Extraction of Acrylic, Propionic, and Butyric Acid Using Aliquat 336 in

Dec 2, 2008 - Department of Chemical Engineering, Indian Institute of Technology (IIT) ... Differences in the hydrophobicity and octanol-water partiti...
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Ind. Eng. Chem. Res. 2009, 48, 888–893

Extraction of Acrylic, Propionic, and Butyric Acid Using Aliquat 336 in Oleyl Alcohol: Equilibria and Effect of Temperature Amit Keshav, Kailas L. Wasewar,* and Shri Chand Department of Chemical Engineering, Indian Institute of Technology (IIT) Roorkee, Uttarakhand, 247667 India

Reactive extractions of acrylic, propionic, and butyric acids using Aliquat 336 in oleyl alcohol were carried out to study the effects of temperature (305-333 K). The effects of temperature on the partition (P) and dimerization (D) coefficients were evaluated, and it was found that P decreases with increasing temperature, whereas the effects of temperature on D vary. Chemical extractions using Aliquat 336 in oleyl alcohol at temperatures ranging from 305 to 333 K show an increase in KE(1:1) values with temperature up to 313 K for acrylic and propionic acids but a decrease with increasing temperature for butyric acid over the range studied. Differences in the hydrophobicity and octanol-water partition coefficient are suggested as the reasons for the differences in extraction by the respective acids using Aliquat 336 in oleyl alcohol. The enthalpy (∆H) and entropy (∆S) of reaction were evaluated at different temperatures, and their difference was assessed in terms of different parameters. 1. Introduction Reactive extraction is receiving increasing attention for the recovery of carboxylic acids from fermentation broth or dilute aqueous streams because of its various advantages over conventional and other recovery methods.1-17 Different extractants (tri-n-butyl phosphate, trioctylphosphine oxide, Alamine 336, tri-n-octylamine, Aliquat 336, etc.) are used in reactive extraction recovery processes that can chemically complex with the acid to form acid-extractant complexes. The complexation provides higher yields and selectivities of acid over the nonacidic components. Because these extractants are either solid or viscous, they are dissolved/mixed in diluents, which provide the solvation of the complexes formed and also improves the physical properties of the extractants (viscosity, surface tension at the interfaces, etc.). The effect of temperature is an important subject of study in reactive extraction processes in view of operating temperatures and back-extraction/regeneration steps. This study thus had two objectives: first, to determine the effects of temperature on extraction of the acid from fermentation broth (where temperature usually varies in the range from 308 to 323 K) and, second, to determine whether temperature-swing extraction/regeneration can be employed for recovery of the acid (as discussed later). Tamada and King5 studied the effects of temperature on the extraction of succinic and lactic acids by Alamine 336 in methyl isobutyl ketone (MIBK) (0-348 K) and in chloroform (0-328 K). It was found that extraction decreases with increasing temperature. The apparent enthalpies of association were found to be more exothermic for succinic acid than for lactic acid and more exothermic in chloroform than in MIBK. The decrease in entropy of reaction was higher for succinic acid than for lactic acid and was greater in chloroform than in MIBK. For the systems studied, 1:1 acid-extractant complexation was found to be much more exothermic and to involve a much greater loss of entropy than the formation of 2:l or 3:l complexes. This is reasonable when related to the findings of the authors in an earlier article6 in which it was concluded that 1:1 complexation * To whom correspondence should be addressed. E-mail: [email protected], [email protected]. Tel.: +91-1332285347. Fax: +91-1332-276535.

involves the formation of an ion pair, but higher complexes involve hydrogen-bond formation. Baniel et al.7,8 proposed a temperature-swing extraction/ regeneration scheme. In this approach, the extraction was carried out at a relatively low temperature, producing an acid-loaded organic extract and an aqueous raffinate waste stream containing the unwanted feed components. During regeneration, the extract was contacted with a fresh aqueous stream at a higher temperature to produce an acid-loaded aqueous product stream and an acid-free organic phase. The concentration of the acid achievable in this stream was found to depend on the amount of change in the extraction equilibrium between temperatures and could be higher than that in the original aqueous feed stream. Thus, temperature-swing extraction/regeneration can be attractive for the design of complete reactive extraction process. Harington and Hossain9 studied the effect of temperature (283-313 K) on the extraction of lactic acid using 20% tri-noctylamine in sunflower oil at natural pH. Increasing the operating temperature was found to increase the distribution coefficient in the organic phase. The trend was suggested to be a positive result because the optimal temperature of fermentation broths is around 38 °C. Therefore, performing the extraction at this temperature would allow greater extraction than at room temperature. Aliquat 336 is a quaternary amine that extracts carboxylic acids by an ion-exchange mechanism. A good amount of work on reactive extraction of different carboxylic acids using Aliquat 336 can be found in the literature. Lazarova and Peeva10 found that, in the extraction of lactic acid by Aliquat 336, the logarithm of the distribution coefficient increases with increasing pH up to pH of 6-7 and decreases thereafter. Kyuchoukov et al.11 studied the extraction of lactic acid with Aliquat 336 dissolved in dodecane and decanol under various experimental conditions. The effect of pH was also studied. Yang et al.12 studied the extraction of carboxylic acids with tertiary and quaternary amines under various pH values ranging from 2.0 to 8.5. Pure quaternary amines were found to provide higher distribution coefficients than tertiary amines. Extraction was conducted with two diluents: kerosene and 2-octanol. Neither diluent was found to be active when used with Aliquat 336. Uslu et al.3 studied the reactive extraction of propionic acid using Aliquat 336

10.1021/ie8010337 CCC: $40.75  2009 American Chemical Society Published on Web 12/02/2008

Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 889

(quaternary amine) dissolved in five pure solvents (cyclohexane, hexane, toluene, methyl isobutyl ketone, and ethyl acetate) and binary solvents (hexane + MIBK, hexane + toluene, and MIBK + toluene) under various experimental conditions. Their results and the observed phenomena were discussed in terms of the mechanism of extraction and the concentrations of the interaction products in the aqueous phase. In all cases, 1:1 acid-amine complexes were formed with no overloading. Although much work on the extraction equilibria of monocarboxylic acids can be found in literature,3,13-19 no studies on the effects of temperature on the extraction of these acid (acrylic, propionic, and butyric acids), particularly using quaternary amines, could be identified. In the present work, the effects of temperature on the extractions of acrylic, propionic, and butyric acids using Aliquat 336 in oleyl alcohol were studied. Data on the effects of temperature on the physical and chemical extraction, the equilibrium complexation constant, the distribution coefficient, and the enthalpy and entropy of reaction are reported.

Figure 1. Physical equilibria for the extraction of acrylic, propionic, and butyric acids using oleyl alcohol at 305 K. for the Extraction of Table 1. Effects of Temperature on Kdiluent D Different Monocarboxylic Acids (Acrylic, Propionic, and Butyric Acids) Using Oleyl Alcohol Kdiluent D

2. Materials and Methods

acid

Aliquat 336 (methyltricaprylammonium chloride, C25H54NCl) (molecular mass, 404.17 g/mol; density, 0.889 g/cm3), a quaternary amine, was used as an extractant, and oleyl alcohol (molecular mass, 268.49 g/mol; density, 0.85 g/cm3) was used as a diluent. The choice of using oleyl alcohol as a diluent was based on its nontoxic characteristic (log P ) 7.69, where log P is the logarithm of the distribution coefficient of the solvent in a standard octanol-water two-phase system).20 Acrylic acid (99%), propionic acid (99%) and butyric acid (99%) (Himedia, Mumbai, India) were of technical grade and were used without pretreatment. Aqueous solutions of acid (0.05-0.4 mol/L) were prepared using distilled water. Low concentrations of acid were used because the acid concentration in the fermentation broth is no greater than 0.5 mol/L. 21 Extraction experiments involved the shaking of equal volumes (20 cm3) of aqueous and organic phases for 12 h at constant temperature (305-333 K) in a water bath (Remi Instruments Ltd., Mumbai, India), followed by settling of the mixture for at least 2 h at the same temperature in a separating funnel. Aqueous phase-acid concentrations were determined with a highperformance liquid chromatography system (Waters 1523) composed of a binary pump, a refractive index detector (Waters 2414), and dual λ absorbance detectors (Waters 2487). The column used was C-18. The sample was eluted by 0.25 mol/L aqueous ammonium dihydrogen phosphate solution adjusted to a pH of 2.2 by an aqueous phosphoric acid solution at a rate of 2.0 mL/min in a reverse-phase C-18 column (4-mm i.d. × 150mm length). The acid content in the organic phase was determined by mass balance. A few experiments were carried out in duplicate, and consistency was found to within (2%.

acrylic

propionic

butyric acid

[HA]0 (mol/L)

305

0.05 0.10 0.20 0.40 0.05 0.10 0.20 0.40 0.05 0.10 0.20 0.40

K

1.14 1.73 0.94 0.89 0.82 0.69 0.71 0.84 2.75 2.16 2.33 2.64

313

K

1.08 2.98 1.48 1.38 1.08 1.08 0.98 1.05 2.65 4.25 3.02 2.89

Extraction of acid by diluent alone was explained in terms of three phenomena: (1) ionization of acid in the aqueous phase (KHA), (2) partition of the undissociated acid into the organic phase (P), and (3) dimerization of acid in the organic phase (D). 22 These can be described in terms of the following reactions: (1) Ionization of the acid in the aqueous solution [HA]aq T H+ + A-

(1)

KHA ) [H+][A-]/[HA]

(2)

K

1.08 2.24 1.08 1.12 0.82 0.82 0.72 0.85 2.65 4.47 2.89 2.89

333

K

1.24 2.50 1.38 1.17 0.88 1.16 0.98 1.10 3.17 5.03 3.17 3.24

(2) Partition of the undissociated molecular acid between the two phases, aqueous (aq) and organic (org) [HA]aq T [HA]org

(3)

P ) [HA]org/[HA]aq

(4)

(3) Dimerization of the acid in the organic phase 2[HA]org T [HA]2org

(5)

D ) [HA]2,org/[HA]2org

(6)

The overall distribution coefficient for physical extraction (KDdiluent) can be written in terms of these parameters as KDdiluent )

P + 2P2D[HA]aq 1 + KHA/[H+]aq

(7)

For the dilute concentrations of acid used in this study, it can fairly be assumed that the second term in the denominator of eq 7 can be neglected. Thus KDdiluent ) P + 2P2D[HA]aq

3. Results and Discussion

323

(8)

The extractions of acrylic, propionic, and butyric acids using oleyl alcohol are presented in Figure 1. Typical values of Kdiluent D at 305 K for the extraction of 0.2 mol/L acid using oleyl alcohol alone were found to be 0.94, 0.71, and 2.33 for acrylic, propionic, and butyric acids, respectively. As shown in Table 1, at T ) 305-333 K, except for butyric acid, the physical extraction was poor, with KDdiluent values less than 1. Further, there was only a slight effect of temperature on physical extraction. Table 2 lists the values of the partition (P) and dimerization (D) coefficients from eq 8 for the extractions of acrylic, propionic, and butyric acids using oleyl alcohol at

890 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 Table 2. Partition (P) and Dimerization Coefficient (D) Values for the Extraction of Different Carboxylic Acids at Different Temperatures (305-333 K) acid acrylic

propionic

butyric acid

temperature (K)

P

D (L/mol)

305 313 323 333 305 313 323 333 305 313 323 333

1.45 1.60 1.63 1.89 0.74 0.75 0.76 0.97 2.42 2.51 2.66 3.14

0.37 0.35 0.25 0.23 0.07 0.16 0.18 0.22

different temperatures (T ) 305-333 K). The partition coefficient (P) was found to increase with increasing temperature for all of the acids studied (Figure 2). The partition coefficients follow the trend butyric acid > acrylic acid > propionic acid. No dimerization was observed for acrylic acid. For propionic acid, D was found to decrease with increasing temperature, whereas for butyric acid, an increase in D with temperature was observed (Figure 3) A similar trend was obtained for the extraction of lactic acid by MIBK, where an increase in temperature from 273 to 348 K resulted in an increase in the P value and a decrease in the D value.5 Chemical extraction equilibria were studied using 20% and 30% Aliquat 336 in oleyl alcohol. Higher concentrations of Aliquat 336 were not chosen because of the high viscosity of Aliquat 336, which results in formation of a three-phase mixture during extraction. Chemical extraction occurred through a

Figure 4. Chemical equilibria for the extraction of 0.05-0.4 mol/L acrylic, propionic, and butyric acid using 30% Aliquat 336 in oleyl alcohol at 305 K.

complexation reaction between acid ([HA]) and Aliquat 336 ([R4N+Cl-]), which can result in 1:1 ([R4N+Cl-:HA]), 2:1 ([R4N+Cl-:(HA)2]), or n:1 ([R4N+Cl-:(HA)n]) acid-Aliquat 336 complexes according to the following reactions1 KE(1:1)

[HA]aq + [R4N+Cl-]org T [R4N+Cl-:HA]org

(9)

KE(2:1)

[HA]aq + [R4N+Cl-:HA]org T [R4N+Cl-:(HA)2]org (10) l KE(n:1)

[HA]aq + [R4N+Cl-:(HA)n-1]org T [R4N+Cl-:(HA)n]org (11) The equilibrium complexation constant (KE(n:1)) for the reactions represented by the above equations can be written as KE(n:1) )

[R4N+Cl-:HA]org [R4N+Cl-]org[HA]aqn

(12)

The overall distribution coefficient (KDoverall) is evaluated as KDoverall )

[HA]org [HA]aq

(13)

The degree of extraction (E, %) is expressed as E (%) ) KDoverall × 100/(1 + KDoverall)

Figure 2. Effects of temperature (305-333 K) on physical partition coefficient for the extraction of different carboxylic acids using oleyl alcohol.

(14)

The extent to which the organic phase (amine + diluent) can be loaded with carboxylic acid is expressed in terms of the loading ratio, z z)

[HA]org o [R4N+Cl-]org

(15)

The value of z in eq 15 was found to be dependent on the strength of the acid-extractant interaction and the aqueous concentration of acid. The extractions of acrylic, propionic, and butyric acids using Aliquat 336 in oleyl alcohol resulted in loading ratios of less than 0.5 at all acid/extractant concentration ratios. Therefore, 1:1 complexes were assumed to be formed, and the stoichiometry of the overall extraction reaction was found to be function of z as

Figure 3. Effects of temperature (305-333 K) on physical dimerization coefficient for the extraction of different carboxylic acids using oleyl alcohol.

z ) KE(1:1)[HA]aq (16) 1-z z/(1 - z) can thus be plotted against the aqueous concentration of acid ([HA]aq) to obtain the equilibrium complexation values. Figure 4 shows the chemical extraction equilibria plots for the extractions of acrylic, propionic, and butyric acids using

Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 891 Table 3. Effects of Temperature (305-333 K) on Reactive Extraction of Different Monocarboxylic Acids Using Aliquat 336 in Oleyl Alcohol [B]0 ) 0.44 mol/L acid acrylic

[HA]0 (mol/L)

KD

E (%)

z

KD

E (%)

z

KE(1:1) (L/mol)

∆H (kcal/mol)

305

0.05 0.1 0.2 0.4 0.05 0.1 0.2 0.4 0.05 0.1 0.2 0.4 0.05 0.1 0.2 0.4 0.05 0.1 0.2 0.4 0.05 0.1 0.2 0.4 0.05 0.1 0.2 0.4 0.05 0.2 0.3

0.92 1.13 1.29 1.42 1.10 1.17 1.38 1.22 1.12 1.32 1.50 1.49 0.86 0.95 1.09 0.99 0.72 0.82 0.94 1.54 0.73 0.86 1.05 1.42 3.21 5.49 8.44 9.12 3.12 5.34 5.29 6.58 2.73 3.67 4.93 5.49

47.92 53.14 56.37 58.69 52.43 53.94 57.98 54.95 52.83 56.91 60.00 59.80 46.18 48.75 52.26 49.66 41.89 45.00 48.48 60.63 42.07 46.10 51.23 58.68 76.25 84.60 89.40 90.12 75.73 84.23 84.11 86.80 73.20 78.60 83.13 84.60

0.086 0.121 0.256 0.677 0.064 0.123 0.264 0.500 0.091 0.194 0.273 0.544 0.052 0.154 0.238 0.451 0.152 0.205 0.317 0.689 0.142 0.210 0.699 0.741 0.109 0.192 0.406 0.705 0.095 0.383 0.573 0.789 0.104 0.179 0.378 0.769

1.00 1.53 1.48 1.62 1.81 1.48 1.69 1.75 1.38 1.58 1.53 1.38 1.13 1.43 2.03 1.55 0.73 0.90 1.00 1.10 0.74 0.89 1.13 1.31 6.41 6.58 7.02 7.05 4.24 5.82 5.34 5.49 4.24 4.45 5.49 5.34

49.90 60.41 59.60 61.83 64.45 59.60 62.83 63.64 57.98 61.22 60.41 57.98 53.13 58.76 67.02 60.72 42.27 47.23 50.09 52.33 42.60 47.20 52.98 56.75 86.51 86.80 87.53 87.57 80.93 85.33 84.23 84.60 80.93 81.67 84.60 84.23

0.038 0.092 0.181 0.344 0.049 0.090 0.190 0.386 0.044 0.093 0.183 0.351 0.040 0.124 0.203 0.368 0.142 0.186 0.228 0.357 0.087 0.107 0.241 0.369 0.066 0.132 0.265 0.531 0.061 0.129 0.255 0.513 0.061 0.124 0.256 0.511

3.10

-12.65

4.38

-0.871

-12.95

3.93

-0.887

-36.83

-27.42

-0.915

313

333

propionic

305

313

333

butyric

[B]0 ) 0.66 mol/L

T (K)

305

313

333

0.05 0.1 0.2 0.4

30% Aliquat 336 in oleyl alcohol at 305 K. Chemical extraction is far better than physical extraction, with KDoverall following the trend butyric acid > propionic acid > acrylic acid. The values of E (%) for acrylic, propionic, and butyric acids were found to lie in the ranges of 50-60%, 45-65%, and 75-90%, respectively. Higher loading ratios were obtained for higher concentrations of acid at both Aliquat 336 concentrations (20% and 30% in oleyl alcohol). Further, it was found that higher concentrations of Aliquat 336 gave higher extractions at all acid concentrations. This suggests that 30% Aliquat 336 can be chosen as the optimum concentration of Aliquat 336 at which high extractions can be obtained with no problems of threephase formation, difficulty in phase separation, high turbidity, etc. Equilibrium complexation constants were evaluated to be 3.1, 3.76, and 15.23 L/mol for the extractions of acrylic, propionic, and butyric acids, respectively, using Aliquat 336 in oleyl alcohol. Again, the higher KE(1:1) value for butyric acid suggests that Aliquat 336 can extract butyric acid better than the other two acids. Generally, the value of E (%) depends on three major factors: (1) the association ability between acid and Aliquat 336, pKb - pKa; (2) the hydrophobicity of the solute, log P; and (3) the steric effect between the solute and the extractant.23 The hydrophobicity of the solute should be the key factor because of the similar molecular structures. The trend of hydrophobicity values for the three acids [acrylic acid (pKa ) 4.2, log P ) 0.16), propionic acid (pKa ) 4.67, log P ) 0.29), and butyric acid (pKa ) 4.78, log P ) 0.751)] clearly signifies that the higher the pKa and log P values of the acid, the higher the equilibrium complexation constant and, hence, the higher the extraction.

∆S [cal/(mol K)]

∆Htrans

4.69

3.11

3.78

3.94

3.71

15.25

12.31

11.07

Higher KE(1:1) values can also be correlated with the properties of the acids. In the present case, at 305 K, it was found that higher molecular weight and lower density of the acid led to a higher KE(1:1) value [butyric acid (molar mass, 88.11; density, 0.957 g/cm3), propionic acid (molar mass, 74.08; density, 0.99 g/cm3), acrylic acid (molar mass, 72.06; density, 1.05 g/cm3)]. The effect of temperature on the reactive extractions of acrylic, propionic, and butyric acids using Aliquat 336 in oleyl alcohol was studied (Table 3) as the temperature was varied from 305 to 333 K. The extraction of carboxylic acids by extractant occurs by intermolecular hydrogen bonding or ion exchange of the extractant group with the acid. The extraction of propionic acid by extractant-acid complexation is expected to be exothermic and to make the system more ordered. This increase in order means that a decrease in entropy is expected for the reaction.5 Further, the system becomes more ordered if the interaction between the acid and the extractant is strong. Figures 5-7 show plots of eq 16 for acrylic, propionic, and butyric acids, respectively, to determine KE values at different temperatures. If the enthalpy and entropy of reaction are assumed to be constant over the temperature range, the equilibrium complexation constant is related to temperature by the equation5 -∆H ∆S + (17) RT R The above expression indicates that a plot of ln KE vs 1/T should give a straight line. The more exothermic the reaction, the more sensitive the equilibrium to changes in the temperature. The slope is proportional to the enthalpy of reaction (∆H), and ln KE )

892 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009

Figure 5. Plot of z/(1 - z) vs [HA]aq for the determination of the 1:1 equilibrium complexation constant for the extraction of 0.05-0.4 mol/L acrylic acid using Aliquat 336 in oleyl alcohol at different temperatures (305-333 K).

336 in oleyl alcohol follow the trend acrylic acid > propionic acid > butyric acid. The overall effect of temperature is attributed to the effects of different parameters such as the acid pKa, the amine-acid interaction, the solubility of the acid in both phases, the extractant basicity, and water coextraction. These parameters were studied by Canari and Eyal,24 who concluded that the pKa values of common carboxylic acids decrease only slightly as the temperature is increased. Similarly, pKa measurements25 for carboxylic acids exhibit a small decrease as the temperature is increased but only up to a given point. Above this point, the pKa value increases. Similarly, the solubilities of acids in both the aqueous and extractant phases are affected by temperature. The solubilities of oxalic, malonic, succinic, adipic, maleic, malic, citric, and tartaric acids in water increase as the temperature is increased from 278.15 to 338.15 K.26 In all of these cases, the solubility in water increases as the temperature is increased. A more negative value of ∆H for a particular acid suggests that the extraction is more exothermic. This might be due to differences in the partial molar heat of mixing of the complex in the solvent, the partial molar heat of mixing of the acid in the aqueous phase, or ∆Htrue. King et al.5 evaluated the organic-phase heat of mixing using partition coefficient values (P) for the extraction of acids by diluents alone by the following relation d ln P ) -∆Htransfer d(1/T)

Figure 6. Plot of z/(1 - z) against [HA]aq for the determination of the 1:1 equilibrium complexation constant value for the extraction of 0.05-0.4 mol/L propionic acid using Aliquat 336 in oleyl alcohol at different temperatures (305-333 K).

(18)

where ∆Htransfer is the heat of transfer from the organic to the aqueous phase. Table 3 lists ∆Htransfer values for the extractions of different carboxylic acids using oleyl alcohol only. It can be seen that -∆Htransfer for all of the studied acids follows the trend butyric acid > propionic acid > acrylic acid. Thus, this suggests that the higher ∆H value of the acid can be attributed to the higher value of -∆Htransfer. 4. Conclusions

Figure 7. Plot of z/(1 - z) against [HA]aq for the determination of the 1:1 equilibrium complexation constant value for the extraction of 0.05-0.4 mol/L butyric acid using Aliquat 336 in oleyl alcohol at different temperatures (305-333 K).

the intercept is proportional to the entropy (∆S). Table 3 reports the values of ∆H and ∆S obtained according to the fit of eq 17 for the extractions of different carboxylic acids using Aliquat 336 in oleyl alcohol. The relatively large differences in enthalpy and entropy loss between the acids for 1:1 complexation are consistent with the interaction of oleyl alcohol with the complexes, which is specific hydrogen bonding. Butyric acid-Aliquat 336 complexation was found to be more exothermic and to lead to higher increases in the order (larger decreases in the entropy) of the system than that for the other two acids. The values of ∆H and ∆S for extraction using Aliquat

The effects of temperature on the extractions of different monocarboxylic acids (acrylic, propionic, and butyric acids) were studied, and the following results were found: (1) Partition (P) and dimerization (D) coefficients are affected by temperature. P increases with increasing temperature with the trend butyric acid > acrylic acid > propionic acid. D could not be found for acrylic acid. For propionic acid, it decreased with increasing temperature, whereas the opposite trend was observed for butyric acid. (2) Chemical extraction is better than physical extraction, with higher extraction for 30% Aliquat 336 in oleyl alcohol. Using Aliquat 336 concentrations above 30% resulted in the formation of three-phase mixtures because of the high viscosity of Aliquat 336. (3) Extraction equilibrium constant for 1:1 acid-extractant complexes (KE(1:1)) were evaluated for the extractions at T) 305, 313, and 333 K. For acrylic and propionic acids, KE(1:1) increased from 305 to 313 K, after which it decreased. However, for butyric acid, KE(1:1) decreased with increasing temperature for the range studied. The difference in the KE(1:1) values of the acids studied can be explained of the differences in their hydrophobicity values. (4) ∆H values for the extractions of acrylic, propionic, and butyric acids were determined to be -12.65, -12.95, and -36.83 kcal/mol, respectively, and the corresponding ∆S values were 4.38, 3.93, and -27.42 cal/(mol K). Butyric acid-Aliquat 336 complexation was found to be more exothermic and to result

Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 893

in higher increases in the order (greater decreases in the entropy) of the system than for the other two acids. The higher ∆H value of butyric acid can be attributed of its higher value of -∆Htransfer. Acknowledgment K. L. Wasewar acknowledges the Department of Science and Technology (DST), Government of India, for financial support under Young Scientist Project SR/FTP/ETA-43/2005, Reactive Extraction of Propionic Acid. Nomenclature [A-] ) concentration of anion (mol/L) D ) dimerization coefficient of acid in the organic phase (L/mol) E ) degree of extraction [H+] ) concentration of hydroxyl ion (mol/L) [HA] ) concentration of acid (mol/L) KDdiluent ) distribution coefficient of acid using diluent alone KDoverall ) overall distribution coefficient KE or KE(1:1) ) extraction equilibrium constant for 1:1 acid-extractant complex (L/mol) KE(2;1) ) extraction equilibrium constant for 2:1 acid-extractant complex [(L/mol)2] KE(n:1) ) extraction equilibrium constant for n:1 acid-extractant complex [(L/mol)n] log P ) hydrophobicity of the solute P ) partition coefficient of the undissociated acid in the organic phase [R4N+Cl-] ) concentration of extractant (mol/L) [R4N+Cl-:HA] ) concentration of 1:1 acid-extractant complex (mol/L) [R4N+Cl-:(HA)n] ) concentration of n:1 acid-extractant complex (mol/L) [R4N+Cl-:(HA)2] ) concentration of 2:1 acid-extractant complex (mol/L) z ) loading ratio ∆H ) enthalpy of reaction (kcal/mol) ∆Htransfer ) heat of transfer from the organic phase to the aqueous phase ∆S ) entropy of reaction [cal/(mol K)] Subscripts 0 ) initial aq ) aqueous phase org ) organic phase

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ReceiVed for reView July 3, 2008 ReVised manuscript receiVed October 15, 2008 Accepted October 15, 2008 IE8010337