Mechanism of Lactic Acid Extraction with Quaternary Ammonium

Jun 2, 2005 - Institut National Polytechnique de Toulouse, 118 route de Narbonne, 31077 Toulouse, .... Ecole Nationale Supérieure des Ingénieurs en ...
0 downloads 0 Views 134KB Size
Download PDF
Ind. Eng. Chem. Res. 2005, 44, 5733-5739

5733

Mechanism of Lactic Acid Extraction with Quaternary Ammonium Chloride (Aliquat 336) George Kyuchoukov,*,† Dragomir Yankov,† Joe1 l Albet,‡ and Jacques Molinier‡ Institute of Chemical Engineering, Bulgarian Academy of Sciences, Academic George Bonchev Street, Building 103, 1113 Sofia, Bulgaria, and Ecole Nationale Supe´ rieure des Inge´ nieurs en Arts Chimiques et Technologiques, Equipe Ge´ nie Chimique, Laboratoire de Chimie Agro-Industrielle, UMR 1010 INRA, Institut National Polytechnique de Toulouse, 118 route de Narbonne, 31077 Toulouse, France

The extraction of lactic acid with Aliquat 336 dissolved in dodecane and decanol was investigated at various experimental conditions. To determine the concentration of extracted anions and undissociated molecules in the organic phase, the experimental results were treated by mass balance equations of lactic acid and hydrogen concentration in both phases. Thus, the ratio between the extracted anions and whole (undissociated) molecules was calculated. This ratio depends strongly on the pH and lactic acid concentration. The overall distribution coefficient rises or decreases with the pH increase in dependence on lactic acid concentration. The obtained linear dependence of the acid molecules concentration in the organic phase on their concentration in the aqueous phase is a strong argument to suppose physical extraction by Aliquat 336. Introduction Lactic acid is an important chemical with wide use in the chemical, pharmaceutical, food, and other industries. About half of the world’s lactic acid production is obtained by fermentation of different sugars by the Lactobacillus strain. The low final concentration of the acid due, on one hand, to the product inhibition and, on the other hand, to the change of pH caused by acid accumulation in the system is the major obstacle for the realization of effective industrial lactic acid production by fermentation. The traditional method of lactic acid purification, precipitation with bases, needs improvement, because it does not ensure the desired purity. The methods based on extraction of the acid with tertiary amines or quaternary ammonium salts seem to be the most promising, because they give the opportunity to realize in situ extractive fermentation. For the purpose of extractive fermentation of organic acids, a number of investigations by means of quaternary ammonium salts (Aliquat 336 and TOMAC) have been conducted.1-10 Some of them7-9 ignore the removal of the undissociated molecules and suppose only an anion exchange mechanism, where the chloride anion of the extractant is replaced with the anion of the acid. In the studies of Matsumoto et al.,7 the experiments were carried out at high pH value, low concentration of the natural acid, and high concentration of the chloride anions. These conditions give the opportunity to determinate the extraction constant and the complex formed between the extractant and removed acid. Lazarova and Peeva8 calculated the extraction constant of lactic acid, taking into account the concentration of chloride ions and the acidity of the aqueous solution (the concentration of lactate anion). In all studies when the experiments were carried out at low lactic acid concentration, the experimental results coincide very well with the * Corresponding author. Tel.: +359 2 720230. Fax: +359 2 8707523. E-mail: [email protected]. † Bulgarian Academy of Sciences. ‡ Ecole Nationale Supe´rieure des Inge´nieurs en Arts Chimiques et Technologiques.

anion exchange mechanism, i.e., the extraction increases with the rise of pH.7-9 Yang et al.11 have investigated the extraction of different organic acids with Aliquat 336 at various conditions (acid and extractant concentration, pH, and effect of diluent). They have presented large and interesting experimental results. They ascertained that the quaternary ammonium chloride extracts both dissociated and undissociated forms of acids, and with an increase of the equilibrium pH, the overall distribution coefficient decreases. Here it must be emphasized that this conclusion is drawn out when the initial acid concentration is selected to be high enough so that all final aqueous acid concentrations are above 10 g/L. Their assumption is that it is necessary to avoid the concentration effect that exists at a low acid concentration, but why this concentration effect influences the extraction is not explained or related to the mechanism of extraction. They develop a theory according to which the carboxylic acids distribute as an undissociated form with distribution coefficient K1 ) [HA]org/[HA]aq and as anions with distribution coefficient K2 ) [A-]org/[A-]aq. They accept constant distribution coefficients for a defined extraction system. In our point of view, this assumption is correct only in the case of physical extraction. When a chemical interaction takes place, the above-mentioned ratio depends on the experimental conditions, as can be seen in the theoretical and experimental part of this study. Eyal and Canari12 repeated Yang’s experiments with Aliquat 336 and propionic acid, measuring the total acid concentration. The anion exchange is determined by the analysis of chloride anions in the aqueous phase. From the obtained experimental data, they assume H-bonding of undissociated molecules to the anion of the ion-pair, forming R4N+Cl-:HA at low pH. The experimentally proved extraction of undissociated and dissociated forms of carboxylic acids poses some questions. First, which type of extraction dominates in the dependence of experimental conditions (composition of the aqueous and organic phase). Second, what is the value of the ratio between the concentration of extracted anions and whole molecules in the case of lactic acid

10.1021/ie0402721 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/02/2005

5734

Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005

extraction with quaternary ammonium chloride. Another fundamental question is how the undissociated molecules are extracted: by chemical interaction with extractant or by physical extraction. The answers of these questions are the purpose of this work. Definition of the Distribution Coefficients. The overall distribution coefficient expresses the ratio between the total concentration of extracted substance in all of its possible forms in the organic and the aqueous phase:

m(CHL) )

CHL CHL

where CHL ) [HL] + [L-]. The square brackets mean molar concentration, and the over bar means the species in the organic phase. The partial distribution coefficients represent the ratio of the concentration of extracted species in the organic phase to the total (analytical) concentration of acid in the aqueous phase. For the case of lactic acid extraction by Aliquat with chemical interaction, the partial distribution coefficient of lactate anions is

R4N+Cl- + L- T R4N+L- + Clthe equilibrium extraction constant is

[R4N +L-][Cl-]

KE(L-) )

m(L

[R4N+L-][Cl-](Ka + [H+])

m(HL) )

[R4N Cl :HL] CHL

For the case of physical extraction by extractant, modifier, and diluent, the partial distribution coefficients represent the ratio between the concentration of extracted species in the volume of corresponding solvent (extractant, modifier, or diluent) and the total (analytical) concentration of the acid in the aqueous phase. They can be expressed by

me )

[HL]e [HL]m [HL]d , mm ) , md ) CHL CHL CHL

where [HL]e, [HL]m, and [HL]d represent the concentration of undissociated acid molecules in the volume of extractant, diluent, and modifier, respectively. The particular (intrinsic) distribution coefficients represent the concentration of extracted species in the organic phase to the concentration of its extracted form in the aqueous phase. In the case of chemical interaction, they are equal to p(L-) ) [R4N+L-]/[L-] and p(HL) ) [R4N+Cl-:HL]/[HL] for the extracted anions and the whole (undissociated) molecules, respectively. For the physical extraction, the particular distribution coefficients of extractant, modifier, and diluent are equal to: pe ) [HL]e/[HL], pm ) [HL]m/[HL], and pd ) [HL]d/[HL], respectively. Theoretical Part. When the lactate anions [L-] are extracted by chemical interaction with the quaternary ammonium chloride [R4N+Cl-] according to the reaction

m(L-)[Cl-]

(2)

R[R4N+Cl-]

where Ka is the dissociation constant of lactic acid and R ) Ka/(Ka + [H+]) represents the part of dissociated form of acid. From eq 2 the partial distribution coefficient of lactate anions (m(L-)) can be expressed as follows

m(L ) ) -

RKE(L-)[R4N+Cl-]

(3)

[Cl-]

and

m(L-)[Cl-] R

) KE(L-)[R4N+Cl-]

(3a)

or in logarithmic form,

and of the undissociated molecules is -

)

CHLKa[R4N+Cl-]

-

+

)

[L-][R4N+Cl-]

+ -

[R4N L ] ) ) CHL

(1)

log

(

)

m(L-)[Cl-] R

) log KE(L-) + log ([R4N+Cl-]) (4)

Equation 2 can also be presented with the particular distribution coefficient of lactate anions (p(L-)) In this case,

p(L-) )

m(L-) R

)

KE(L-)[R4N+Cl-] [Cl-]

(5)

From eqs 2 and 3, it is easy to see that the partial distribution coefficient of lactate anions(m(L-)) depends on the equilibrium (free) extractant concentration in the organic phase, the dissociation constant of acid, the pH, and the concentration of total chloride anions in the aqueous phase. The particular (intrinsic) distribution coefficient (p(L-)) also depends on the above-mentioned parameters except pH and the dissociation constant of extracted acid. The easier way to determine the extraction constant is to carry out the experiments at R ) 1, high (constant) chloride concentration, and different extractant concentrations in conditions such that its final concentration does not essentially differ from the initial one (low concentration of removed anions), as in the works of Matsumoto et al.7 To calculate the distribution coefficient of lactate anions (m(L-)) in the general case, it is necessary to know the real concentrations of extractant and chloride anions. Even when neglecting the extraction of undissociated molecules and without adding chloride anions, it is difficult to calculate the equilibrium concentration of extractant ([R4N+Cl-] ) [R4N+Cl-]in - [R4N+L-]). Unfortunately, the removed lactate anions are not equal to the exchanged chloride anions, by reason of the extractant solubility in the aqueous phase.

Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005 5735

If we assume that the undissociated molecules are removed by chemical interaction according to

R4N+Cl- + HL T R4N+Cl-:HL

(6)

m(CHL) )

with the extraction constant

[R4N+Cl-:HL]

KE(HL) )

+

[R4N+L-] + (1 - βd - βm)[HL]e + βd[HL]d + βm[HL]m CHL (13)

)

-

[HL][R4N Cl ] +

[R4N Cl-:HL](Ka + [H+]) CHL[H+][R4N+Cl-]

m(HL)

)

(1 - R)[R4N+Cl-]

(7)

the following expressions take place:

[R4N+Cl-:HL] m(HL) ) ) KE(HL)(1 - R)[R4N+Cl-] (8) CHL and

m(HL) (1 - R)

)

[R4N+Cl-:HL] (1 - R)CHL

) KE(HL)[R4N+Cl-] (8a)

(

m(HL)

)

(1 - R)

) log KE(HL) + log[R4N+Cl-]

(9)

The partial distribution coefficient (m(HL)) and the extraction constant (KE(HL)) of undissociated molecules can be determined under the condition that R ) 0, i.e., mineral acid must be added, but in this way, the extraction system changes and m(HL) will be not the same as in the absence of mineral acid. As in the case of eq 2, in eq 7 the particular distribution coefficient of lactic acid molecules removed with extractant by chemical interaction (p(HL)) can be introduced:

p(HL) )

m(HL) (1 - R)

) KE(HL)[R4N+Cl-]

(10)

If the physical extraction by the diluent and modifier is negligible and can be neglected, the overall distribution coefficient (m(CHL)) can be presented as

m(CHL) )

[R4N+L-] + [R4N+Cl-:HL] ) m(L-) + m(HL) CHL (11)

When the organic phase consists of extractant, diluent, and modifier and the physical extraction by diluent and/or modifier cannot be neglected, as in the case of lactic acid, it must be taken into account:

m(CHL) ) [R4N+L-] + [R4N+Cl-:HL] + βd[HL]d + βm[HL]m CHL ) m(L-) + m(HL) + βdmd + βmmm

) m(L-) + (1 - βd - βm)me + βdmd + βmmm where me ) [HL]e/CHL ) pe(1 - R) is the partial distribution coefficient of extracted undissociated molecules by physical extraction with extractant and pe ) [HL]e/[HL] is the particular distribution coefficient. From the above-mentioned equations, it is clearly seen that, to predict the extraction efficiency, it is necessary to know the parts of carboxylic acid removed as the undissociated and dissociated forms. When the organic phase does not contain carboxylic acid and hydrogen before the extraction, their concentrations after the extraction of a monoacid can be calculated from the mass balance according to the following equations:

CHL(in)Vin - CHLV ) ([HL] + [R4N+L-])V h

or in logarithmic expression,

log

h d/V h and βm ) V h m/V h are the volume fractions where βd ) V of diluent and modifier, respectively. If the undissociated molecules are extracted with extractant by physical extraction (i.e., chemical interaction does not occur), eq 10 is modified to:

(12)

(14)

h ) ([HL]in + [H+]in)Vin - ([HL] + [H+])V ([HL])V (15) where in the case of chemical interaction between the extractant and undissociated molecules,

[HL] ) [R4N+Cl-:HL] + βd[HL]d + βm[HL]m ) pHL[HL] + βd pd[HL] + βm pm[HL] ) mHL md mm [HL] + βd [HL] + βm [HL] (16) 1-R 1-R 1-R and in the case of physical extraction of undissociated molecules,

[HL] ) (1 - βd - βm)[HL]e + βd[HL]d + βm[HL]m ) (1 - βd - βm)pe[HL] + βd pd[HL] + βm pm[HL] ) me md (1 - βd - βm) [HL] + βd [HL] + 1-R 1-R mm βm [HL] (17) 1-R Materials and Methods All chemicals used were produced by Acros Organics (France). As extractant, quaternary ammonium salt Aliquat 336 (tri(C8C10)methylammonium chloride) was used. As inert and active diluents, n-dodecane and 1-decanol were used. The organic phase was composed from extractant (5-30% v/v) dissolved in n-dodecane (50-75% v/v) and 1-decanol (20% v/v). The 90% (mass) L-(+)-lactic acid was used for the extraction studies. Because of the presence of dimers of the acid under these conditions (about 25% of total concentration), a 10-fold diluted solution was boiled under reflux for 8-10 h for dimer hydrolysis. The resulting solutions were used for model medium prepa-

5736

Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005

ration. Pure (98% mass), crystalline L-(+)-lactic acid was used to prepare standard solutions for HPLC analyses. 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 15 min at ambient temperature on the shaking machine IKA HS501 digital (IKA Labortechnique). Our preliminary experiments showed that a maximum of 5 min was sufficient time for attaining the equilibrium. After separation of the phases, the volumes, the pH value of the aqueous phase, and the chloride ion concentration were measured. 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. When necessary, the initial pH value of the aqueous solution was adjusted with solutions of NaOH and lactic acid with appropriate concentrations. The HPLC system was composed of pump, autosampler, and UV-detector (all from Spectra Physics) and integrator (Hewlett-Packard). The column used was Aminex HPX-87H (Bio-Rad). As the mobile phase, a 0.005 M solution of H2SO4 was used at a flow rate of 0.6 mL/min. The chloride ion concentrations were determined by means of an ion-selective electrode and Meter Lab (Radiometer Analytical) M 240 pH/ion meter. The concentration of transferred undissociated acid molecules in the organic phase ([HL]) was calculated according to eq 15. The obtained value was introduced in eq 14 for the determination of lactate anion concentration in the organic phase ([R4N+L-]) extracted by the anion exchange mechanism. The distribution coefficients of lactic acid when the organic phase is pure decanol or dodecane were determined experimentally. The obtained values were pm ) 0.152 for decanol and pd ) 0.034 for dodecane. Using these data, the values of pHL and pe (and the corresponding values of m) can be calculated from eqs 16 and 17. Results and Discussion All our results were treated according to the mass balance of carboxylic acid and hydrogen taking into account eqs 14 and 15 and the measured chloride ion concentration in the aqueous phase. This provided us the opportunity to know the concentration of different species in the organic phase. Parts of our experiments were carried out at different pH values and at a large range of lactic acid concentrations (from 5 to 100 g/L). When the influence of pH was studied, the organic phase was composed of 30% v/v Aliquat 336, 20% v/v decanol, and 50% v/v dodecane. Figure 1a elucidates the influence of pH on the overall distribution coefficient (m(CHL)) (curve 1), the partial distribution coefficients of whole molecules (m(HL) or me) and anions (m(L-)) extracted by Aliquat 336 (curves 2 and 3, respectively), and the sum of the partial distribution coefficients of undissociated molecules (curve 4) removed with modifier and diluent (m(m+d) ) mm + md). In these experiments, the initial acid concentrations were ∼11 g/L (from 10.99 to 11.62 g/L). The same shape of curves was obtained when the initial acid concentration was ∼5 g/L (results not presented here). It is evident that, at a low pH value (pH ) 2.15), the part of the whole molecules removed

Figure 1. (a) Influence of pH on the distribution coefficients at a low lactic acid concentration. Curves: 1, m(CHL); 2, m(HL) or βeme; 3, m(L-); 4, βmmm + βdmd. Solvent composition: 30% v/v Aliquat 336, 20% v/v decanol, and 50% v/v dodecane. The initial acid concentration in the aqueous phase is ∼11 g/L. (b) Influence of pH on the distribution coefficients at a high lactic acid concentration. Curves: 1, m(CHL); 2, m(HL) or βeme; 3, m(L-); 4, βmmm + βdmd. Solvent composition: 30% v/v Aliquat 336, 20% v/v decanol, and 50% v/v dodecane. The initial acid concentration in the aqueous phase is ∼100 g/L.

by the extractant is a few times bigger than the part of the extracted anions, but with the rise of pH, this ratio turns. The concentration of extracted molecules by physical extraction with modifier and diluent decreases with increasing pH, as the concentration of extracted molecules by extractant decreases with increasing pH, because their concentration in the aqueous phase decreases with the rise of pH. For the extracted anions, the dependence is vice versa, due to the increase of anion concentration in the aqueous phase with pH increasing. The same tendency was observed for a relatively high initial acid concentration in the aqueous phase (∼50 and 100 g/L) for the extracted species, but the tendency was reversed for the overall distribution coefficient as it was obtained in the work of Yang et al.11 Figure 1b shows our results when the initial concentration of lactic acid is ∼100 g/L (from 96.2 to 112.8 g/L). In the beginning of curve 1, the overall distribution coefficient increases, but after that it decreases with the rise of pH value. Similar behavior of the distribution coefficients for lactic acid molecules extracted by extractant (curve 2) and modifier and diluent (curve 4) is observed. The distribution coefficient of lactic acid anions (curve 3) increases

Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005 5737

Figure 2. Effect of pH on the ratio [HL]/[L-] at different initial lactic acid concentrations in the aqueous phase. Curve 1, initial lactic acid concentration 11.0 g/L; Curve 2, initial lactic acid concentration 100 g/L (from 96 to 113 g/L). Solvent composition: 30% v/v Aliquat 336, 20% v/v decanol, and 50% v/v dodecane.

sharply up to pH ≈ 4. This contributes to the increase in the beginning in curve 1. A similar effect of pH on the overall distribution coefficients for the extraction of different carboxylic acids with 50% v/v Aliquat 336 in kerosene can be observed in the experimental points of Yang et al.11 The reason why, at low acid concentrations, the dependence of the overall distribution coefficient on pH corresponds to that of the distribution coefficient of lactate anions and, at a high concentration of the acid, the dependence corresponds to that of lactic acid molecules is that the pH of an aqueous solution depends on the concentration of the acid. With the rise of its concentration, the pH decreases and it contributes to the rise of acid molecule concentrations. From the results presented in Figure 2, one can see that, whereas at 11 g/L (curve 1) the part of the extracted molecules at low pH is ∼ five times bigger than that of the extracted anions, at a concentration of 100 g/L (curve 2), this ratio is >16. If the extraction constant KE is determined with the assumption that only the anion exchange mechanism takes place even at a low acid concentration and with the measurement of the concentration of chloride anions in the aqueous phase, it contributes to a significant error as can be seen in Figure 3a. Line 1 and line 2 express the influence of free extractant concentration on m(CHL)[Cl-]/R and m(L-)[Cl-]/R, respectively, but line 3 represents m(HL)/(1 - R) and corresponds to eq 8a, which concerns the removal of whole molecules. According to eq 3a, if only the anion exchange mechanism takes place, lines 1 and 2 must be represented by one straight line with a slope equal to the extraction constant (KE(L-)). Line 3 also must be a straight line with a slope corresponding to the extraction constant of undissociated molecules (KE(HL)), according to eq 8a. Generally, the logarithmic form of these dependences is used (eqs 4 and 9), which provides the opportunity to directly determine the extraction constant (log KE) and the number of molecules of extractant that interact with one molecule of acid from the slope of the straight line, as in Figure 3b. The determined extraction constant from line 1 is 0.220, whereas the extraction constant of lactate anions KE(L-) from straight line 2 is 0.032. The difference between the two values is obvious. The slope of line 2 and line 3 are close to 1.0 (0.9 and 1.05, respectively).

Figure 3. (a) Different terms depending on the equilibrium (free) extractant concentration ([R4N+Cl-]). Curve 1, m(CHL)[Cl-]/R; Curve 2, m(L-)[Cl-]/R; Curve 3, m(HL)/(1 - R). Solvent composition: from 5% v/v to 30% v/v Aliquat 336, a constant extractant-tomodifier ratio ) 1.5. The initial acid concentration in the aqueous phase is 5.704 g/L. (b) Determination of equilibrium extraction constant. Curve 1, log m(CHL)[Cl-]/R vs log[R4N+Cl-] accepting exchange of anions, only. Curve 2, log m(L-)[Cl-]/R vs log[R4N+Cl-] taking into account the real concentration of R4N+L-. Curve 3, log m(HL)/(1 - R) vs log[R4N+Cl-]. Experimental conditions as in Figure 3a.

Line 1 does not express the real mechanism (the slope is not 1.0 but 1.27), and its use for the determination of the extraction mechanism and extraction constant is incorrect. For line 3, the equilibrium extraction constant of whole molecules (KE(HL) ) 0.613) is much bigger than the extraction constant of lactate anions KE(L-). The experiments for Figure 3 were carried out at a low acid concentration (5.704 g/L ) 0.063 M). The concentration of extractant was varied (from 5 to 30% v/v or 0.09 to 0.54M) at a constant volume ratio between the extractant and modifier )1.5. How the particular distribution coefficients of lactate anions (p(L-) ) [R4N+L-]/[L-]) and of lactic acid molecules, removed with extractant by chemical interaction (p(HL) ) [R4N+Cl-:HL]/[HL]) or by physical extraction (pe ) [HL]e/[HL]), and the particular distribution coefficient of lactic acid extracted with modifier and diluent by physical extraction (p(m+d) ) pm + pd) depend on pH is shown in Figure 4. The experiments are carried out at the same condition as for Figure 1. Curve 1 shows that p(L-) ) [R4N+L-]/[L-] is not constant and strongly depends on the pH of the aqueous solution. The quantity

5738

Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005

Figure 4. Dependence of particular distribution coefficients on pH. Curve 1, p(L-); Curve 2, p(L-)[Cl-]/[R4N+Cl-]; Curve 3, p(HL); Curve 4, p(m+d). Experimental conditions as in Figure 1a.

Figure 5. Influence of the undissociated molecule concentration in the aqueous phase on their concentration in the organic phase. Curve 1, [HL]; Curve 2, [HL] - βm[HL]m - βd[HL]d; Curve 3, βm[HL]m; Curve 4, βd[HL]d. Solvent composition: 30% v/v Aliquat 336, 20% v/v decanol, and 50% v/v dodecane.

p(L-)[Cl-]/[R4N+L-] ) m(L-)[Cl-]/R‚[R4N+L-] (curve 2) is constant, which corresponds to the extraction constant of lactate anions (KE(L-)) according to eq 2. Line 4 represents the particular distribution coefficient of physical extraction effectuated by diluent and modifier. Line 3 corresponds to p(HL) ) [R4N+Cl-:HL]/[HL] ) m(HL)/(1-R), and according to eq 10, in the case of chemical interaction, it depends only on the extraction constant and the extractant concentration, or in the case of physical extraction, pe ) [HL]e/[HL] is constant. In these experimental conditions, the extractant concentration does not change significantly (from 0.487 to 0.514 M), and we cannot draw a conclusion about which mechanism occurs, chemical interaction or physical extraction. This conclusion also cannot be drawn if the initial extractant concentration is changed because, in both cases, the concentration in the organic phase depends on the extractant concentration. This was the reason to perform an investigation in which the initial lactic acid concentration in the aqueous phase varied from 1.140 g/L (0.013 M) to 168.4 g/L (1.869 M) and the composition of the organic phase was 30% v/v Aliquat 336 (0.540 M), 20% v/v decanol, and 50% v/v dodecane. At these conditions, if we take into account the chemical interaction between the extractant and undissociated molecules of lactic acid (according to eq 6) as well as with the lactate anions (according to eq 1), in these cases the equilibrium (free) extractant concentration changes from 0.538 to 0.262 M and from 0.540 to 0.521 M, respectively. When the physical extraction takes place, the change of initial extractant concentration due to chemical reaction with lactate anions is small and the concentration of lactic acid molecules in the organic phase is proportional to their concentration in the aqueous phase. This dependence must be represented by a straight line with a slope equal to the particular distribution coefficient. Figure 5 shows the total concentration of lactic acid molecules in the organic phase (line 1) and their concentration due to the extraction by the extractant (line 2), modifier (line 3), and diluent (line 4) in terms of their dependence on the undissociated lactic acid molecule concentration in the aqueous phase. They are straight lines. This linear dependence of the concentration of extracted molecules on their

Figure 6. Percentage of different types of contributions to the global extraction yield. Curve 1, [HL] - βm[HL]m - βd[HL]d; Curve 2, [R4N+L-]; Curve 3, βm[HL]m; Curve 4, βd[HL]d. Solvent composition: 30% v/v Aliquat 336, 20% v/v decanol, and 50% v/v dodecane.

concentration in the aqueous phase ([HL] - βdpd[HL] - βmpm[HL]) ) (1 - βm - βd)pe[HL] provides the opportunity to assume removal of the undissociated molecules of lactic acid with Aliquat 336 by physical extraction because (1 - βm - βd), βdpd, βmpm, and pe are constants. If chemical interaction occurs, then line 2 must be curved because, in the investigated acid concentration range, [R4N+Cl-:HL] ) KE(HL)[R4N+Cl-][HL] ) KE(HL)([R4N+Cl-]in - [R4N+Cl-:HL] - [R4N+L-])[HL] changes significantly, due to the variation of [R4N+Cl-] (from 0.538 to 0.262 M). The percentage of each type of contribution to the global extraction yield of lactic acid in terms of its dependence on its molecular concentration in the aqueous phase is elucidated in Figure 6. From the figure, it is clearly seen that the main contribution is the physical extraction of undissociated molecules by the extractant (curve 1). The part of the interaction product that is due to the anion exchange mechanism has a maximum (curve 2). Its part at a high molecular acid concentration

Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005 5739

in the aqueous phase is less than the part of the molecular concentration in the organic phase due to the physical extraction by modifier (curve 3) and diluent (curve 4). Conclusions The obtained results from the experimental data of this study showed that the extraction of lactic acid by Aliquat 336 is a complex process which depends strongly on the experimental conditions. Using the mass balance of lactic acid and hydrogen, it is possible to calculate the concentrations of different species in the organic phase. The following conclusions can be drawn out: (i) The ratio between the concentrations of extracted undissociated molecules and the extracted anions depends not only on pH but also on the total concentration of the acid in the aqueous phase. (ii) Depending on the lactic acid concentration in the aqueous phase, the overall distribution coefficient can rise or decrease with the increase of pH at a low and high acid concentration, respectively. (iii) The ratio between the concentrations of extracted acid anions and those in the aqueous phase depends significantly on the concentration of chloride anions in the aqueous phase. (iv) The constant ratio between the concentrations of extracted molecules and those in the aqueous phase at relatively high concentrations of extracted molecules by Aliquat 336 (important change of free extractant concentration if a chemical interaction occurs) provides a substantial reason to assume physical extraction. Acknowledgment This work was supported by the Ministry of Education and Science of Bulgaria under Grant Rila 13/03 and Grant PAI RILA No. 06298TC, which are greatly appreciated. D.Y. gratefully acknowledges the Agence Universitaire de la Francophonie for the provided financial support. Notation CHL ) total lactic acid concentration in the aqueous phase CHL ) total lactic acid concentration in the organic phase [L-], [HL] ) concentration of the lactate anions and undissociated lactic acid molecules in the aqueous phase [HL]e, [HL]m, [HL]d ) concentration of undissociated lactic acid molecules in the volume of extractant, modifier, and diluent in the case of physical extraction [R4N+Cl-] ) concentration of the quaternary ammonium chloride in the organic phase [R4N+L-], [R4N+Cl-:HL] ) concentration of lactate anions and undissociated lactic acid molecules in the organic phase in the case of chemical interaction m(CHL) ) CHL/CHL, ) overall distribution coefficient m(L-) ) [R4N+L-]/CHL, ) partial distribution coefficient of lactate anions (result of exchange of anions) m(HL) ) [R4N+Cl-:HL]/CHL, ) partial distribution coefficient of undissociated lactic acid molecules in the case of chemical interaction me ) [HL]e/CHL, mm ) [HL]m/CHL, md ) [HL]d/CHL ) partial distribution coefficients for extractant, modifier, and diluent in the case of physical extraction

p(L-) ) [R4N+L-]/[HL], p(HL) ) [R4N+Cl-:HL]/[HL] ) particular distribution coefficients for lactate anions and undissociated lactic acid molecules in the case of chemical interaction pe ) [HL]e/[HL], pm ) [HL]m/[HL], pd ) [HL]d/[HL] ) particular distribution coefficients for extractant, modifier, and diluent in the case of physical extraction KE(L-), KE(HL) ) equilibrium extraction constants of lactate anions and undissociated lactic acid molecules in the case of chemical interaction Ka ) dissociation constant of lactic acid R ) part of the dissociated form of lactic acid. βd, βm ) volume fractions of diluent and modifier V, V h, V h d, V h m ) volumes of aqueous phase, total organic phase, diluent, and modifier Indices in ) initial values e ) extractant d ) diluent m ) modifier CHL ) total acid [HL] ) undissociated acid molecules [L-] ) lactate anions

Literature Cited (1) Choudhury, B.; Basha, A.; Swaminathan, T. Study of lactic acid extraction with higher molecular weight amines. J. Chem. Technol. Biotechnol. 1988, 72, 111-116. (2) Hano, T.; Matsumoto, M.; Uenoyama, S.; Ohtake, T.; Kawano, Y.; Miura, S. Separation of lactic acid from fermentation broth by solvent extraction. Bioseparation 1993, 3, 321-326. (3) Matsumoto, M.; Takagi, T.; Kondo, K. Separation of lactic acid using polymeric membrane containing mobile carrier. J. Ferment. Bioeng. 1998, 85 (5), 483-487. (4) Hironaka, M.; Hirata, M.; Takanashi, H.; Hano, T. Contribution of water in the extraction and stripping kinetics of lactic acid with tri-n-octylmethylammonium chloride. Solv. Extr. Res. Dev., Jpn. 2001, 8, 103-112. (5) Sai, P.; Katikaneni, R.; Cherian, M. Purification of fermentation derived acetic acid by liquid-liquid extraction and esterification. Ind. Eng. Chem. Res. 2002, 41, 2745-2752. (6) Frieling, P.; Schugerl, K. Recovery of lactic acid from aqueous model solutions and fermentation broths. Process Biochem. 1999, 34, 685-696. (7) Matsumoto, M.; Takagi, T.; Nakaso, T.; Kondo, K. Extraction equilibria of some organic acids with tri-n-octylmethylammonium chloride. Solv. Extr. Res. Dev., Jpn. 1999, 6, 144-150 (8) Lazarova, Z.; Peeva, L. Solvent extraction of lactic acid from aqueous solution. J. Biotechnol. 1994, 32 (1), 75-82. (9) Kyuchoukov, G.; Marinova, M.; Molinier, J.; Albet, J.; Malmary, G. Extraction of lactic acid by means of a mixed extractant. Ind. Eng Chem. Res. 2001, 40 (23), 5635-5639. (10) Kyuchoukov, G.; Marinova, M.; Albet, J.; Molinier, J. New method for the extraction of lactic acid by means of a modified extractant (Aliquat 336). Ind. Eng. Chem. Res. 2004, 43, 11791184. (11) Yang, Sh.-T.; White, S.; Hsu, Sh.-T. Extraction of carboxylic acids with tertiary and quaternary amines: effect of pH. Ind. Eng. Chem. Res. 1991, 30, 1335-1342. (12) Eyal, A. M.; Canari, 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, 1789-1798.

Received for review November 4, 2004 Accepted April 20, 2005 IE0402721