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Ind. Eng. Chem. Res. 2008, 47, 6192–6196
Extraction of Propionic Acid Using Different Extractants (Tri-n-butylphosphate, Tri-n-octylamine, and Aliquat 336) Amit Keshav, Kailas L. Wasewar,* and Shri Chand Department of Chemical Engineering, Indian Institute of Technology (IIT), Roorkee, Uttarakhand, 247667, India
Reactive extraction of propionic acid from aqueous solution containing 0.05 to 0.4 kmol/m3 acid using trin-butylphosphate (TBP) (organophosphorous compound), tri-n-octylamine (TOA) (tertiary amine), and Aliquat 336 (quaternary amine) as extractants in 1-octanol was studied. The comparison among the different categories of extractants was made. The order of extraction power was found to be TOA > Aliquat 336 > TBP with KE values of 25.67, 3.58, and 2.36 m3/kmol, respectively. The highest value of equilibrium complexation constant (25.67 m3/kmol) and distribution coefficient (14.09) of TOA suggested it to be the best extractant among the three. The greatest support to highest recovery was also due to the choice of diluent for the system. 1-Octanol was an active diluent which provided good solvation of the acid-extractant complexes. In all the extractions (1:1) acid-extractant complexes were formed with loading ratios less than 0.5, except in Aliquat where values higher than 0.5 were also obtained. This led to the definition of an another equilibrium constant (KE(2:1)) as (2:1) acid-Aliquat complexes were formed, with the value of KE(2:1) ) 26.54 (m3/kmol)2. 1. Introduction Fermentation is an important replacement to the petrochemical route for the production of a wide range of carboxylic acids such as propionic acid, lactic acid, acetic acid, citric acid, etc. A stumbling block in the use of fermentation to produce these acids is the difficulty in recovery from dilute solutions in which they are produced. A number of recovery methods are available such as precipitation, distillation, membrane bioreactor, liquid surfactant membrane extraction, adsorption, direct distillation, electrodialysis, reactive extraction, reverse osmosis, anion exchange, etc. Conventionally, recovery has been carried out through calcium hydroxide precipitations, which have had a number of shortcomings, such as being environmentally unfriendly, energy inextensive, and difficult to handle. Recovery of the solute species can be achieved by solvent extraction, and the reactive extraction has received increasing attention.1–7 Propionic acid is an important acid widely used in chemical, food, and pharmaceutical industries. It is used in grain preservation, manufacture of cellulose plastics, herbicides, antiarthritic drugs, perfumes, flavors, plasticizers, and mold preventatives in silage and hays. Its calcium and sodium salts are used as antifungal agents in bread and other foods.8 The acid content in fermentation broth/waste streams sources is less than 10% w/w.8 The separation, purification, and preconcentration of propionic acid are rather difficult because of its chemical behavior, as it has a strong affinity to water. The high solubility in water renders traditional solvents such as alcohols, ethers, esters, and inert diluents (hexane, n-heptane, etc.) to give low distribution coefficients. Thus physical extraction with conventional solvents is not an efficient method for recovery of propionic acid. To increase selectivity and yield of an acid, a combination of diluent with an extractant, which can chemically complex with acid, was tried.1–7 The improved results established the technology of reactive extraction for the recovery of carboxylic acids. The advantages for extractive fermentation include high reactor productivity, ease in reactor pH control without requiring base addition, and use of a high * To whom correspondence should be addressed. E-mail: k_wasewar@ rediffmail.com,
[email protected]. Tel.: +91-1332-285347. Fax: +91-1332-276535
concentration substrate as the process feed to reduce process wastes and production costs. Extractive fermentation also allows the process to produce and recover the fermentation product in one continuous step and reduce the down stream processing load and recovery costs. Reactive extraction involves the use of an extractant-diluent system to extract the acid. Extractants are usually classified as (i) anion exchange extractants (e.g., aliphatic primary, secondary, and tertiary amines), which form ion pairs (salts) in acidic medium; (ii) cation exchange extractants (e.g., phosphine and phosphoric acid), which exchange proton against the cation; (iii) solvating extractants (e.g., phosphoric and phosphinic acid, esters, and phosphinoxides), which are lewis bases and form nonstoichiometric compounds with neutral solutes; and (iv) chelate forming extractants (e.g., aliphatic and aromatic hydroxyines), which exchange the cation and form coordinate binding. Tri-n-octylphosphine oxide (TOPO),9,10 tri-n-butylphosphate (TBP),4,11 tri-n-octylamine (TOA),12,13 Alamine 336,7,14 and Aliquat 15,16 are some of the widely used extractants employed for reactive extraction of carboxylic acids. The extractant interacts with acid to form an acid-extractant complex and thus provide a high distribution of acid. The high affinity of acid to base provides an additional advantage of high selectivity over the nonacidic components in the mixture. Extractants are generally viscous or solid, so they are dissolved in diluents, which improve their physical properties like surface tension and viscosity. Diluents provide the solution of the extractants and also general and specific solvation to the acid-extractant complexes formed. Polar diluents are more favorable than zero polarity, low dielectric constant, or aliphatic or aromatic hydrocarbons. The solvation of the whole extractantacid complex is based on dipole-dipole interaction and was found to play an important role in the neutralization reaction between acid and extractant, which is promoted by increasing the polarity of the diluent. Peter,11 has studied the extraction using strongly solvating extractants like organophosphorous compounds and sulfoxides for the extraction of organic compounds. A systematic comparison of the extracting powers of TBP, TOPO, and di-n-octyl sulfoxides in the extraction of aromatic carboxylic acids and
10.1021/ie800006r CCC: $40.75 2008 American Chemical Society Published on Web 07/12/2008
Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 6193 Table 1. Chemical Equilibrium Data for Extraction of Propionic Acid Using Different Extractants in 1-Octanol extractant TBP + 1octanol
Aliquat 336 + 1octanol
Figure 1. Chemical equilibria for extraction of propionic acid using different extractants in 1-octanol.
Figure 2. Equilibrium complexation constants for extraction of propionic acid using different extractants in 1-octanol.
phenols was studied. Zhong et al.17 studied the reactive extraction of propionic acid using Alamine 304-1 in 2-octanol, 1-dodecanol, and Withohol 85 NF as diluents at various amine concentrations (0-100%) and found extraction to be maximized at the amine concentration between 20-40%. Diluent significantly effects the extraction power of the extractant, with 2-octanol providing the highest distribution coefficient of the three. The extraction of different carboxylic acids using TBP, TOA, and their mixtures was investigated by Matsumato et al.,18 and the synergism effect was highlighted in improving the extraction. Although some work has been done on the recovery of carboxylic acid using TBP, Aliquat, and TOA, studies involving comparison are scanty, specifically for propionic acid. In this context, the reactive extractions of propionic acid using TBP, Aliquat 336, and TOA in 1-octanol, respectively, were discussed in the present paper to find the most suitable extractant to recover propionic acid from dilute aqueous solutions. Also a reactive extraction equilibrium mechanism was proposed and equilibrium complexation constants were evaluated.
TOA + 1octanol
[T]o (kmol/m3)
[HA]o (kmol/m3)
KD
E%
z
1.65
0.05
2.59
72.18
0.021
0.66
0.1 0.15 0.2 0.3 0.4 0.05
2.98 3.17 3.13 3.22 3.34 1.81
74.86 76.03 75.76 76.30 76.97 64.45
0.045 0.069 0.092 0.139 0.187 0.049
1.108
0.1 0.15 0.2 0.3 0.4 0.05
1.57 1.45 1.32 1.50 1.21 14.47
61.12 59.26 56.95 60.00 54.73 93.54
0.093 0.135 0.173 0.273 0.332 0.042
0.1 0.15 0.2 0.3 0.4
17.01 13.28 25.06 20.58 22.57
94.45 93.00 96.16 95.37 95.76
0.085 0.126 0.174 0.258 0.346
g/cm3. Propionic acid (99%) (Himedia, India) and diluent 1-octanol (supplied by S. d. Fine-Chem Ltd., Mumbai, India) are of technical grade and were used without pretreatment. Distilled water was used to prepare the solutions of various concentrations of propionic acid solutions. NaOH used for the titration is of analytical grade and was supplied by Ranbaxy, India. For the standardization of the NaOH, oxalic acid (99.8%) was obtained from S. d. Fine-Chem Ltd., India. Phenolphthalein solution (pH range 8.2-10.0) was used as an indicator for titration and was obtained from Ranbaxy, India. The range of pH for the experiment was 2.65-3.14 at 303 K. A low concentration of propionic acid was used because in waste streams and in fermentation broths its concentration is not expected to be greater than 0.5 kmol/m.3,8 2.2. Extraction Method. Extraction experiments involve shaking equal volumes (20 cm3) of aqueous and organic phases for 12 h in a constant temperature (305 K) water bath, followed by settling of the mixture for at least 2 h at a same temperature (305 K).19 Aqueous phase pH was measured by an Orion 3 star pH benchtop (Thermo Electro Corporation). Aqueous phase acid concentration was determined by titration with NaOH and by HPLC. Fresh NaOH was prepared every time prior to titration. HPLC system (WATERS 1523) was composed of binary pump, refractive index detector (WATERS 2414) and dual λ absorbance detector (WATERS 2487). The sample was eluted by 0.25 kmol m-3 aqueous ammonium dihydrogen phosphate solution adjusted to pH 2.2 by an aqueous phosphoric acid solution endowing at a rate of 1.0 mL min-1 in a reverse phase C-18 column (4 mm i.d. × 150 mm in length). Propionic acid was detected at 236 nm. The results of the above methods were comparable with error less than ( 2%. The data reported in the present work is from a HPLC system. The acid content in the organic phase was determined with a mass balance.
2. Experimental Section 2.1. Chemicals. TBP (Himedia, India) is a phosphorusbonded oxygen donor, a light colorless liquid with the molecular weight of 266.32 g/mol and density of 0.92 g/cm3. Aliquat 336 (Methyltricaprylammonium chloride), a quaternary amine, is a mixture of C8-C10 with a minimum assay of 80% and with molecular weight of 404.17 and density of 0.888 g/cm3. TOA (C24H51N) (ACROS, India), a tertiary amine, is a light colorless liquid with the molecular weight of 353.66 and density of 0.809
3. Results and Discussion Propionic acid in the concentration ranges of 0.05 to 0.4 kmol/ m3 was equilibrated with 10 to 40% extractant (TBP, Aliquat, and TOA) in 1-octanol. The choice of 1-octanol was in account of higher alcohols being among the best diluents that provide higher distribution coefficients. The aim of the work was to find out the best extractant, among all of the named, in the recovery of propionic acid from dilute streams. It is well-known
6194 Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008
that propionic acid dissociates in aqueous solution. Under the experimental condition that the pH of the aqueous solution was smaller than the pKa of the acid (4.67), the effect of the acid dissociation was negligibly small. Thus only the undissociate form of acid was expected to exist in the aqueous phase. 3.1. Extraction Using Tri-n-butylphosphate (TBP). TBP alone gives a good extraction, but since it has a relatively high viscosity (3.56 × 10-3 Pa s) and specific gravity close to unity (0.98), it was used along with low viscosity and low density diluent, which could facilitate good phase separation in continuous extraction process. The mass law equilibria describing the extraction of propionic acid by extractant (T) (in the present case, TBP) in diluent can be represented by the following set of equations: [HA]aq + [T]org
KE(1:1)
798 [T : HA]org
[HA]aq + [T : HA]org
KE(2:1)
798 [T : (HA)2]org
(1)
(2)
: [HA]aq + [T : (HA)n-1]org
KE(n:1)
798 [T : (HA)n]org
(3)
The equilibrium extraction constant and the number of reacting molecules of extractant were computed by applying the law of mass action that was the ratio between concentrations of product molecules and concentration of the reactant species, according to the general equation of interaction between the extractant and the extracted species. [T : (HA)n]org [T]org[HA]aqn
(4)
KE(n:1) was expected to depend on properties of the acid and the solvation efficiency of diluent used. The experimentally accessible distribution ratio KD can be given as KD )
[HA]org [HA]aq
)
[HA.T]org [HA]aq + [A-]aq
(5)
The extent to which the organic phase (extractant + diluent) can be loaded with carboxylic acid was expressed as the loading ratio, z, z)
extractant Alamine 304-1 Alamine 304-1 Alamine 304-1 Alamine 336 Alamine 336 TOA (30%) TOA (20%) TOA (20%) TOA (20%) TOA (20%) TOA (20%) TOA (20%) Amberlite LA-2 Aliquat 336 (50%) Aliquat 336 (50%) Aliquat 336 (50%) TBP (40%)
diluents
KD (-)
KE (m3/kmol)
2-octanol 1-dodecanol with alcohol 85 NF 2-octanol kerosene 1-octanol petroleum ether butyl acetate TBP hexanol xylene MIBK benzene kerosene 2-octanol 1-octanol 1-octanol
1.25 1.20 1.17
-
7.10 2.00 14.09 0.75
7.17 2.19 25.67 2.57
[22] [22] present work [20]
3.31 7.53 17.20 3.70 2.60 1.66 2.79
13.74
[20] [23] [20] [23] [23] [24] [22] [22] present work present work
83.40 0.51 1.91 7.41 3.16 3.10 3.58 2.36
reference [17] [17] [17]
(KD(1-octanol) ) 1.73). The equilibrium complexation constant form the plot of eq 7 obtained from Figure 2 as
:
KE(n:1) )
Table 2. Distribution Coefficients and Equilibrium Complexation Constants for the Extraction of Propionic Acid Using Different Extractants and Diluents
[HA]org [T]o,org
(6)
The value of z in eq 6 depends on extractability of acid (strength of the acid-base interaction) and its aqueous concentration. In extraction involving TBP, it was always less that 0.5, thus ascertaining only (1:1) acid-TBP complexes to be formed and hence no overloading. The stoichiometry of the overall extraction reaction depends on the loading ratio in the organic phase, z, as z ) KE[HA]aq (7) 1-z It was found that increasing the TBP concentration increases the distribution coefficient obtained. However since TBP was relatively viscous, concentrations of 1.1 and 1.65 kmol/m3 (30 and 40%) were employed in the present study. The distribution coefficient value for 40% TBP in 1-octanol was obtained from Figure 1 as 2.79. The value was surely higher that pure diluent
KE ) 2.36 m3/kmol
(8)
Degree of extraction is defined as the ratio of propionic acid concentration in organic phase to the sum of acid concentration in organic and aqueous phase and is defined in term of KD as E)
KD × 100 1 + KD
(9)
E was found to increase with increase in acid concentration and the maximum value of 76.97% was obtained (Table 1). 3.2. Extraction Using Aliquat 336. The extraction of the undissociated molecules of propionic acid by chemical interaction can be represented by similar eqs 1-3 where “T” becomes Aliquat 336. Figure 1 shows the distribution coefficient for 40% Aliquat in 1-octanol to be 1.66, which is lower than in the earlier case. Since Aliquat is highly viscous, diluent was used in association with it to improve its physical properties, thus allowing its easier handling. The diluent lowers the viscosity of Aliquat 336 and decreases the surface tension at the interface. The KD value of the acid decreased with increasing Aliquat concentration in a diluent for higher acid concentrations, whereas for low acid concentrations, it was nearly constant for all Aliquat concentrations. At higher acid concentrations, more acid-Aliquat complexes were formed and if diluent was present in a large extent these complexes were easily solvated and penetrated into the organic phase from the interface, in comparison to when diluent was present in lower concentration, where high viscosity at the interface hindered the transfer of the complex and also the reaction. At lower acid concentrations, there was no apparent effect on the extraction performance because in that case it was acid which becomes limiting. Another advantage of 1-octanol is the clear phase separation obtained, which was not found in extraction involving other diluents such as aliphatic or aromatic hydrocarbons, etc. Equation 7 was plotted, as shown in Figure 2, to obtain the KE value for (1:1) acid-amine complexes as 3.58 m3/kmol (Table 2). However in the present case, z values greater than 0.5 were also obtained, thus along with the (1:1) complex, (2: 1) or higher acid-amine complexes can be assumed to be formed. The equilibrium complexation values for the (2:1) complex were obtained20 as
Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 6195
zt ) KE(2:1)[HA]2aq 2 - zt
(10)
for the (2:1) acid-extractant complex. The values of KE(2:1) were obtained for the fit of the above equation for values of z > 0.5 as KE(2:1) ) 26.54
(m3/kmol)2
(11)
The value of the equilibrium complexation constant was higher than that of TBP, thus ascertaining it to be a better diluent than TBP. The degree of extraction was found to be higher at the initial acid concentration and found to decrease with an increase from 0.05 to 0.4 kmol/m3 (Table 1). The reason for this being that Aliquat can load more acid, yet it is the limit of diluent (1-octanol) to solvate these complexes. 3.3. Extraction Using Tri-n-octylamine (TOA). TOA by itself is a relatively poor solvation medium for a polar complex, so it is preferable to use it with an active diluent like 1-octanol. TOA concentration was used in the concentration range of 10-90% in 1-octanol. In 1-octanol, with high TOA content, loading decreases with increasing amine concentration as the solvent becomes a less favorable solvating medium. It was found that the distribution coefficient increases up to 40%, after which it falls. So it was decided to use TOA concentrations of 30 and 40% in 1-octanol to find the optimum recovery. The extraction mechanism involves the interaction of basic nitrogen of TOA and the acidic portion of a carboxyl group and is devised according to eqs 1-7. The nature of acid-amine interaction depends on strength of acid and type of amines.22 Organic versus aqueous concentrations were plotted to obtain the distribution coefficient value KD ) 14.09 (Figure 1). The value was higher than organophosphrous (TBP) and quaternary amine (Aliquat 336). Thus tertiary amine (TOA) was the best extractant among all. The declaration was also verified by the equilibrium complexation constant value obtained (Table 2): KE ) 25.67
m3/kmol
(12)
which was also highest among all. Degree of extraction value (E) of greater than 95% was obtained for all acid concentration (Figure 3). Loading ratio in all extraction involving TOA was less than 0.5 (Table 1), thus only (1:1) acid-TOA complexes were formed with no overloading. The difference in extraction between the above extractants was explained by the behavior of the acid portion as it transfers from aqueous to organic phase. Organophosphorous compounds were effective extractants and provided higher distribution coefficients than carbon-bonded oxygen bearing extractants. Being chemically stable, a high possibility to be an efficient extracting solution with good separation effect with solutions
Figure 3. Degree of extraction (E) for extraction of propionic acid using 40% TBP, Aliquat 336, and TOA in 1-octanol.
containing chemically similar elements was shown. TBP, an organophosphorous compound, contains a phosphoryl group which was a stronger lewis base than the carbonyl one. This led to a high distribution coefficient. TBP was selected because of low water coextraction (4.67 mass %) and very low solubility in the aqueous phase (w ) 0.039 mass %). TBP contains a dP (O) OH group, which has a marked tendency toward an intermolecular hydrogen bonding. Because of the presence of both electron donor and electron acceptor groups in the )P (O) OH grouping, it underwent specific interactions like selfassociation and molecular complex formation with diluents or other solutes. The knowledge of these factors was necessary for understanding the mechanism of extraction, the effect of diluents, and the role of additional reagents. The quaternary ammonium chloride extracts both dissociated and undissociated forms of acid. Kyuchoukov et al.15 experimented with the lactic acid extraction with quaternary ammonium chloride (Aliquat 336) and found that at low pH value and low acid concentration (5 g · L-1), the part of whole molecule removed by the extractant was bigger than the part of the extracted anions, but with the rise of pH, the ratio turns. Thus the concentration of undissociated acid was the function of the pH of the aqueous phase. Since the present study was at low pH, it can be assumed that only undissociated acid was being involved in the extraction. In the present case, Aliquat form both (1:1) and (2:1) complexes with propionic acid. This was the reason why the extraction using Aliquat 336 was higher than that using TBP, where the extraction was mainly by the 1:1 acid-extractant mechanism only. In TBP, the acid strength in aqueous phase and the hydrogen binding in organic phase was the measure of extractability. However in amines, the ammonium salt formation of acid by the ion pair association of alkylammonium cation and the acid radical decides the stability of complexes in the organic phase, which was by acid-base-type reaction. The proton association constant was highest for TOA and was dependent on the nature of diluent more than other extraction systems.21 1-Octanol has an acid interaction functional group that provides high specific and general solvation to the acid-TOA complex which leads to a higher distribution than the other two categories. 4. Conclusions Reactive extraction is an emerging separation technique for recovery of acids from dilute/waste streams, where an acid content of less than 10% is expected. Reactive extraction shows its excellency over other separation methods available, in regards to its success in the removal of acid-form dilute solutions. A wide range of extractants are available for this purpose such as conventional carbon-bonded oxygen-bearing extractants, organophosphorous extractants, and higher molecular weight aliphatic amines. In the present paper, attempts were made to find the most suitable extractant for the recovery of propionic acid. The distribution coefficient (KD), degree of extraction (E), and equilibrium complexation constant values (KE) were chosen as a measure of difference in the extractants chosen for study. The results show a wide difference in values of distribution coefficients and equilibrium complexation constants values of extractants (TBP, Aliquat 336, and TOA) in 1-octanol. The higher KD ) 14.09, E ) 65%, and KE ) 25.97 m3/kmol values of the TOA-octanol extraction system, suggested it to be the best extraction system for propionic acid.
6196 Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008
Acknowledgment This work was supported by the Young Scientist Project, SR/ FTP/ETA-43/2005 (DST-276-CHD) and the Department of Science and Technology (DST), India. Nomenclature KD ) distribution coefficient KD(1-octanol) ) distribution coefficient of 1-octanol alone KE or KE(1:1) ) extraction equilibrium constant for (1:1) acidextractant complex, m3/kmol KE(2;1) ) extraction equilibrium constant for (2:1) acid-extractant complex, (m3/kmol)2 KE(n:1) ) extraction equilibrium constant for (n:1) acid-extractant complex, (m3/kmol)n [HA] ) concentration of acid, kmol/m3 [T] ) concentration of extractant, kmol/m3 [HA · T] ) concentration of acid-extractant complex, kmol/m3 [A-] ) concentration of anion, kmol/m3 E ) degree of extraction z ) loading ratio Subscripts aq ) aqueous phase org ) organic phase o ) initial
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ReceiVed for reView January 2, 2008 ReVised manuscript receiVed April 7, 2008 Accepted May 6, 2008 IE800006R
