Modeling and Optimization of Reactive Extraction of Citric Acid

Article pubs.acs.org/jced

Modeling and Optimization of Reactive Extraction of Citric Acid Niraj Thakre,† Abhinesh K. Prajapati,‡ Shyama P. Mahapatra,§ Awanish Kumar,∥ Akhilesh Khapre,⊥ and Dharm Pal*,⊥ †

Department of Chemical Engineering, IIT Kharagpur, Kharagpur, West Bengal 721302, India Department of Chemical Engineering, IESI, Indore 452012, India § Department of Chemistry, NIT Raipur, Raipur 492010, India ∥ Department of Biotechnology, NIT Raipur, Raipur 492010, India ⊥ Department of Chemical Engineering, NIT Raipur, Raipur 492010, India ‡

ABSTRACT: Reactive extraction of citric acid from dilute aqueous solutions was studied using three different extractants, namely tri-n-butylphosphate (TBP), tri-n-octylamine (TOA), and Aliquat 336 (A336), dissolved in three different diluents: butyl acetate, decanol, and benzene. The isothermal batch equilibrium experiments were carried out at T = 300.15 ± 1 K. The extraction was interpreted in terms of the distribution coefficient (KD). Maximum extraction efficiency (E = 95.5%) was obtained at 20% (v/v) TOA in butyl acetate with complexation constant KE2 = 1039.7 (kg mol−1)2 for the (2:1) complex. In addition to having a higher loading ratio (Z > 0.5), the overloading of amine(TOA) in the case of the citric acid + TOA + decanol system was also confirmed by spectroscopic (FTIR) analysis. The linear solvation energy relationship model was successfully applied to predict the distribution coefficient. The complex stoichiometry was also optimized using differential evolution. A close resemblance was observed between experimental and model values. (diluents).6 The diluents provide the desired solvation media to the extractants by modifying their physicochemical properties. The diluents may be inactive diluents comprising inert aliphatic hydrocarbons, vegetable oils,7,8 kerosene,9 etc., or active diluents that include solvents with active functional groups. With no doubt, active diluents have been proven to be more effective than inactive diluents and provide better extraction media when mixed with extractants.10−13 Also, there are some inherent measures to predict the diluents’ affinity to acid:extractant complexes such as polarity, dielectric constant, aromaticity, and so forth.11 The dipole−dipole interaction plays an important role in acid:extractant complex formation by neutralization reactions.5 Several studies can be found on the reactive extraction of citric acid using the nitrogen-based extractants such as Nsubstituted alkyl amides,4 long chain (about 8−12 carbon atoms) alkyl amines,7,11−15 and so forth. They have reported extraction efficiency as high as 90% using the tertiary amines diluted in active diluents.11−15 Studies on the reactive extraction of citric acid related to equilibrium,7,11−14 kinetics,14−16 effects of pH,17 back extraction,17 etc., can be found in the literature. Different modeling approaches have also been reported; for example, a mathematical model was obtained by Bizek et al.6,13

1. INTRODUCTION Among various aerobic fermentation products, organic acids are one of the most important and useful products.1 Citric acid (tricarboxylic acid) has wide applications in the food and pharmaceutical industries. The acid is abundant in many citrus fruits and can also be produced through biological fermentation.2 However, there are several synthetic means available for the production of citric acid. Due to the ever increasing prices of petroleum-derived feedstock, production through bioroutes is most economic.3 The acid is produced in low concentrations in the fermentation broth and needs recovery for end use. Recovery of acid makes up almost half of the production cost; therefore, efficient separation technology is required to develop an economically feasible process. There are several recovery methods available, such as precipitation, extraction, adsorption, and so forth.3 However, in the recent past, reactive extraction has emerged as the most promising and economically viable option among different processes studied.4 Reactive extraction involves the use of specific extractants, such as various nitrogen- or phosphorus-based extractants, that are employed with the different diluents. Kertes and King1 categorized the extractants into three broad classes: (i) carbonbonded oxygen donor extractants that include hydrocarbons and substituted hydrocarbon solvents, (ii) phosphorus-bonded oxygen donor extractants, and (iii) aliphatic amine extractants. As these extractants are costly as well as viscous,5 they are often used as mixed extractants6 or mixed with conventional solvents © XXXX American Chemical Society

Received: March 30, 2016 Accepted: June 14, 2016

A

DOI: 10.1021/acs.jced.6b00274 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. Physicochemical Properties of Chemicals Used in the Present Work

taken in a conical flask and allowed to mix in a water bath shaker (Remi Equipment Pvt. Ltd., India) for 2 h at T = 300.15 ± 1 K. The solution was allowed to settle for a specific time period to obtain clear distinct phases. Citric acid concentration in the aqueous phase was determined by an HPLC system (Young Lin Instruments, South Korea) equipped with a quaternary pump and vacuum degasser. A reverse phase column, C-18 columns, and a UV−vis detector at 210 nm were used. The mobile phase was an aqueous solution of phosphoric acid and potassium dihydrogen phosphate with a flow rate of 0.6 mL/min. Samples were sonicated and filtered through a Ran Disc 0.22 μm PTFE membrane before the injection into HPLC. Acid concentration in the organic phase was obtained by material balance. 2.2.2. Fourier Transform Infrared (FTIR) Analysis of Acid:Extractant Complex Formation. The acid:extractant complex formation in the organic phase were interpreted by spectroscopic techniques using a Fourier transform infrared (FTIR) unit (Bruker, model alpha, laser class 1). All measurements were done in a cell equipped with potassium bromide windows. The acid:amine complexation in the reactive extraction of citric acid with TOA diluted in decanol was examined. The comparative IR spectra of the initial aqueous phases along with the organic phases in the equilibrium state were obtained and analyzed with respect to the stoichiometry of the acid:amine complex. The intensity of the peaks also gave information about the concentration of species with the respective chemical bond. The IR range of functional groups of chemicals used in this work is shown in Table 2.

using the correlation between solvatochromic properties of diluents.18 Also, Poposka et al.15 interpreted equilibrium isotherms with the modified Langmuir isotherm model. The stoichiometric parameters have been also predicted by researchers using an evolutionary-based algorithm, i.e. differential evolution, for various carboxylic acids.19−21 However, to the best of our knowledge, a study on the application of differential evolution to optimize reactive extraction of citric acid has not been reported previously. In the present work, the effect of extractants (TBP, TOA, and A336) and diluents (decanol, butyl acetate, and benzene) on the extraction equilibria was studied. Also, the linear solvation energy relationship (LSER) model and differential evolution (DE) were successfully applied for modeling and optimization of the extraction equilibrium.

2. MATERIALS AND METHODS 2.1. Materials. Citric acid was received from Merck Co Ltd. (Germany). Tri-n-octylamine (TOA) and Aliquat 336 (A336) were obtained from Sigma-Aldrich (India), and tri-nbutylphosphate (TBP) was supplied by Loba Chemie (India). Distilled water (Millipore) was used for preparing the aqueous solutions. The physicochemical properties of the reagents used in the present study are presented in Table 1. 2.2. Methods. 2.2.1. Equilibrium Studies. The aqueous solutions of citric acid were prepared in the concentration range of 0.20.8 mol kg−1, as the yield of citric acid is about 0.53 mol kg−1 in fermentation broth.22 The extraction media (organic phase) was prepared by mixing specific amounts of extractants into diluents. The organic and aqueous phases were B



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Table 2. IR Range of Functional Groups of Chemicals Used in This Work functional group carboxylic acids amines (primary) amines (secondary) alcohols

Zm = Z − vμ; where μ =

−1

type of vibration

IR range (cm )

hydrogen-bonded O−H stretch CO stretch N−H stretch N−H bend N−H stretch N−H bend hydrogen-bonded O−H stretch

3400−2400 1730−1650 3500−3100 1640−1560 3500−3100 1550−1450 3600−3100

[CA mEn](1 + K a /[H+])m [CA mEn] m n = [CA] [E ̅ ] [CA]mT [E ̅ ]n

KE =

Zm = KE1[CA] 1 − Zm

[CA]T [CA mEn] =m [CA]T [CA]T

Zm = KE2[CA]2 2 − Zm

(1)

(2)

XYZ = XYZo + s(π * + dδ) + aα + bβ + hδ H + eξ (11)

where XYZ is the physicochemical property of the solvent. XYZo represents the value of the parameter at the free state. The π* scale of solvent polarizability is an index of the ability of the solvent to stabilize a charge with its dielectric effect. The α scale solvent hydrogen-bond donor (HBD) acidities is the measure of the ability of the solvent to donate a proton in a solvent−solute hydrogen bond. The β scale of hydrogen-bond acceptor (HBA) basicities quantifies the solvent’s capability to accept a proton, i.e., to donate an electron pair in a solute− solvent hydrogen bond. The δH term is the Hildebrand solubility parameter, which is a measure of the solute/solvent interactions, and the ξ parameter is a measure of coordinate covalence. The model equation can be fitted for the natural logarithm of the distribution coefficient as eq 12:

(3)

KD(1 + K a /[H+])m m[CA]T m − 1 [E ̅ ]n

(4)

The concentration of extractant in the organic phase can be calculated as eq 5. K n[CA]T [E ̅ ] = [E ̅ ]in − D m

ln KD = ln KD0 + s(π * + dδ) + aα + bβ

(6)

The stoichiometry of the acid:extractant complex formed can be predicted by the loading ratio. It is defined as the extent to which extractant can be loaded with acid (eq 7).

Z=

[CA] [E ̅ ]

(12)

where KD is the distribution coefficient of citric acid between the aqueous and organic phase. The δH and ξ terms are eliminated because they do not affect the value of the objective function (ln KD) significantly. Eq 12 is regressed for optimum values of the coefficients against the set of experimental distribution coefficients. The values of regression coefficients s, d, a, and b were utilized for comparing solvent properties. Hence, by a well-judged choice of solvents, eq 12 can be reduced to obtain the KD value of citric acid between the organic and aqueous phases. Solvatochromic parameter correlations and values were determined by measuring intensities of maximal absorption in NMR, ESR, IR, and UV−vis spectra. The values of the solvatochromic parameters of solvents used in this work were taken from the literature18,19 and are given in Table 3. Deviations between observed and predicted values were assessed in terms of root-mean-square deviation (RMSD) using mod experimental (Kexp D ) and model (KD ) values of distribution coefficients (KD) as

(5)

When eq 4 and eq 5 are solved for KD, a relation between KD and KE is obtained as eq 6. n ⎛ [CA]T ⎞ [CA]T m − 1 KD = mKE⎜[E ̅ ]in − KDn ⎟ ⎝ m ⎠ (1 + K a /[H+])m

(10)

Here, KE2 is the equilibrium complexation constant for the 2:1 acid:extractant complex. 3.2. Linear Solvation Energy Relationship (LSER) Modeling. Kamlet et al.18 provided thoughts on the solvation properties of different solvents that can be predicted on the basis of different solvatochromic parameters. In general, the extraction capability of solvents can be characterized, and equilibrium distribution can be predicted, by a linear fit given by eq 10:

A combination of eq 2 and eq 3 gives the relation between the equilibrium constant, KE, and the distribution coefficient (eq 4). KE =

(9)

Higher loading values (Z > 0.5) assumes higher stoichiometry complexes. For the 2:1 acid:extractant complex formation, the corresponding relation can be expressed as eq 10.

Here, KE represents the reaction equilibrium constant for the complexation reaction (eq 2). [CA]T denotes the total (ionized and un-ionized) acid concentration of citric acid. The distribution coefficient, KD, of acid is defined as the ratio of total concentration of acid in the organic phase to that in the aqueous phase (eq 3). KD =

(8)

Thus, for Zm < 0.5, a 1:1 acid:TOA complex formation is assumed, and KE1 is related to Zm as eq 9:

3. THEORY 3.1. Reactive Extraction Equilibrium. The distribution of citric acid (CA) between the aqueous phase and organic phase via formation of the acid:extractant complex can be represented by eq 1: mCA + nE ̅ ↔ CA mEn

[CA]diluent [E ̅ ]o

(7)

Because the contribution of active diluents was also associated with the degree of extractant, the modified loading ratio (Zm) was introduced to evaluate the loading by extractant only. This can be obtained by subtracting the contribution by diluents alone from the total loading value, Z, as eq 8. C



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where xi is the experimental value, x̅ is the mean of the experimental values, and N is the number of experimental values.

Table 3. Solvatochromic Parameters for Various Solvents Used

a

diluents

π*

δ

α

β

decanola n-butyl acetateb benzeneb

0.40 0.46 0.59

0 0 0

0.00 0.00 0.00

0.00 0.00 0.10

4. RESULTS AND DISCUSSION 4.1. Physical Extraction of Citric Acid. The phase distribution of citric acid (0.2−0.8 mol kg−1) between the aqueous and organic phases for the physical extraction using various diluents is given in Table 4. To visualize the effect of physicochemical properties on physical extraction, solvents (diluents) were chosen from different categories, namely decanol, butyl acetate, and benzene. The distribution coefficients were found to be in the order of decanol > butyl acetate > benzene. The active diluents, such as decanol and butyl acetate, provided significant extraction efficiency (E = 10.54−13.25% and E = 9.45−12.84%, respectively), whereas the inactive diluent (benzene in present case) was able to provide extraction merely up to E = 6.187.75%. Also, the extraction efficiency was found to decrease with an increase in initial acid concentration, signifying the suitability of the extraction, as carboxylic acids are mostly produced in low concentrations in fermentation broths. 4.2. Reactive Extraction of Citric Acid. Diluents alone were not able to provide considerable extraction (Table 4); hence, reactive extraction involving specific extractants was needed. 4.2.1. Effect of Diluents. The reactive extraction using TBP, TOA, and A336 diluted in various diluents was studied. Table 5 represents the equilibrium distribution of citric acid using 30% TBP in different diluents, namely decanol, butyl acetate, and benzene. When compared to the physical extraction with these diluents, no significant improvement was observed when TBP was diluted in butyl acetate. Benzene and decanol were used for citric acid recovery. The extraction efficiency obtained with decanol alone is 6.11−7.00% that is merely improved to 9.16− 13.54% when decanol was used with TBP (Table 5). The trend in Table 5 shows that extraction efficiency is in the order of butyl acetate (KD = 0.127−0.185) > decanol (KD = 0.100−0.156) > benzene (KD = 0.032−0.052). Also, with lower acid concentrations (0.2−0.4 mol kg−1), the extraction efficiency becomes higher; that is, it decreases with as the initial acid concentration increases. The extraction efficiency increases from 9.26% to 12.13% when initial acid concentration was increased from 0.8 mol kg−1 to 0.2 mol kg−1. That envisaged the favorable result of higher extraction at a lower acid concentration because the acid concentration produced in fermentation broth is not higher than 10 wt %. The chemical extraction through solvation mechanism with TBP was found to be poor for citric acid extraction due to weak van der Waals forces caused by the presence of three bulky carboxylic groups. Thus, TBP is not recommended as a good extractant for the recovery of citric acid from dilute aqueous solutions. Further, the tertiary amine extractant TOA was employed in 20% concentration with the same categories of diluents (Table 6) and was found to be highly efficient compared to the physical extraction (diluents alone) of citric acid. The distribution coefficients are in the order of butyl acetate (KD = 13.64−15.62) > decanol (KD = 7.11−14.73) > benzene (KD = 5.98−8.77). The KD values were found to decrease with an increase in acid concentration, hence suggesting that reactive extractions using TOA could be successfully employed for the removal of acid from dilute solutions. All of the diluents chosen in this study were almost equally good in providing the

Datta and Kumar (ref 21). bKamlet et al. (ref 18).

RMSD =

1 (KDexp − KD mod)2 n

(13)

3.3. Optimization of Complex Stoichiometry and Equilibrium Constant Using Differential Evolution (DE). The differential evolution19 (DE) technique is based on the genetic algorithm20 (GA) that is introduced to overcome the problems associated with traditional approaches. The conventional gradient-based optimization techniques may trap the local minima, whereas DE seeks the global minimum. DE follows evolutionary computation that is focused on survival of the fittest. A schematic diagram of the DE algorithm is presented in Figure 1. The DE algorithm seeks to evolve a

Figure 1. Schematic diagram for the DE algorithm.

population of size N. The population vector (Xi = {X1, X2, X3, ...}) of dimension D is generated on a random basis. The most appropriate vector, Xbest, is chosen. The crossover ratio (CR) is taken between 0 and 1. 3.4. Experimental Uncertainty. To ensure the reproducibility of the results, all analyses were repeated twice under identical conditions, and the mean value was used for further calculations. The uncertainty in the experimental results was analyzed in accordance with National Institute of Standards and Technology (NIST) principle guidelines. Standard uncertainty was evaluated to be within x ± 0.001 using eq 14: ⎛ ∑N (x − x )2 ⎞1/2 i ̅ ⎟ μ(x) = ⎜⎜ i = 1 ⎟ ( N 1) − ⎝ ⎠

(14) D



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Table 4. Physical Extraction of Citric Acid (0.2−0.8 mol kg−1) with Different Solvents at T = 300.15 ± 1 Ka solvents

[CA] (mol kg−1)

[CA] (mol kg−1)

KD,exp

E (%)

P

D (kg mol−1)

0.8 0.6 0.5 0.4 0.2 0.8 0.6 0.5 0.4 0.2 0.8 0.6 0.5 0.4 0.2

0.107 0.067 0.055 0.058 0.030 0.086 0.067 0.050 0.037 0.026 0.048 0.042 0.042 0.024 0.013

0.160 0.128 0.124 0.175 0.176 0.124 0.128 0.113 0.105 0.147 0.065 0.075 0.092 0.066 0.069

13.80 11.33 11.05 14.91 14.97 11.05 11.33 10.19 9.46 12.84 6.11 7.00 8.46 6.19 6.46

0.078

9.13

0.087

3.92

0.048

11.07

decanol

n-butyl acetate

benzene

a

Standard uncertainties (u) are u(T) = ± 1 K; u(CA) = ± 0.001 mol kg−1.

Table 5. Reactive Extraction of Citric Acid (0.2−0.8 mol kg−1) with 30% TBP Dissolved in Various Diluents at T = 300.15 ± 1 Ka diluents decanol

n-butyl acetate

benzene

[CA] [CA] (mol kg−1) (mol kg−1) 0.8 0.6 0.5 0.4 0.2 0.8 0.6 0.5 0.4 0.2 0.8 0.6 0.5 0.4 0.2

0.068 0.079 0.050 0.035 0.024 0.100 0.091 0.070 0.043 0.024 0.025 0.025 0.025 0.015 0.010

KD,exp

KD,mod (DE)

E (%)

pH

0.102 0.157 0.111 0.101 0.138 0.158 0.185 0.163 0.127 0.138 0.033 0.044 0.052 0.040 0.050

0.124 0.118 0.119 0.116 0.106 0.173 0.157 0.151 0.139 0.109 0.038 0.040 0.041 0.043 0.049

9.26 13.55 10.02 9.17 12.13 13.61 15.61 14.03 11.28 12.13 3.18 4.17 4.95 3.80 4.76

2.08 2.11 2.12 2.14 2.19 2.08 2.13 2.17 2.22 2.31 1.77 1.86 1.91 1.94 2.01

Table 6. Reactive Extraction of Citric Acid (0.2−0.8 mol kg−1) with 20% TOA Dissolved in Various Diluents at T = 300.15 ± 1 Ka diluents decanol

n-butyl acetate

benzene

[CA] [CA] (mol kg−1) (mol kg−1) 0.8 0.6 0.5 0.4 0.2 0.8 0.6 0.5 0.4 0.2 0.8 0.6 0.5 0.4 0.2

0.646 0.522 0.466 0.358 0.181 0.689 0.557 0.468 0.358 0.182 0.669 0.523 0.437 0.353 0.187

KD,exp

KD,mod (DE)

E (%)

pH

7.12 8.53 14.73 16.95 12.25 14.64 21.25 15.63 17.16 13.64 5.99 7.01 6.65 8.78 8.69

5.04 13.61 9.95 4.69 3.22 14.64 21.05 28.03 19.17 13.27 5.23 8.43 9.40 7.75 5.66

87.68 89.51 93.64 94.43 92.46 93.61 95.51 93.99 94.49 93.17 85.69 87.52 86.93 89.77 89.68

3.06 3.18 3.32 3.71 3.93 2.83 2.93 3.02 3.24 3.41 2.23 2.41 2.54 2.62 2.81

a Standard uncertainties (u) are u(T) = ± 1 K; u(CA) = ± 0.001 mol kg−1.


solvation of acid:TOA complexes, which is a critical factor in the extraction of acid. The interaction between the complex and diluents can be divided into general solvation and specific interactions. Aromatic diluents (benzene in the present case) provided higher distribution (E = 87.02−89.67%) due to solvation caused by the interaction of the aromatic π electrons with the complex. Alcohols (decanol in the present case) are polar diluents and can promote extraction by providing a good dissolving media for the ion pair. However, polarity alone does not completely account for the solubility. Butyl acetate provided up to 94% extraction efficiency and was found to be the best diluent in the entire range of acid concentrations examined. Extraction using a 20% TOA concentration at lower acid concentrations provided the maximum degree of extractions (up to 96%). Therefore, even if an excess amount of TOA is present, it would have been wasted without recovering the acid; hence, the cost of the extraction system would increase by recycling excess TOA back to the broth. Therefore, it is

optimum to use 20% TOA. Further, it has been mentioned that the distribution coefficient of an acid using various concentrations of amine goes through a maximum.17 The reason for this is that TOA alone is a relatively poor solvation medium for the complexes. Aliquat 336 (trioctylmethylammonium chloride) is a quaternary ammonium salt that is liquid probably because the hydrocarbon chains cannot effectively pack to form crystallites. A336 has much longer hydrocarbon chains that mask the ionic site in nonpolar media, which enhances its solubility in the organic phase. The longer hydrocarbon chains might also anchor A336 to the interface, preventing them from diffusing too far into the polar phase to get back to the nonpolar phase efficiently while still being long enough chains to allow the ionic site into the polar phase to grab a reactant. The distribution of acid using 30% (v/v) A336 in decanol, butyl acetate, and benzene was studied. This is evident from Table 7, wherein the extraction efficiency varies with the diluents in the order of decanol > butyl acetate> benzene (KD = E



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Table 7. Reactive Extraction of Citric Acid (0.2−0.8 mol kg−1) with 30% A336 Dissolved in Various Diluents at T = 300.15 ± 1 Ka diluents decanol

n-butyl acetate

benzene

[CA] [CA] (mol kg−1) (mol kg−1) 0.8 0.6 0.5 0.4 0.2 0.8 0.6 0.5 0.4 0.2 0.8 0.6 0.5 0.4 0.2

0.171 0.150 0.122 0.093 0.047 0.195 0.137 0.108 0.093 0.049 0.082 0.078 0.064 0.049 0.030

KD,exp

KD,mod (DE)

E (%)

pH

0.282 0.339 0.326 0.312 0.303 0.334 0.300 0.278 0.310 0.323 0.118 0.152 0.147 0.142 0.172

0.302 0.306 0.316 0.325 0.329 0.321 0.323 0.320 0.300 0.257 0.127 0.133 0.129 0.123 0.115

22.02 25.30 24.60 23.80 23.28 25.06 23.09 21.74 23.67 24.41 10.57 13.17 12.83 12.41 14.66

1.38 1.41 1.44 1.48 1.56 1.34 1.36 1.37 1.42 1.48 1.49 1.51 1.52 1.57 1.65

Table 8. Reactive Extraction of Citric Acid (0.2−0.8 mol kg−1) with TBP (10−50 mol kg−1) Dissolved in Decanol at T = 300.15 ± 1 Ka TBP conc % (v/v) 10

30

50

a Standard uncertainties u are u(T) = ± 1 K; u(CA) = ± 0.001 mol kg−1.

[CA] [CA] (mol kg−1) (mol kg−1) 0.8 0.6 0.5 0.4 0.2 0.8 0.6 0.5 0.4 0.2 0.8 0.6 0.5 0.4 0.2

0.018 0.021 0.015 0.009 0.007 0.068 0.079 0.050 0.035 0.024 0.152 0.123 0.114 0.083 0.044

KD,exp

E (%)

Z

pH

0.024 0.037 0.032 0.024 0.037 0.102 0.157 0.111 0.101 0.138 0.261 0.267 0.297 0.280 0.288

2.33 3.60 3.08 2.32 3.56 9.26 13.55 10.02 9.17 12.13 20.67 21.10 22.87 21.84 22.35

0.050 0.058 0.042 0.025 0.020 1.493 1.730 1.093 0.761 0.520 0.083 0.067 0.062 0.045 0.024

2.11 2.14 2.16 2.19 2.22 2.08 2.11 2.12 2.14 2.19 1.87 1.91 1.98 2.02 2.06


Table 9. Reactive Extraction of Citric Acid (0.2−0.8 mol kg−1) with TOA (5−20% v/v) Dissolved in Decanol at T = 300.15 ± 1 Ka

0.28−0.33 for decanol, KD = 0.27−0.33 for butyl acetate, and KD = 0.11−0.15 for benzene). Also, in this case, the aromatic hydrocarbon (benzene) provided a significant distribution coefficient due to a similar interaction of the aromatic π electron with the complex. However, considerable improvement in extraction efficiency was observed when decanol was used. This shows that polarity is the crucial factor for the solvation of the acid:A336 complex. The alkyl chain adjacent to the functional group also affects the polarity of the diluents. The KD values were found to increase with a decrease in acid concentration, hence suggesting the employability of A336 for the recovery of acid from dilute solutions. 4.2.2. Effect of Extractant Concentration. The concentration of extractant (TBP) was varied from 10−50% in decanol, and the equilibrium distribution of citric acid (0.2−0.8 mol kg−1) was obtained. A higher distribution coefficient (KD) was observed at higher TBP concentrations (Table 8). The KD value with 10% TBP in decanol was very low (E = 2.33− 3.60%), which lies in the range physical extraction with decanol. However, the use of 30% TBP in decanol showed enhanced extraction efficiency (E = 9.16−13.54%). A significant increment in extraction efficiency was observed with 50% TBP in decanol (E = 20.67−22.34%) as the percent E is almost thrice that of using decanol alone (E = 6.11−7.00%). Yet, the extraction efficiency does not meet the commercial requirement. Therefore, TBP is not recommended as an extractant for the recovery of citric acid. However, TBP can be used a diluent with specific extractants. The mechanism involved in the extraction of carboxylic acids using TBP is mainly based on solvation that is a physical interaction only. Additionally, it was observed that KD values remain constant over the entire range of initial acid concentrations (0.2−0.8 mol kg−1) using the TBP + decanol extraction system. The equilibrium distribution of citric acid (0.2−0.8 mol kg−1) between the aqueous and organic phases was studied by varying the concentration of TOA (5−20% v/v) in the organic phase (TOA + decanol). The equilibrium distribution showing the effect of TOA concentration on extraction efficiency is given in Table 9. The increases in TOA concentration lead to a

TOA conc % (v/v) 5

10

20

[CA] [CA] (mol kg−1) (mol kg−1) 0.8 0.6 0.5 0.4 0.2 0.8 0.6 0.5 0.4 0.2 0.8 0.6 0.5 0.4 0.2

0.224 0.253 0.245 0.199 0.111 0.434 0.353 0.322 0.244 0.124 0.646 0.522 0.466 0.358 0.181

KD,exp

E (%)

Z

pH

0.436 0.767 0.972 1.104 1.303 1.436 1.536 1.836 1.810 1.729 7.115 8.529 14.733 16.952 12.255

30.38 43.39 49.28 52.47 56.58 58.95 60.57 64.74 64.41 63.35 87.68 89.51 93.64 94.43 92.46

0.978 1.106 1.073 0.869 0.484 1.897 1.773 1.591 1.409 1.066 2.822 2.281 2.038 1.563 0.791

2.17 2.26 2.35 2.41 2.45 2.08 2.16 2.21 2.62 2.91 3.06 3.18 3.32 3.71 3.93


significant augmentation in extraction efficiency. The extraction efficiency was found to be in the range of E = 30.38−56.57% when 5% TOA was employed with decanol. Further increases in TOA concentration (10%) leads to higher percent E (58.95−64.74%). However, when TOA concentration was 20%, a remarkable enhancement in extraction efficiency (E = 87.67− 94.42%) occurred. This could be explained as the higher concentration of TOA fulfils the stoichiometric requirement of TOA. It offers more complexation and thus higher extraction efficiency (>94%) could be obtained. However, use of a higher TOA concentration results in a higher overall process cost due to its high price. Moreover, the higher the concentration of these extractants, the higher its toxic effects to the microorganisms present in the fermentation broth. The higher amine concentration also increases the pH of extraction system, which is undesirable as most of the amine extractants (including F



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TOA) are active in only acidic conditions.18 Therefore, the use of an optimum amount of extractant is recommended, considering all factors such as cost, toxicity, back-extraction, and extent of extraction. The equilibrium distribution of citric acid between the aqueous and organic phases was examined by investigating the effect of A336 concentration in the organic phase (A336 + decanol) (Table 10). Because higher concentrations (>30%) of

are presented in Figure 2. The equilibrium complexation constants (KE1) were analyzed with linear regression analysis

Table 10. Reactive Extraction of Citric Acid (0.2−0.8 mol kg−1) with A336 (10−30% v/v) Dissolved in Decanol at T = 300.15 ± 1 Ka A336 conc % (v/v) 10

20

30

[CA] [CA] (mol kg−1) (mol kg−1) 0.8 0.6 0.5 0.4 0.2 0.8 0.6 0.5 0.4 0.2 0.8 0.6 0.5 0.4 0.2

0.0309 0.0258 0.0155 0.0155 0.0103 0.0619 0.0361 0.0258 0.0258 0.0155 0.0823 0.0781 0.0635 0.0486 0.0295

KD,exp

E (%)

Z

pH

0.0414 0.0454 0.0323 0.0411 0.0540 0.0863 0.0648 0.0549 0.0704 0.0833 0.1182 0.1517 0.1471 0.1417 0.1717

3.973 4.347 3.124 3.947 5.127 7.946 6.086 5.208 6.578 7.692 10.57 13.17 12.83 12.41 14.65

0.0471 0.0393 0.0236 0.0236 0.0157 0.0943 0.0550 0.0393 0.0393 0.0236 0.1468 0.1231 0.1125 0.0819 0.0458

1.87 1.91 1.97 2.02 2.12 2.08 2.21 2.45 2.62 2.88 1.49 1.51 1.52 1.57 1.65

Figure 2. Estimation of extraction equilibrium constant for the 1:1 acid:amine complex in the reactive extraction of citric acid (0.2−0.8 mol kg−1) using 30% A336 and 30% TBP diluted in decanol. Solid lines represent the best fit of the experimental data.

(R2 ∼ 0.98−0.99). The slope of these loading curve (eq 9) directly gives the KE1 values that were estimated to be 0.1648 kg mol−1 and 0.1285 kg mol−1 for the 30% A336 + decanol and 30% TBP + decanol extraction systems, respectively. The loading ratios were analyzed for TOA diluted in three different diluents (decanol, butyl acetate, and benzene). In the extraction involving TOA, there is a marked effect of amine concentration on the loading of extractant. Loading curves for extraction with TOA are presented in Figure 3. Higher loading


A336 in decanol did not result in a significant improvement of the distribution coefficient of the acid, only 10−30% (v/v) A336 was used in the present work. Table 10 represents the effect of A336 concentration (10− 30%) in the decanol extraction system on the extraction efficiency of citric acid. Lower concentrations of A336 provide a much lower distribution coefficient (E = 3.12−5.12%). Thus, A336 is not effective in low concentrations. However, at higher concentrations of A336, high extraction was obtained at all initial acid concentrations (0.2−0.8 mol kg−1) studied. However, when citric acid is recovered from fermentation broth, where the problem of toxicity is a major concern, it is preferable to use extractant in low concentrations. Further, A336 is highly viscous (μ = 1500 Pa·s at 30 °C), so a volume percentage above 30% could create problems of an undesirable third phase formation, especially with inert diluents. The third phase formation could result in delayed settling of phases that leads to difficulty in the separation of the aqueous and organic phases. Also, when 30% A336 was used, the third phase formation was encountered at higher concentrations of citric acid. Consequently, for clear phase separation, centrifugation at 10 000 rpm for 10 min was necessary. 4.3. Stoichiometry of the Acid:Extractant Complex. Loading ratios (Z) for the reactive extraction of citric acid using 30% TBP and 30% A336 diluted in decanol were obtained using eq 7. The loading factor values lie below 0.5 for TBP and A336; thus, only 1:1 acid:extractant complex formation was assumed.1 As decanol is an active diluent, the exact extractant loading can be calculated using the modified loading ratio (Zm) (eq 8). The plots of Zm/(1−Zm) vs [HA]aq for A336 and TBP

Figure 3. Estimation of extraction equilibrium constant for the 2:1 acid:amine complex in the reactive extraction of citric acid (0.2−0.8 mol kg−1) using 20% TOA in different diluents. Solid lines represent the best fit of the experimental data.

ratios were obtained with TOA (Z > 0.5). Kertes and King1 suggested that if Z values are greater than 0.5, overloading of amine can be predicted. Thus, higher complexation is possible in extractions involving TOA. The equilibrium complexation constant (KE2) for the 2:1 complex can be estimated by plotting Zm/(2 − Zm) vs [HA]aq2 (eq 10). The KE2 obtained were 1039.7 (kg mol−1)2, 540.15 (kg mol−1)2, and 184.09 (kg mol−1)2 for butyl acetate, decanol, and benzene, respectively. The highest KE value obtained with butyl acetate divulges its tendency to provide better stability for the acid:amine complex. Also, the alcohols (decanol) could provide significantly higher G



Article

Figure 4. FT-IR image of citric acid (aqueous phase), TOA + decanol (organic phase), and TOA + decanol + citric acid (organic phase) after complexation.

KE values compared to those with inert diluent (benzene) (Figure 3). 4.4. FT-IR Analysis of Acid:Extractant Complexation. The IR plots obtained for the organic phase before and after complexation are shown in Figure 4. The major peaks lying in the range 2800−3000 cm−1 shows the H−C−H asymmetric and symmetric stretch that ensures the presence of alkane chains in the extraction system. The valence oscillation of the hydrogen-bonded O−H stretch was identified at 3400−3500 cm−1. This range confirms the hydrogen bonding of citric acid with the extractant, TOA. There are no peaks in the range of 3100−3500 cm−1 for the organic phase before complexation. This is due to the presence of the tertiary amine tri-noctylamine, for which the N−H stretch is absent. As the tertiary amine behaves like a primary amine after complexation, a broad peak can be seen in the range of 34003500 cm−1. The band at 1717 cm−1 results from the CO stretching vibration in the carboxyl group in the IR curve for the citric acid aqueous solution. This band almost disappears after complexation, which confirms formation of the 2:1 acid:amine complex. 4.5. Modeling and Optimization. 4.5.1. Linear Solvation Energy Relationship Modeling. The LSER model was applied for the reactive extraction of citric acid from an aqueous solution. The regression coefficients are presented in Table 11. The RMSD value was zero for the model fit. The LSER results show the difference in solvatochromic behavior of solvent used in the extraction process. The solvents used in the physical and reactive extraction of citric acid act as basic in nature that results in zero value of a. However, significant values of b show the high proton donor capability of the solvents. The solute solvent interaction is more feasible than the dipole−dipole interactions for physical extraction as the values of s are significant whereas d is zero in all cases. This signifies the presence of dipole−dipole interactions over dipole−induced dipole interactions. The model values of distribution coefficient (Kmod D ) are comparable with experimental values (Kexp D ) for the reactive extraction with TBP,

Table 11. LSER Modeling Results for Physical and Reactive Extraction of Citric Acid [HA]0aq 0.8 0.6 0.4 0.8 0.6 0.4 0.8 0.6 0.4 0.8 0.6 0.4

ln K0D

s

d

physical extraction −5.07696 6.50372 0 −6.11168 8.81263 0 −5.78741 7.67038 0 reactive extraction with 30% TBP −5.17530 7.23255 0 −2.95703 2.75908 0 −3.83232 3.84772 0 reactive extraction with 20% TOA −2.84980 12.03001 0 −3.94220 15.21408 0 2.74912 0.20308 0 reactive extraction with 30% A336 −9.06751 17.33002 0 −6.43889 11.38236 0 −7.17716 13.05726 0

a

b

0 0 0

−6.3926 −10.7577 −8.7586

0 0 0

−25.0635 −18.0632 −16.6996

0 0 0

−24.5802 −30.8641 −6.9670

0 0 0

−24.2179 −13.5920 −16.9022

TOA, and A336. This means that the presence of reactive species does not impart the induced dipole in the solvents. 4.5.2. Optimization of Stoichiometric Parameters Using Differential Evolution (DE). The model eq 6, correlating the distribution coefficient (KD) to the stoichiometries of the acid:extractant complexation reaction, was solved with the differential evolution technique. The objective function based on least-squares error between the experimental and model value of KD is minimized for predicting the stoichiometry of the complexation reaction. The optimized values of m, n, and the equilibrium constants (KE) are presented in Table 12, and consequently the model eq 6 was used for the calculation of the model value of KD. For each extraction system, predicted values of KD (obtained by the DE approach) show good resemblance with the experimental values of KD (Tables 4−7). In the extraction of citric acid with TBP, TOA, and A336, the predicted values of H



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Table 12. Differential Evolution Optimization for Reactive Extraction of Citric Acid extractants

diluents

30% TBP

decanol n-butyl acetate benzene decanol n-butyl acetate benzene decanol n-butyl acetate benzene

20% TOA

30% A336

KE 0.1231 0.1647 0.0433 1035.0 2801.1 273.626 0.2515 0.5831 0.5951

m

n

SE

1.1239 1.3389 0.7925 1.5000 1.7000 1.7000 0.8935 1.3076 1.0824

1 1 1 1 1 1 1 1 1

0.0004 0.0001 0.0000 35.2000 0.0004 0.7269 0.0001 0.0002 0.0003

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], Tel.: + 91-771-2254200, Fax: + 91-771-2254600. Funding

We thank CCOST, Raipur, India for the financial support provided to carry out this work through a sponsored research project (Grant 435/CCOST/MRP/2014). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to the NIT Raipur for infrastructural and analytical support.

KD show little deviation from experimental values of KD at lower acid concentrations. This may be correlated to the minor errors in the measurement of concentration of the aqueous phase when equilibrium extraction is carried out with very low concentrations of acids. Because the higher extent of extraction of citric acid with TOA used in higher concentrations leads to very low equilibrium aqueous phase concentrations, the standard error was observed to be high for the same reason. The stoichiometry of the acid:extractant complex can be predicted with the optimized values of KE, m, and n, which are listed in Table 12. In the extraction of citric acid using TOA, a high stability for the complex was found for all of the diluents, as the KE values obtained are extremely large. That also promotes higher extractant loading, resulting in 2:1 acid:amine complex formation. The formation of acid:amine complexes also depends on the nature of diluents, which affect the basicity of the amine and the stability of the ion pair formed in the extract phase. In the case of active diluents (n-butyl acetate and decanol), the synergistic extraction is remarkably larger due to the simultaneous effect of the physical extraction and chemical interaction through hydrogen bonding. Carboxylic acids are physically more easily extracted by the active diluents alone (nbutyl acetate and decanol) as compared to in inactive diluents (benzene) (Table 12); however, the magnitude of the acid:amine complexation was found to be larger for n-butyl acetate. TOA + n-butyl acetate affects the complex aggregation through hydrogen bonding and dipole−dipole interactions more readily than in the amine-free diluent/acid association. These results show that the diluents are not only involved in the physical extraction but also influence the polarity of the extractant.

ABBREVIATIONS [E] extractant concentration (mol kg−1) [CA] citric acid concentration (mol kg−1) [(CA)m·En] citric acid:extractant complex concentration (mol kg−1) D dimmerization constant (mol kg−1) Ka citric acid disscociation constant KD distribution coefficient KE overall equilibrium complexation constant (mol kg−1) KE1 1:1 acid:extractant equilibrium complexation constant (mol kg−1) KE2 2:1 acid:extractant equilibrium complexation constant (kg mol−1)2 m number of acid molecules in the complex n number of extractant molecules in the complex P partition coefficient Z loading ratio Zm modified loading ratio μ acid taken by diluent alone per unit extractant concentration ν diluent’s fraction in the organic phase aq aqueous phase org organic phase exp experimental value mod model value 0 initial value

5. CONCLUSION A comprehensive study of the reactive extraction equilibria of citric acid along with LSER modeling and process optimization using differential evolution is reported. TOA turned out to be the most efficient extractant (E ∼ 98%) for the recovery of citric acid. However, A336 provided extraction (E ∼ 30%) slightly better than that of TBP (E% ∼ 22%). High removal of acid obtained at lower acid concentrations signifies the success of reactive extraction for the removal of citric acid from dilute aqueous solutions. In cases of TBP and A336, the loading ratio was found to be very low (Z < 0.5); hence, only the 1:1 complex is proposed. However, in case of TOA, based on overloading (Z > 0.5), the 2:1 complex formation was also considered; this was further confirmed by FT-IR analysis. The population-based differential evolution optimization technique proved to be successful tool for predicting the stoichiometry of acid:extractant complex formation during reactive extraction.

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Modeling and Optimization of Reactive Extraction of Citric Acid

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