The reason is that one part of hydrogen is bounded with extractant (EH+ + EH+A− + E:HA) and the measured hydrogen ion which corresponds to the free acid concentration decreases. From eq 14 we cannot calculate the exact values of [EH+], [EH+A−], and [E:HA], but it gives a possibility of determining the minimal and maximal values of free (unbounded with extractant) monocarboxylic acid in the aqueous phase. Depending on which side is shifted, for the equilibrium for every species in the aqueous phase, we can distinguish two boundary cases. If the extraction products are fully dissociated, then [EH+] ≫ [EH+A−] + [E:HA] and [EH+A−] + [E:HA] can be neglected, and from measured values of CHA and [H+] (pH) it is possible to calculate the value of [EH+] and then the maximal possible value of the free acid CHAmax free = CHA − [EH+]. The other case is when [EH+] ≪ [EH+A−] + [E:HA] and [EH+] can be neglected (the extraction products are not dissociated). From eq 14 the sum [EH+A−] + [E:HA] can be calculated, taking into account the real concentration of [H+] and the minimal possible value of free monocarboxylic acid + − (CHAmin free ) is equal to CHA − [EH A ] − [E:HA].

Thus, finally, we obtain

= [E](1 − α)(K(E:HA) + K aK(EH+A−)) ̅

(13)

[H+]([H+] + [EH+]) + [H+] + [EH+] + [EH+A−] Ka

and from eq 4 K(EH+A−)[E]̅ K a =

[H+]([H+] + [EH+]) [H+][A−] = Ka Ka

(12)

Equation 11 transforms to

concentrations. From eq 3 we have K(E:HA)[E]̅ =

(11)

(10) 9118

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20 min at ambient temperature on the shaking machine IKA HS501 Digital (IKA Labortechnique). Our preliminary experiments showed that a maximum 5 min was sufficient time for attaining equilibrium. After phase separation the volume of the phases and the pH value of the aqueous phase were measured. Pure (98%) crystalline L-(+)-lactic acid (Sigma) was used as a standard for HPLC analyses. The lactic acid concentrations of the aqueous phase were determined by HPLC, and the corresponding concentrations in the organic phase were calculated by mass balance. The HPLC system was composed of a pump (Knauer), a refractive index detector (Perkin-Elmer), and the chromatographic software EuroChrom (Knauer). The column used was Aminex HPX-87H (Bio-Rad). As mobile phase, 0.005 M H2SO4 solution was used at a flow rate of 0.6 mL/min.

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RESULTS AND DISCUSSION As an example, in Figure 1 are shown the calculated values of max CHAmin free /CHA (curve 1) and CHAfree /CHA (curve 2) for the extraction system whose organic phase is composed of TOA and dodecane.

Figure 2. Influence of amine concentration for different amines, when the organic phase is composed of 20% v/v dodecane, amine, and OA, max on the (a) minimal (CHAmin free /CHA) and (b) maximal (CHAfree / CHA) fractions of free lactic acid in the aqueous phase.

On the basis of the results shown in Figure 2, it could be concluded that the influence of amine (OA, respectively) concentration on the free (or bounded) acid concentration depends on the amine used. For example, it is very strong for Alamine 304 and Alamine 310 and not so strong for TOA and Alamine 336. From Figure 2b it is clearly seen that even in the case of Alamine 304, where the maximal concentration of the free acid in the aqueous phase (at 40% amine) is about 80%, the minimal concentration of acid bounded with extractant (extraction product) in the aqueous phase (about 20%) cannot be neglected. The influence of the concentration of bounded acid [EH+A−] + [E:HA] on the measured and calculated values of pH is presented in Figure 3. To avoid overcrowding Figure 3, only a part of the data is shown. The presented data depict measured (points) and calculated (lines) values of pH (eq 10) in relation to amine (or oleyl alcohol) concentration at a constant concentration of dodecane (20% v/v). The data shown are for three amines (Alamine 304, 308, and 310) for which the maximal value of free acid is greater in comparison with those of the others. The measured values of the pH are greater than the calculated ones according to eq 10, supposing the absence of H+ bounded to the extractant and passing through the maximum. This maximum is attained at 40% v/v amine, where

Figure 1. Influence of TOA concentration on the minimal (CHAmin free / CHA), curve 1, and maximal (CHAmax free /CHA), curve 2, fractions of free lactic acid in the aqueous phase. The points represent the calculated values according to eq 14. The organic phase is composed of TOA, dissolved in dodecane.

From Figure 1 it is obvious that with the increase of amine concentration the concentration of free acid concentration decreases. It can be expected because with the rise of extractant concentration the concentration of acid bounded with amine also increases. As explained above, the curves in Figure 1 show the particulate cases, but in general, the real curve would be situated between the two curves with the same tendency. Another shape of curves was observed in the presence of active diluent (oleyl alcohol, OA) for all studied amines at a constant concentration of dodecane (20% v/v), as shown in Figure 2. They pass through a minimum at 40% (v/v) amine (or at the volume concentration of OA equal to 40% v/v). This means that OA also influences the concentration of the acid bounded with amine, probably by changing the amine basicity. The minimal and maximal values for bounded acid are not max shown in Figure 2 because CHAmin bound/CHA = 1 − CHAfree / max min CHA, CHAbound/CHA = 1 − CHAfree /CHA, and the curves will pass through a maximum. 9119

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and Alamine 310 the maximum is at about 35−40% v/v amine, for Alamine 308 and TDA it is at 30% v/v amine, and for TOA and Alamine 336 it is around 25% v/v amine. Obviously the role of active diluent is not only to increase the basicity of amine; it also changes the solubility of the extraction product in the organic phase by solvation or as a result of different compositions of amine:acid:diluent complexes formed. A general conclusion about the extraction ability of different amines to lactic acid cannot be drawn because it depends on the composition of the organic phase. Consequently, the comparison must be made for the same concentration of the amine and diluents, or the maximal values of the overall distribution coefficient (m) at different compositions of the organic phase must be compared. For the studied amines the range of m at constant composition of the organic phase (20% v/v dodecane, 40% v/v amine, and 40% v/v OA) is Alamine 336 > TOA> Alamine 308 > TDA ≈ Alamine 304 ≈ Alamine 310. For us it was interesting to see the influence of inert and active diluents at a constant (for each case) concentration of amine. Three amines were used with a concentration which corresponds to the maximal value of m (40% v/v Alamine 304, 30% v/v Alamine 308, and 25% v/v TOA). The obtained results are shown in Figure 5. The strong positive influence of

Figure 3. Comparison between calculated (lines) and measured (points) values of pH in dependence on amine concentration. The organic phase composition is as in Figure 2.

the free lactic acid in the aqueous phase is at a minimum (see Figure 2) and bounded acid is at a maximum and consequently has the greatest influence on the measured value of pH. From Figure 3 it is also seen that the measured initial pH values of lactic acid solutions, before extraction (in the absence of extractant) fit very well the calculated ones. Figure 4 depicts the dependence of the overall distribution coefficient (m) on the amine (OA, respectively) concentration,

Figure 4. Influence of amine concentration on the overall distribution coefficient for lactic acid extraction with different amines. The composition of organic phase is as in Figure 2.

at a constant concentration of dodecane (20% v/v) in the organic phase. The overall distribution coefficient for all studied amines passes through a maximum with the change of amine concentration (or OA, respectively). This phenomenon was observed for the dependence of the fraction of acid bounded with amine in the aqueous phase and it influence the overall distribution coefficient (m), but there is an essential difference. The maximum changes its place in dependence on the studied amine. With the increase of the maximal value of m for a given amine, the maximum moves to a lower concentration of amine (higher concentration of OA). For example, for Alamine 304

Figure 5. Influence of (a) active and (b) inert diluents on the overall distribution coefficient (m) at constant amine concentration. 9120

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interesting conclusion can be drown. The increasing range of m for the amine when the organic phase consists only of an amine turns when the organic phase is composed of 20% v/v dodecane, OA, and amine, where m reaches its maximum. In addition, for amines Alamine 310 and Alamine 304, the influence of active diluent (OA) on the minimal value of free lactic acid (maximal value of interaction product, respectively) leads to lower m values compared to amines TOA and Alamine 308, where this influence is not so expressed. For Alamine 310 and Alamine 304 the maximal value of m coincides with the maximal value of the interaction product. For TOA, Alamine 308, and TDA, the maximal value of m is moved to the lower concentration of amine or to the higher concentration of active diluent, respectively. This means that the active diluent influences not only the basicity of amine; it also takes part in the formation of different complexes in the organic phase, thus changing the extraction ability of the extractant.

active diluent (OA) on m is obvious. The obtained maximal values of m are higher than in Figure 4. In addition (see Figure 5a), at 50% OA the range of m is Alamine 304 > Alamine 308 ≈ TOA due to the different concentrations of amine: 40, 30, and 25% v/v, respectively. At the same constant concentration of amines, when the concentration of dodecane increases, the overall distribution coefficient (m) decreases (see Figure 5b) because the concentration of active diluent (OA) decreases. The curve of amine with lower concentration (25% v/v TOA) is situated above the others, and the curve for the amine with higher concentration is situated below the others. Of course the maximal values of the overall distribution coefficient (m) could be reached when the organic phase is composed only of amine and active diluent, as shown in Figure 6. The presence of the maximum once again elucidates the

important role of the active diluent. The inconvenience of this case is that the separation of the two-phase system is very slow. Our curiosity led us to compare the overall distribution coefficient (m) when the organic phase was composed only of amine with the maximal overall distribution coefficient (m) when the organic phase consisted of 20% v/v dodecane, amine, and OA. The data are shown in Table 1. The amines are arranged in increasing range according to their overall distribution coefficients (m). Among all the studied amines only Alamine 336 is a mixture of two amines (TOA + tridecylamine (2:1)). As mentioned above, at 20% v/v dodecane, 40% v/v amine, and 40% v/v OA, there is a maximum in the value of m; consequently, a synergic effect occurs. If we remove Alamine 336 from Table 1, a very Table 1. Values of the Overall Distribution Coefficient (m) in Relation to Composition of the Organic Phase

TOA Alamine TDA Alamine Alamine Alamine

308 336 304 310

m 0.23 0.35 0.38 0.76 0.80 0.81

amine with diluents Alamine Alamine TDA Alamine Alamine TOA

310 304 308 336

amine (% v/v)

m

40 40 30 30 20 25

2.98 3.11 3.17 3.32 3.79 3.93

CONCLUSIONS

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AUTHOR INFORMATION

This study and the treatment of the experimental data concerning the extraction of lactic acid by different amines and compositions of organic phase permit the following conclusions: • The extraction ability of the studied long chain tertiary amines for lactic acid depends on the composition of the organic phase and passes through the maximum. At constant amine concentration the overall distribution coefficient increases with the rise of active diluent concentration and decreases with the increase of inert diluent concentration. • The concentration of the extraction product in the aqueous phase depends on the composition of the organic phase and cannot be neglected. In our experiments the calculated concentration of the extraction product in the aqueous phase in some cases reaches more than 50%. • The overall distribution coefficient depends not only on the concentration of the extraction product in the aqueous phase but also on the concentration of the active diluent. • In the case of lactic acid extraction with pure amines, the distribution coefficient increase in the order Alamine 310 > Alamine 304 > TDA > TOA. When the extraction system was composed of diluents (dodecane and oleyl alcohol) and amine (at a concentration with a maximal overall distribution coefficient), the values of the distribution coefficient decreased in the same order: Alamine 310 < Alamine 304 < TDA < TOA.

Figure 6. Influence of TOA concentration on the overall distribution coefficient (m) in the case of lactic acid extraction when the organic phase is composed of TOA and OA.

amine only

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Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by the National Science Fund (Ministry of Education, Youth and Science, Bulgaria) under Grant Bg-Sk-211/2009. 9121

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