Ind. Eng. Chem. Res. 2001, 40, 5635-5639
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SEPARATIONS Extraction of Lactic Acid by Means of a Mixed Extractant George Kyuchoukov,*,† Maria Marinova,† Jacques Molinier,‡ Joel Albet,‡ and Guy Malmary‡ Institute of Chemical Engineering, Bulgarian Academy of Sciences, Acad. G. Bonchev str. Bl. 103, Sofia 1113, Bulgaria, and Ecole Nationale Superieure des Inge´ nieurs en Arts Chimiques et Technologiques, Equipe Ge´ nie Chimique, Laboratoire de Chimie Agro-Industrielle, Unite´ associe´ e a` l’INRA No. 31A1010, Institut National Polytechnique de Toulouse, 118 route de Narbonne, 31077 Toulouse, France
Equilibrium studies on the extraction of lactic acid by means of a mixed extractant (a mixture of tri-n-octylamine and Aliquat 336 in decanol) are carried out. Different ratios between the two extractants are investigated, and the obtained results are compared with those using the individual extractants. The effect of the initial pH of the aqueous solution on the extraction extent is followed in a large interval. The equilibrium pH values are also measured. The obtained results give grounds to conclude that the mixture of a tertiary amine and a quaternary ammonium salt is more efficient than the individual extractants. This makes it particularly appropriate for the recovery of lactic acid from fermentation broth. Introduction The increasing interest in the use of extraction processes as an alternative of precipitation processes for the recovery of organic acids from aqueous solutions may be related to the relatively low concentration of the organic acids in the side products of sugar plants and wineries. To cover the broad application field of organic acids in food and chemical industries, a number of them are obtained by bioprocessing of natural products. The most successfully developed macrofermentation processes are those for citric and lactic acids where no serious competing synthetic sources are available.1 From the numerous patented methods proposed for the extraction recovery of citric and lactic acids from fermentation broths, only the extraction of citric acid by a tertiary amine extractant and back extraction with hot water has found practical application so far. The recovery of the organic acids is usually performed through reactive extraction, i.e., through a chemical reaction between the extractant and the organic acid. Basic amines or anion-exchanging quaternary ammonium salts are most often used as extractants. Neutral extractants with oxygen-containing polar groups such as ketones (e.g., methyl isobutyl ketone), alkyl sulfoxides, or esters (e.g., tri-n-butyl phosphate and trioctylphosphine oxide), reacting with the organic acids by a solvation mechanism, are also used. In most studies on the extraction of organic acids, attention is paid to the type of extractant, the effect of its concentration, the roles of the diluent, and the modifier.1-8 The latter is added when a diluent inert with respect to the extractant is used to prevent the * To whom correspondence should be addressed. Phone: (359(2)) 720230. Fax: (359(2)) 707523. E-mail:
[email protected]. † Bulgarian Academy of Sciences. ‡ Institut National Polytechnique de Toulouse.
formation of a second organic phase and to increase the basicity of the extractant. The effects of the composition of the aqueous phase, pH, and temperature are also studied.4-6,8-11 The mechanism of the extraction process is discussed, and mathematical models for its description are developed.11-14 Tertiary amines with a long hydrocarbon chain are most often used as extractants, because they are hardly soluble in aqueous media. With amines and solvating extractants, the extraction of the organic acid decreases upon elevation of the pH of the aqueous phase at fixed concentrations of the free extractant and the organic acid.3,5,6,11,17 With quaternary ammonium salts, which are typical anion-exchanging extractants, the increase in the pH favors the dissociation of the acid, and the anionic form is extracted by the ammonium salt. Therefore, extraction with a tertiary amine or a solvating extractant is more appropriate at low pH values of the aqueous solution, whereas that with a quaternary ammonium salt is appropriate at high pH values. Of paramount importance in the production of lactic acid from fermentation broths is the pH of the medium to be maintained within the range 5-6.2 The increase in the concentration of lactic acid obtained by the fermentation process leads, however, to a decrease in the pH of the medium and, correspondingly, to an inhibition of the fermentation process. According to the classical method, the accumulated lactic acid is removed as calcium lactate by precise dosation of calcium hydroxide, taking care to keep the pH within the optimum range.3,16 When a solvating extractant or amine is used for the removal of the accumulated lactic acid, the same effect is observed. The decrease in the concentration of lactic acid leads to a corresponding pH increase. Higher pH values of the medium do not favor the performance of these types of extractants. Under these conditions, the quaternary ammonium salt reacting with the lactate
10.1021/ie010137d CCC: $20.00 © 2001 American Chemical Society Published on Web 11/07/2001
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anions obtained by the almost complete dissociation of lactic acid at pH 5-6 is more appropriate. As a result of this extraction process, the pH of the medium decreases because of the hydrogen ions remaining in the solution, while the lactate ions are replaced by the anions of the corresponding quaternary ammonium salt (chloride anions for Aliquat 336). The purpose of the present work is to study the extraction of lactic acid by a mixed extractant consisting of a tertiary amine and a quaternary ammonium salt. It is expected that the mixed extractant should manifest better extraction properties than its individual constituents in a broad pH range.
index in ) initial solution, and index eq ) solution after reaching equilibrium.
Experimental Section
where R3N and HA represent the tertiary amine and monobasic organic acid, respectively. Which mechanism will dominate depends on the pH of the aqueous solution, the pKa of HA, and the basicity of the amine with respect to HA. For both mechanisms, an increase in the pH shifts the equilibrium to the left, and as was shown in our former work,17 neither from the theoretical dependences nor from the experimental results may a conclusion about the type of extraction mechanism be drawn. When a quaternary ammonium salt is used for extraction, anion exchange takes place upon its equilibration with an aqueous solution of an acid or its salt:
Materials. The aqueous model medium was prepared by dissolving 98% L-(+)-lactic acid (Sigma Aldrich) in distilled water. The organic phase was a solution of the extractant in n-dodecane (Sigma Aldrich) as the diluent and n-decanol (Sigma Aldrich) as the modifier. As constituents of the extractant mixture, the tertiary amine tri-n-octylamine (TOA) and the quaternary ammonium salt Aliquat 336 [tri(C8C10)methylammonium chloride], both products of Acros Organics, were used. For stripping aqueous solutions, 1 M NaOH (Sigma Aldrich) and 1 M NaCl (Sigma Aldrich) were used. The extent of stripping was over 95%. When 0.5 M Na2(CO3) was used, the extent of stripping was over 90%. Procedure. The experiments were carried out in 125 mL separatory funnels. A total of 50 mL of the aqueous L-(+)-lactic acid solution and 50 mL of the organic phase were shaken for 15 min at ambient temperature on the shaking machine AGITELEC (J. Toulemonde & CIE). The mixing time was sufficient to attain equilibrium. This is shown in Figure 6. After phase separation, the pH of the aqueous phase was measured with the WTW microprocessor pH meter and the lactic acid concentration was determined by high-performance liquid chromatography (HPLC) using a column for organic acid analysis Aminex HPX-87H (Bio-Rad), 0.005 M H2SO4 as a mobile phase, and a Spectra 100 UV-vis detector (Spectra Physics) at 210 nm wavelength. The initial concentration of lactic acid and the pH of the aqueous phase were also determined. The concentration of lactic acid in the organic phase was calculated by mass balance. The pH of the initial aqueous solution was adjusted with appropriate concentrations of NaOH or H2SO4. Definition of the Characteristic Parameters. The extraction is represented by the extent of extraction E (%) and the distribution coefficient m:
E)
Vincin - Veqceq × 100 Vincin
Theory The interaction between an organic acid and a tertiary amine is effected through H bonding of undissociated acid molecules:
R3N + HA T R3N-HA
(4)
or by ion-pair formation:
R3N + H+ + A- T R3NH+A-
R4NH+X- + A- T R4NH+A- + X-
(5)
(6)
where R4N+X- represents the quaternary ammonium salt. The anions of the organic acid are replaced with anions of the quaternary ammonium salt. When HA is a much weaker acid than HX (pKHX , pKHA), the pH decreases, because
KHA )
[H+][A-] [HA]
(7)
and a decrease in A- leads to an increase in H+ as a result of HA dissociation. Ensuing from the dependences (4)-(7), it may be assumed that upon the simultaneous presence of an amine and a quaternary ammonium salt the extraction of the organic acid by the one extractant should favor its extraction by the other one. For example, the extraction of the acid by the amine at pH < pKHA leads to an increase in the pH according to eqs 4 and 5, thus creating favorable conditions for the extraction of the acid by the quaternary ammonium salt according to eq 6, and vice versa, the interaction between R4N+X- and A- leads to a decrease in the pH and creates favorable conditions for the interactions (4) and (5) to take place.
(1) Results and Discussion
ceq Vincin - Veqceq ) m) ceq V h eqceq
(2)
c ) [HL] + [L-]
(3)
where an overbar refers to the organic phase, V ) volume of the phase, c ) total concentration of lactic acid, [HL] ) concentration of undissociated molecules of lactic acid, [L-] ) concentration of lactate anions,
The efficiency of the mixed extractant was compared to that of 30% (v/v) solutions of the separate constituents: TOA and Aliquat 336 in dodecane with modifier decanol. The studies on the extraction of lactic acid of an initial concentration of approximately 8 g/L and pHin ) 2.62-2.68 revealed the strong effect of the modifier on the distribution coefficient. In the absence of the modifier [30% (v/v) TOA in dodecane], the distribution coefficient m ) 0.119 (pHeq ) 2.81); using the modifier as a diluent [30% (v/v) TOA in decanol], the distribution coefficient increases with 2 orders of
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Figure 1. Effect of the composition of the mixed extractant [30% (v/v) TOA and Aliquat 336 in decanol] on the distribution coefficient m at initial pH ) 2.68 and a concentration of lactic acid in the aqueous phase of 8.612 g/L: concentration of Aliquat 336, x; concentration of TOA, 30 - x.
magnitude (m ) 25.48 and pHeq ) 4.02). The effect of decanol on the extraction capacity of Aliquat 336 is insignificant: the values of the distribution coefficient with 30% (v/v) Aliquat 336 in dodecane or in decanol are m ) 0.489 (pHeq ) 2.45) and m ) 0.650 (pHeq ) 2.39), respectively. Because TOA was the more efficient extractant, most of the comparative studies were performed using decanol as the diluent. Figure 1 presents the distribution coefficient of lactic acid as a function of the composition of the organic phase [different ratios between TOA and Aliquat 336 at a total content of 30% (v/v) in decanol]. The aqueous medium was a 8.61 g/L solution of lactic acid of natural pH value (2.68). The better performance of the mixed extractant is clearly seen. Irrespective of the decrease in the concentration of TOA, which is the extractant active in this region, the distribution coefficient increases, passing through a maximum at a ratio of 1:1 between the two constituents, after which it decreases. Even with 10% (v/v) TOA and 20% (v/v) Aliquat 336 in decanol, the distribution coefficient exceeds that with 30% (v/v) TOA in decanol. The same study was performed at a higher pH of the aqueous medium (pH ) 5.0, adjusted by means of NaOH). The concentration of lactic acid in the solution was 7.077 g/L. At this pH value (pH > pKHA), lactic acid (pKHA ) 3.86) exists mainly as dissociated sodium lactate which may be extracted into the organic phase by anion exchange. Only the quaternary ammonium salt (Aliquat 336) acts as an anionexchanging extractant in this case. TOA is not previously treated with acid, and the maximum amount which can be theoretically extracted by it according to eqs 4 and 5 equals the sum of the undissociated and dissociated molecules of lactic acid. The results of these studies are shown in Figure 2. They confirm the above-made theoretical assumptions. The distribution coefficient increases upon elevation of the concentration of the anion-exchanging extractant Aliquat 336 which is active under these conditions. It should be mentioned that in the presence of the two extractants the equilibrium pH value (pH ) 6.51-6.74) is higher than that in the initial solution, whereas in the absence of TOA (extraction with 30% (v/v) Aliquat
Figure 2. Effect of the composition of the mixed extractant [30% (v/v) TOA and Aliquat 336 in decanol] on the distribution coefficient m at initial pH ) 5.0 and a concentration of lactic acid in the aqueous phase of 7.077 g/L: concentration of Aliquat 336, x; concentration of TOA, 30 - x.
Figure 3. Effect of the equilibrium pH on the extent of extraction E (%). Initial lactic acid concentration in the aqueous phase ) 7.024 ( 0.837 g/L. Curve 1: (() composition of the organic phase of 15% (v/v) TOA, 15% (v/v) Aliquat 336, and 70% (v/v) decanol. Curve 2: ([) composition of the organic phase of 30% (v/v) TOA and 70% (v/v) decanol. Curve 3: (9) composition of the organic phase of 30% (v/v) Aliquat 336 and 70% (v/v) decanol.
in decanol), there is a slight decrease (pH ) 4.9). As was already stressed, this is the main advantage of the mixed extractant for the recovery of lactic acid from fermentation broths: the pH decrease by the formation of lactic acid is compensated for by the pH increase after its extraction. The efficiency of the mixed extractant was compared with that of its separate constituents in the extraction of lactic acid of initial concentration 7.024 ( 0.837 g/L in a broad pH range. The lower pH values were adjusted with solutions of sulfuric acid, and the higher values, with solutions of sodium hydroxide of appropriate concentrations. The extraction extent as a function of the pH is presented in Figure 3, and the equilibrium pH values as a function of pH of the initial solution, in Figure 4.
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Figure 4. Dependence of the equilibrium pH on the initial pH of the aqueous solution. Initial lactic acid concentration in the aqueous phase ) 7.024 ( 0.837 g/L. Curve 1: (() composition of the organic phase of 15% (v/v) TOA, 15% (v/v) Aliquat 336, and 70% (v/v) decanol. Curve 2: ([) composition of the organic phase of 30% (v/v) TOA and 70% (v/v) decanol. Curve 3: (9) composition of the organic phase of 30% (v/v) Aliquat 336 and 70% (v/v) decanol.
Irrespective of the 2 times lower concentrations of the constituents in the mixed extractant (curve 1 in Figure 3) compared with those of the individual extractants (curves 2 and 3 in Figure 3), the former offers definite advantages, mainly in regards to the extraction of lactic acid solutions of natural pH (points indicated with v) and lactic acid solutions of pH adjusted by NaOH. The differences between curves 1 and 2 (Figure 3) are more pronounced at the higher pH values. This is due to the fact that upon lowering the concentration of hydrogen ions the equilibrium in reactions (4) and (5) is shifted to the left and the extraction by the amine decreases (curve 2 in Figure 3). This favors the extraction by the quaternary ammonium salt (Aliquat 336) because the concentration of lactate anions increases at the higher pH values and the equilibrium of reaction (6) is shifted to the right. In both cases (curves 1 and 2 in Figure 4), the extraction of lactic acid leads to an increase in the pH of the initial aqueous solution. For curve 2 (Figure 4) this is due to a decrease in the concentration of lactic acid. For curve 1 (Figure 4) it is assumed that, along with lactic acid, TOA also extracts the hydrochloric acid formed by the hydrogen cations and the chloride anions replacing the exchanged lactate anions according to reaction (6):
R3N + H+Cl- T R3NH+Cl-
(8)
An evidence for the higher affinity of the amine to the strong inorganic acids is the region of curves 1 and 2 (Figure 3), where the initial solutions are acidified with sulfuric acid (to the left of the points indicated with v). The extraction extent of lactic acid sharply decreases as a result of the lower concentration of TOA active in this region, which is consumed for the extraction of sulfuric acid. The preferential extraction of sulfuric acid in the presence of tartaric acid was reported in ref 11. The present results for lactic acid confirm this conclusion. The shape of curve 3 (Figure 3), where a 30% solution of Aliquat 336 in decanol is used, is essentially different from that of curves 2 and 3 (Figure 3) because of the different extraction mechanism in this case (anion
Figure 5. Effect of the initial lactic acid concentration in the aqueous phase on the extent of extraction E (%). Curve 1: (() composition of the organic phase of 15% (v/v) TOA, 15% (v/v) Aliquat 336, and 70% (v/v) decanol. Curve 2: ([) composition of the organic phase of 30% (v/v) TOA and 70% (v/v) decanol. Curve 3: (9) composition of the organic phase of 30% (v/v) Aliquat 336 and 70% (v/v) decanol.
exchange according to eq 6). The acidification with sulfuric acid lowers the concentration of lactate anions and correspondingly their extraction extent, whereas the alkalization with NaOH favors their extraction. There is a pronounced effect of the alkalization up to pH 4.3 of the raffinate. The further pH increase does not affect the extraction of the lactate anions; there is even a slight decrease. The latter may be related to the competitive extraction of hydroxyl anions according to
R4N+Cl- + OH- T R4N+OH- + Cl-
(9)
As an evidence of reaction (9), curve 3 in Figure 4 may be considered which shows that a raffinate of pH close to 6 is obtained with initial solutions of higher pH values as well, irrespective of the slightly decreasing extraction extent in this region (see curve 3 in Figure 3). The experimental results for the extraction of lactic acid with Aliquat 336 are in accordance with those of ref 2 and are incompatible with those of ref 18 where the final aqueous concentration is above 10 g/L. Figure 5 shows the extraction extent as a function of the initial concentration of lactic acid in the range of 0.5-8 g/L (0.0055-0.088 mol/L). Here, too, the advantages of the mixed extractant [15% (v/v) Aliquat 336 and 15% (v/v) TOA in decanol, curve 1] over the 30% (v/v) TOA in decanol (curve 2) are clearly manifested. For the comparison, TOA is used because of its high efficiency in the aqueous solutions of lactic acid with natural pH used in this experiment. Curve 2 reflects the increase in the extraction extent with the concentration of lactic acid in the initial solution, which is due to the lowering of the pH of the aqueous solution upon elevation of the concentration of lactic acid. It is in good agreement with the results of Figure 3 (effect of the pH on the extraction extent for the different extraction systems). Irrespective of the higher pH values of the aqueous solutions with lower concentrations of lactic acid (0.5 and 1.0 g/L), the latter is completely extracted with the mixed extractant (curve 1, Figure 5). This is due to the high efficiency of Aliquat 336 at the high pH
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Acknowledgment M.M. gratefully acknowledges financial support from the Centre Culturel Francais in Bulgaria for her cosupervised Ph.D. Thesis. Literature Cited
Figure 6. Effect of mixing time on the extent of extraction E (%). Initial lactic acid concentration in the aqueous phase ) 7.549 ( 0.204 g/L and pH ) 2.75 ( 0.07. Curve 1: (() composition of the organic phase of 15% (v/v) TOA, 15% (v/v) Aliquat 336, and 70% (v/v) decanol. Curve 2: ([) composition of the organic phase of 30% (v/v) TOA and 70% (v/v) decanol.
values. The good performance of the mixed extractant at low concentrations of lactic acid is particularly advantageous when complete recovery of lactic acid using countercurrent extraction is aimed at. The kinetics of the process using the mixed extractant [15% (v/v) Aliquat 336 and 15% (v/v) TOA in decanol, curve 1] and 30% (v/v) TOA in decanol (curve 2) is shown in Figure 6. As can be seen, in both cases the equilibrium is reached for less that 60 s after mixing; i.e., the use of the mixed extractant does not affect the rate of the process. Conclusions The mixed extractant consisting of a tertiary amine (TOA) and a quaternary ammonium salt (Aliquat 336) is found to be more efficient than its separate constituents in the extraction of lactic acid. The extraction of the organic acid by the one extractant favors its extraction by the other one. After extraction, the aqueous phase has a higher pH value in comparison with that of the initial solution. This fact is particularly favorable for the recovery of lactic acid from fermentation broths where an optimal pH for the fermentation process should be maintained. With a view to the practical application of the system, studies on real fermentation broths should be performed, and the optimum concentrations of extractants, modifier, and diluent avoiding toxic effects, found.
(1) Kertes, A. S.; King, C. J. Extraction chemistry of fermentation product carboxylic acids. Biotechnol. Bioeng. 1986, 28, 269. (2) Lazarova, Z.; Peeva, L. Solvent extraction of lactic acid from aqueous solution. J. Biotechnol. 1994, 32, 75. (3) von Frieling, P.; Schugerl, K. Recovery of lactic acid from aqueous model solution and fermentation broths. Proc. Biochem. 1999, 34, 685. (4) Juang, R. S.; Huang, R. H. Equilibrium studies on reactive extraction of lactic acid an amine extractant. Chem. Eng. J. 1997, 65, 47. (5) Tung, L. A.; King, C. J. Sorption and extraction of lactic and succinic acids at pH > pKa1. 1. Factors governing equilibria. Ind. Eng. Chem. Res. 1994, 33, 3217. (6) Eyal, A. M.; Kanari, R. pH dependence of carboxylic and mineral acid extraction by amine-based extractants: Effects of pKa, amine basicity, and diluent properties. Ind. Eng. Chem. Res. 1995, 34, 1789. (7) Tamada, J. A.; Kertes, A. S.; King, C. J. Extraction of carboxylic acids with amine extractants. 1. Equilibria and law of mass action modeling. Ind. Eng. Chem. Res. 1990, 29, 1319. (8) Tamada, J. A.; King, C. J. Extraction of carboxylic acids with amine extractants. 2. Chemical interactions and interpretation of data. Ind. Eng. Chem. Res. 1990, 29, 1327. (9) Tamada, J. A.; King, C. J. Extraction of carboxylic acids with amine extractants. 3. Effect of temperature, water coextraction, and process considerations. Ind. Eng. Chem. Res. 1990, 29, 1333. (10) San-Martin, M.; Pazos, C.; Coca, J. Liquid-liquid extraction of lactic acid with Alamine 336. J. Chem. Technol. Biotechnol. 1996, 65, 281. (11) Tomovska, R.; Poposka, F.; Heyberger, A.; Prochazka, J. pH dependence of tartaric acid extraction. Chem. Biochem. Eng. Q. 1999, 13, 185. (12) Prochazka, J.; Heyberger, A.; Bizek, V.; Kousova, M.; Vilafova, E. Amine extraction of Hydroxycarboxylic acids. 2. Comparison of equilibria for lactic, malic, and citric acids. Ind. Eng. Chem. Res. 1994, 33, 1565. (13) Poposka, F. A.; Nikolovski, K.; Tomovska, R. Kinetics, mechanism and mathematical modeling of extraction of citric acid with isodecanol/n-paraffins solutions of trioctylamine. Chem. Eng. Sci. 1998, 53, 3227. (14) Bizek, V.; Horacek, J.; Kousova, M.; Heyberger, A.; Prochazka, J. Mathematical model of extraction of citric acid with amine. Chem. Eng. Sci. 1992, 47, 1433. (15) Yabannavar, V. M.; Wang, D. I. C. Extractive fermentation for lactic acid production. Biotechnol. Bioeng. 1991, 37, 1095. (16) San-Martin, M.; Pazos, C.; Coca, J. Reactive extraction of lactic acid with Alamine 336 in the presence of salts and lactose. J. Chem. Technol. Biotechnol. 1992, 54, 1. (17) Yankov, D.; Molinier, J.; Kyuchoukov, G. Extraction of tartaric acid by trioctylamin. Bulg. Chem. Commun. 1999, 31, 446. (18) Yang, S. T.; White, S. A.; Hsu, S. T. Extraction of carboxylic acids with tertiary and quaternary amines: Effect of pH. Ind. Eng. Chem. Res. 1991, 30, 1342.
Received for review February 12, 2001 Revised manuscript received August 7, 2001 Accepted August 9, 2001 IE010137D
