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Equilibrium and Kinetic Studies on Reactive Extraction of Pyruvic Acid with Trioctylamine in 1-Octanol Mustafa E. Marti,†,‡,* Turker Gurkan,† and L. K. Doraiswamy§ †
Department of Chemical Engineering, Middle East Technical University, Ankara, Turkey Department of Chemical Engineering, Selcuk University, Konya, Turkey § Department of Chemical and Biological Engineering, Iowa State University, Ames, Iowa, United States ‡
ABSTRACT: Interest in pyruvic acid has been growing due to the increase in its potential areas of use and its importance in metabolic reactions. These reasons along with the limitations on recovery have prompted researchers to consider novel recovery techniques. Reactive extraction has been proposed as a promising approach to the recovery of carboxylic acids. In this study, equilibrium and kinetic data were obtained for reactive extraction of pyruvic acid using trioctylamine (TOA) or Alamine 336 in 1-octanol or oleyl alcohol. The results showed that, without pH adjustment in the aqueous phase, and without the use of an extractant, 1-octanol extracted more pyruvic acid than oleyl alcohol with a distribution coefficient (KD) of 0.30. This trend remained the same when tertiary amines were used as an extractant. The KD values did not significantly differ with TOA or Alamine 336. The recovery of pyruvic acid was observed to increase as a function of TOA concentration and the stoichiometry of the reaction was mainly 1:1. As tertiary amines react only with undissociated acids, an increase in the initial pH of the aqueous phase lowered the KD values. When the pH was 4.0, the effect of TOA concentration on pyruvic acid extraction disappeared and for all concentration levels a distribution coefficient of 0.10 was obtained. Kinetic measurements showed that the reaction between pyruvic acid and TOA in 1-octanol is first order with respect to the two reactants with a rate constant of 0.94 L mol1 s1. The enhancement factor was calculated as 25.
1. INTRODUCTION Pyruvic acid is an important product of the industry. It is used in the biosynthesis of pharmaceuticals and agrochemicals; and as a precursor of some amino acids.1,2 As an ingredient in foods, it has a role in fat metabolism.3 Pyruvic acid has been produced by chemical methods; however, biocatalytic reactions and fermentation technology have started to replace conventional syntheses in the industry.4 It has been reported that downstream processes in biotechnological productions account for 60% of the total cost.5 The increase in demand and production cost of conventional processes have prompted researchers to consider novel production and recovery strategies. Synthesis of chemicals by means of biotechnological approaches is still restricted due to limitations on product recovery. An alternative separation technique for the recovery of carboxylic acids is reactive extraction, which has been used successfully to recover acetic, lactic, citric, propionic, tartaric, malonic, and other carboxylic acids.613 This method has the advantages of simplicity, energy efficiency, and providing high distribution coefficient (KD) values. Moreover, thermal stability of the products is not affected by the recovery system during the separation process.14 A reactive extraction system must achieve a high distribution coefficient value with good selectivity. Thus the most appropriate chemicals need to be selected for the recovery system. Kertes and King (1986) classified extractants as (1) oxygen-bearing hydrocarbons (MIBK, octanol, etc.); (2) phosphorus-bonded oxygenbearing extractants (tributyl phosphate, trioctyl phosphineoxide etc.); (3) high-molecular-weight aliphatic amines (alamine, TOA, aliquat, etc.).15 Solvents from the first two categories are nonreactive and perform the extraction by forming donor bonds, r 2011 American Chemical Society
while aliphatic amines form acidamine complexes with carboxylic acids which significantly increase recoveries. Tertiary amines (TOA, Alamine 336, etc.) and quaternary amines (Aliquat 336) are stated to be effective for the extraction of carboxylic acids, and their use is becoming widespread.1517 The stability of the acidamine complex is affected by the basicity of the amine and can be altered using various diluents. Moreover, solvent improves reactive extraction of the acid by reducing the viscosity. It has been shown that aliphatic amines dissolved in proper diluents yield higher KD values for lactic acid than they do without a solvent.18 Polar diluents such as alcohols, ketones, and ethers provide higher solvation for the acidamine complex and, due to their ability to accept and donate protons; alcohols yield the highest partition coefficients.15 There has been an increased interest in recovery of carboxylic acids by reactive extraction in recent years with the focus on reaction parameters as well as optimization of the equilibrium and kinetic conditions of the recovery systems.17,19,20 Investigators examined the effects of pH of the aqueous phase, temperature of the recovery system, types of diluents and extractants, composition of the phases and the concentrations of the components on the recovery of carboxylic acids by reactive extraction.11,12,15,21 Others have attempted to produce carboxylic acids by extractive fermentation.3,14,2224 Since fermentation broths often contain several acids, attempts were made to Received: March 29, 2011 Accepted: October 10, 2011 Revised: October 7, 2011 Published: October 10, 2011 13518
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selectively recover carboxylic acids from mixed acid solutions.7,8,25 It was shown that the pKA of the acid and initial pH of the aqueous solution affect the amount of acid extracted during reactive extraction.2628 In the present study, optimization of process conditions for reactive extraction of pyruvic acid was carried out using TOA and Alamine 336 (a mixture of trioctyl/decyl-amine) dissolved in 1-octanol and oleyl alcohol. 1-Octanol, TOA, and Alamine 336 were chosen due to their demonstrated effectiveness in extraction of the carboxylic acids.10,11,20,26,29 Oleyl alcohol was selected as it is known to be a nontoxic solvent to many carboxylic acid producing microorganisms.12,22 Distribution coefficient and loading ratio values were determined through equilibrium studies while the rate constant and the order of the reaction were estimated using the experimental kinetic data and a model proposed by Doraiswamy and Sharma (1984).30
diluent (HAd) is calculated by using eq E3 and subtracted from the total acid concentration in the organic phase to determine the amount of acid reacted with the extractant (HAex). In eq E3, ν is the volume fraction of the diluent in the organic phase and KD,d is the diluent/water distribution coefficient of the acid. ½HA d org ¼
HA aq þ TA org h ðHA TAÞorg ½ðHA TAÞorg
2. THEORETICAL APPROACH
HA aq þ H2 O h H3 O
þ
þ
A aq
HA aq h HA org
ðR2Þ
(3) Complex formation between the carboxylic acid and tertiary amine (TA) in the organic phase to form an ammonium salt. HA org þ TA org h ðHA TAÞorg
KD ¼
½HA t org
ðE1Þ
½HA t aq
pH ¼ pKA þ log
½A aq ½HAaq
z¼
While undissociated carboxylic acid reacts with aliphatic amine, it can also be extracted by active solvents such as alcohols, ketones, and ethers.11 Hence, the concentration of the acid extracted by
ðE4Þ
½ðHA m TAÞorg ½HAaq ½ðHA m1 TAÞorg
ðR5Þ ðE5Þ
½HA ex org ½TAorg, 0
ðE6Þ
Kertes and King (1986) proposed eq E7 (a combination of eqs E4 and E6) to calculate the equilibrium complexation constant using the loading ratio and equilibrium concentration of the acid in the aqueous phase.15 For loading ratio values less than 0.5, a straight line passing through the origin yields the equilibrium complexation constant for a 1:1 complex. For higher values of z, a plot of the data in the form of eq E8 may yield a straight line, whose slope gives the equilibrium complexation constant, KEm, for a (m:1)-type complex. z ¼ KE ½HAaq ðE7Þ 1z z ¼ KEm ½HAmaq mz
! ðE2Þ
ðR4Þ
To quantify the relative excess of the extractant, a loading ratio (z) is defined and calculated using eq E6. A plot of the loading ratio versus the acid and amine concentrations gives an estimate of the stoichiometry and the types of complexes formed in the organic phase during the reactive extraction of the acid.15
ðR3Þ
Equation E1 shown below is used to calculate the experimentally measured distribution coefficient, which is the ratio of the total acid concentration in the organic phase to that in the aqueous phase. The concentration of the undissociated acid in the aqueous phase is critical since it is the only species that can react with the amine in the organic phase. At a given pH and total acid concentration, the amount of undissociated acid can be calculated with the HendersonHasselbach equation (eq E2).
½HAaq ½TAorg
HA aq þ ðHA m1 TAÞorg h ðHA m TAÞorg
ðR1Þ
Partitioning of the undissociated acid between the aqueous and organic phases.
ðE3Þ
Besides a 1:1 type acidamine complex, m:1 type complexes have also been observed with the help of spectroscopic studies. Investigators found that the amine and the extracted acid form a 1:1 acidamine ion-pair. Additionally, a hydrogen bond may form between the CO of the carboxylate of the first acid and the OH of the carboxyl of the second acid to form a 2:1 complex.31 For an m:1 type complex, R5 is valid and the complexation constant can be evaluated by eq E5.
K Em ¼ (2)
ð1 þ vKD, d Þ
The overall equation for the reactive extraction of an undissociated carboxylic acid with a tertiary amine is represented by R4; and the equilibrium complexation constant (KE) can be estimated using eq E4.
KE ¼ 2.1. Equilibrium. Equilibrium data were interpreted by the Mass Action Law. Kertes and King (1986) applied this law to reactive extraction of a carboxylic acid with a tertiary amine dissolved in an organic solvent.15 Only the undissociated portion of the carboxylic acid present in the aqueous phase is extracted by the alcohols and reacts with the tertiary amines. Chemical equilibrium of the system is described in terms of the following steps: (1) Ionization of the carboxylic acid (HA) in the aqueous phase.
KD, d ½HAaq, 0
ðE8Þ
2.2. Kinetics. Doraiswamy and Sharma (1984) proposed a comprehensive theory of extraction accompanied with chemical reaction to determine the effect of reaction on mass transfer.30 They used film and surface renewal theories with the relevant physicochemical and hydrodynamic parameters and classified 13519
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the reactive systems into four categories depending on the relative diffusion and reaction rates: Very Slow, Slow, Fast, and Instantaneous Reaction Regimes. Doraiswamy and Sharma (1984) have also provided guidelines to verify the mechanism of the extraction accompanied with a chemical reaction. To determine the regime of the reaction, effects of speed of agitation and volume ratio of phases should be investigated. The order and rate constant of the reaction can be calculated using the kinetic data.32
3. MATERIALS AND METHODS 3.1. Materials. Pyruvic acid (Merck Co., 98%), trioctylamine (Fluka, 98%), Alamine 336 (Cognis, 95%), oleyl alcohol (Merck Co.) and 1-octanol (Merck Co.) were used as received without any pretreatment. UHP water, produced by Millipore Milli-Q Water System, was utilized in the experiments. To adjust the pH of the aqueous phase, 0.1 M NaOH and HCl were used. 3.2. Equilibrium Studies. Equal volumes (10 mL) of organic and aqueous phases were equilibrated by shaking for 12 h at 298 K in a constant temperature shaker bath (GFL 1083). After reaching equilibrium, the phases were centrifuged at 10000g for 15 min and allowed to settle for 1 h to have a clear separation of the phases. The aqueous phase was sampled by a pipet leaving a portion in the sample tube to prevent contamination. Pyruvic acid concentration in the aqueous phase was measured by HPLC (Shimadzu) with a MetaCarb 87H column (300 7.8 mm, Varian) at 323 K using 0.008 N H2SO4 as the mobile phase with a flow rate of 0.6 mL min1. Concentration determination was performed by means of a PDA detector (Shimadzu SPD-M20A) at 210 nm. Equilibration experiments and the chemical analyses were performed in duplicate. Distribution coefficient and loading ratio values were calculated using the average values of the data. The relative uncertainty of the data was 0.1% and that of replicated experiments was 2%. Pyruvic acid concentration in the organic phase was calculated by a mass balance about the aqueous phase. 3.3. Kinetic Studies. A Lewis-type stirred cell, 0.045 m in diameter (ID) and 0.1 m in height, was used to obtain kinetic data (Figure 1). The temperature of the system was maintained constant by circulating water at 298 K through the jacket of the stirred cell. This was followed by addition of the aqueous phase, adjustment of the speed of the four blade paddle by an overhead mechanic stirrer (IKA RW 20 DZM), and addition of the organic phase. Since an optimum speed of agitation is critical in obtaining an interfacial area equal to the geometric area, a maximum agitation rate of 1.33 rev s1 was selected. When N was less than 1.00 rev s1, lower RHP values were obtained. This was most probably due to incomplete renewal of the surface, resulting in a mass transfer resistance-controlled condition. For this reason, N was varied between 1.00 and 1.33 rev s1. Samples were periodically taken from the aqueous phase and assayed for residual pyruvic acid concentration using an HPLC (Shimadzu) with a IOA-1000 column (300 7.8 mm, Alltech) at 333 K, using 0.008 N H2SO4 as the mobile phase with a flow rate of 0.4 mL min1. Concentration determination was performed by a UV detector (Shimadzu SPD-10A) at 210 nm. Kinetic experiments and sample analyses were performed in duplicate. The relative uncertainties of the kinetic data and the replicated experiments were 0.1% and 2%, respectively. Average values were used, and pyruvic acid concentration in the organic phase was determined by a mass balance about the aqueous phase.
Figure 1. A schematic diagram of a Lewis-type stirred cell used in the study (all dimensions are in millimeters).
Figure 2. Equilibrium isotherms for reactive extraction of pyruvic acid with tertiary amines in 1-octanol and oleyl alcohol.
4. RESULTS AND DISCUSSION 4.1. Equilibrium. 4.1.1. Comparison of Diluents and Extractants. It is known that a higher polarity of the active diluent
results in a higher value of the distribution coefficient. Thus, partitioning of the carboxylic acids is affected by chemical properties such as molecular weight and dielectric constant of the organic solvents. It is seen in Figure 2 that the equilibrium concentration of pyruvic acid in the organic phase increases linearly with that in the aqueous phase. The slope of the line yields the distribution coefficient of pyruvic acid to be 0.30 and 0.07 with 1-octanol and oleyl alcohol, respectively. The higher KD obtained with 1-octanol is due to its lower molecular weight and higher dielectric constant compared to oleyl alcohol. This superiority of 1-octanol is consistent with the results reported for propionic and lactic acids.22,29 It can also be seen in the same figure that even a relatively low concentration of tertiary amine is sufficient to observe its positive effect on the extraction of pyruvic acid. On the other hand KD values obtained using TOA and Alamine 336 are not considerably different, and the trend does not change for higher concentrations of the extractants. During the extraction of pyruvic acid with tertiary amines in oleyl alcohol, an opaque and dense emulsion phase formed at the interface and could not be completely removed by centrifugation at 10000g for 1 h. This was probably due to the clustering of the 13520
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Figure 3. Variation of KD with the equilibrium concentration of pyruvic acid in the aqueous phase for various concentration levels of TOA in 1-octanol.
Figure 4. Variation of z with the equilibrium concentration of pyruvic acid in the aqueous phase.
acidamine complex molecules, since the polarity of oleyl alcohol is of limited magnitude for stabilization of the resultant complex within the organic phase. The extent of this emulsion phase increased with an increase in the concentration of the components. Thus, the amount of the acid kept in the emulsion phase is probably not negligible.11 4.1.2. Effect of Concentration of the Phase Components and Stoichiometric Results. Figure 3 shows that KD increases with an increase in the initial concentration of TOA and decreases with an increase in the equilibrium concentration of pyruvic acid in the aqueous phase. The KD values are extremely high at low equilibrium concentrations of pyruvic acid in the aqueous phase and decrease very sharply with an increase of the latter to 0.2 mol L1. This trend was observed to repeat itself showing only a slight increase in the limiting equilibrium distribution coefficient at higher extractant concentrations. As shown in Figure 4, the loading ratio (z) increases with the equilibrium concentration of pyruvic acid in the aqueous phase and is independent of the initial TOA concentration in the organic phase. These findings are consistent with the results reported for acetic, formic, and lactic acids.11,17 The formation of a 1:1 acidamine complex is expected in the equilibrium concentration range of pyruvic acid between 104 and 103 mol L1, since z is less than 0.5. In the second part of the curve up to about an equilibrium concentration of pyruvic acid of
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Figure 5. Plot of z/(1 z) vs [HP]aq for the estimation of KE for (1:1)type pyruvic acidTOA complex (z < 0.5).
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