1335
I n d . Eng. Chem. Res. 1991,30, 1335-1342
The use of the molar volume (Wilke-Chang and Hayduk-Minhas) or molecular weight (Matthews-Akgerman) of the solute to characterize the solute size has ita limitations, for it has been shown that solutes of similar molar volumes or molecular weight can diffuse at different rates. None of the three correlations studied were able to account for the effects of the structure of the solute molecule on diffusion.
Acknowledgment Financial support of this work was provided by TECHn-Nu P/L and the Australian Government under the auspices of the Australian Research Council, Grant No. A88930202. bgirtry No. COS,124-38-9;Ca+ ethyl ester, 5908-87-2;Cm ethyl ether, 81926-94-5;CW methyl ester, 2566-90-7;Cm5 methyl ester, 2734-47-6;CIS* ethyl ester, 111-61-5.
Literature Cited Alizadeh, A,; Nieto de Castro, C. A,; Wakeman, W. A. The Theory of the Taylor Dispersion Technique for Liquid Diffusivity Measurements. Znt. J. Thermophys. 1980,l, 243. Angus, S.;Armstrong, B.; de Reuck, K. M. Carbon Dioxide: Znternational Thermodynamic Tables of the Fluid State; Pergamon Press: Oxford, 1976;Vol. 3. Aria, R. On the Dispersion of a Solute in a Fluid Flowing Through a Tube. Proc. R. SOC.London 1956,235,67. Chapman, S.; Cowling, T. G. The Mathematical Theory of NonUniform Gases; Cambridge University Press: Cambridge, 1970. Chen, S. H.;Davis, H. T.; Evans, D. F. Tracer Diffusion in Polyatomic Liquids. 111. J. Chem. Phys. 1982,77,2540. Debenedetti, P. G.; Reid, R. C. Diffusion and Mass Transfer in Supercritical Fluids. AZChE J . 1986,32,2034. Dyerberg, J. Linolenate Derived Polyunsaturated Fatty Acids and Prevention of Atherosclerosis. Nutr. Rev. 1986,44,125. Dymond, J. H. Corrected Enskog Theory and the Transport Coefficients of Liquids. J. Chem. Phys. 1974,60, 969. Funazukuri, T.; Hachisu, S.; Wakao, N. Measurement of Diffusion coefficients of C18 Unsaturated Fatty Acid Methyl Esters, Naphthalene, and Benzene in Supercritical Carbon Dioxide by a Tracer Response Technique. Anal. Chem. 1989,61,118.
Groves, F. R.; Brady, B.; Knopf, F. C. State of the Art on the Supercritical Extraction of Organics from Hazardous Wastes. CRC Crit. Rev. Environ. Control 1984, 15, 237. Hayduk, W.; Minhas, B. S. Correlations for Prediction of Molecular Diffusivities in Liquids. Can. J. Chem. Eng. 1982,60, 295. Hildebrand, J. H.Motions of Molecules in Liquids: Viscosity and Diffusivity. Science 1971,174,490. Knaff, G.; Schlunder, E. U. Diffusion Coefficients of Naphthalene and Caffeine in Supercritical Carbon Dioxide. Chem. Eng. Proc. 1987,21,101. Levenspiel, 0.; Smith, W. K. Notes on the Diffusion-Type Model for the Longitudinal Mixing of Fluids in Flow. Chem. Eng. Sci. 1957, 6,227. Lusis,M. A,; Ratcliff, G. A. Diffusion in Binary Liquid Mixtures at Infinite Dilution. Can. J. Chem. Eng. 1968,46,385. Matthews, M. A; Akgerman, A. Diffusion Coefficients for Binary Alkane Mixtures to 573 K and 3.5 MPa. AZChE J. 1987,33,881. Moulijn, J. A.; Spijker, R.; Kolk, J. F. M. Axial Dispersion of Gases Flowing Through Coiled Columns. J. Chromatogr. 1977,142,155. Reddy, K. A.; Doraiswamy, L. K. Estimating Liquid Diffusivity. Znd. Eng. Chem. Res. 1967,6,77. Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Property of Gases and Liquids, 4th ed.; McGraw-HIlk Singapore, 1988. Saseiat, P. R.; Mourier, P.; Caude, M. H.; Roaset, R. H. Measurement of Diffusion Coefficienta in Supercritical Carbon Dioxide and Correlation with the Equation of Wilke and Chang. Anal. Chem. 1987,59,1164. Swaid, I.; Schneider, G. M. Determination of Binary Diffusion Coefficients of Benzene and Some Alkylbenzenes in Supercritical COz between 308 and 328 K in the Pressure Range 80 to 160 bar with Supercritical Fluid Chromatography (SFC). Ber. BunsenGes. Phys. Chem. 1979,83,969. Taylor, G. Dispersion of Soluble Matter in Solvent Flowing Slowly Through a Tube. Proc. R. SOC.London 1953,219,186. Taylor, G. Conditions Under Which Dispersion of a Solute in a Stream of Solvent can be Used to Measure Molecular Diffusion. Proc. R. SOC.London 1954,225,413. Wells, P. A.; Foster, N.R.; Chaplin, R. P. Diffusion in Supercritical Fluids. CHEMECA '88, Z.E. Aust. Prepr. Papers, Sydney 1988, 898. Wilke, C. R.; Chang, P. Correlation of Diffusion Coefficients in Dilute Solutions. AZChE J. 1955,1, 264. Received for review August 13, 1990 Accepted December 10,1990
Extraction of Carboxylic Acids with Tertiary and Quaternary Amines: Effect of pH Shang-Tian Yang,* Scott A. White, and Sheng-Tsiung Hsu Department of Chemical Engineering, The Ohio State University, 140 West 19th Avenue, Columbus, Ohio 43210
Extractions of carboxylic acids by tertiary and quaternary amines were studied under various equilibrium pHs ranging from 2.0 to 8.5. The quaternary amine Aliquat 336 extracted both dissociated and undissociated forms of acids, whereas the tertiary amine Alamine 336 extracted only the undissociated acid. Pure Aliquat 336 also had higher distribution coefficients, KD,than Alamine 336 at all pHs. In the intermediate pH range, KDdecreased with increasing the equilibrium pH of the aqueous phase. However, in the extremely high and low pH ranges, KD remained unchanged with pH. This pH dependency of KDcan be modeled by using a three-parameter equation derived from general chemistry principles. Extractions were also conducted with two diluents, kerosene and 2-octanol. In general, the polar diluent, 2-octanol, increased the extracting power of Alamine 336 by providing more solvating capacity for the nonpolar amine. In contrast, neither the polar nor the nonpolar diluent was active when used with Aliquat 336.
Introduction Recently, extractive recovery of carboxylic acids from dilute, aqueous solutions, such as fermentation broth and wastewater, which have acid concentrations lower than 10% (w/w), has received increasing attention (Helsel, 1977; Jagirdar and Shanna, 1980, Busche et al., 1982; Kertes and King, 1986; Tamada et al., 1990). Organic solvents used
for extraction can be categorized into three major types: (1)conventional oxygen-bearing and hydrocarbon extractanta, (2) phosphorus-bonded oxygen-bearing extractanta, and (3)high molecular weight aliphatic amines (Kertes and King,1986). Solvent extraction with conventional solvents such as alcohols, ketones, ethers, and aliphatic hydrocarbons is not efficient when applied to dilute, carboxylic
0888-5885/91/2630-1335$02.50/00 1991 American Chemical Society
1336 Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991
acid solutions because of the low aqueous activity of carboxylic acids resulting in low distribution coefficients. However, carboxylic acid extractions with organophosphates, such as trioctylphosphine oxide (TOPO) and tri-n-butyl phosphate (TBP), and aliphatic amines have large distribution coefficients. Aliphatic amines are slightly more effective and less expensive than phosphorus-bonded oxygen-bearing extractants (Wardell and King, 1978). Several aliphatic amines have been used successfully to extract carboxylic acids (Wardell and King, 1978;Ricker et al., 1979, 1980; Kertes and King, 1986; Wennersten, 1983; Yabannavar and Wang, 1986; Tamada and King, 1990). The strong amine interactions with the acid allow for formation of acid-amine complexes and thus provide for high distribution coefficients. In addition, the high affinity of the organic base for the acid gives selectivity for the acid over nonacidic components in the mixture. However, primary amines are too soluble in water to be used with aqueous solutions. Secondary amines are subject to amide formation upon regeneration by distillation. Consequently, long-chain tertiary amines have received the most attention. Many factors have important influence on the extraction characteristics-the nature of the acid extracted, concentrations of the acid and the extractant, and the type of diluent used. These factors have been extensively studied for several carboxylic acid extractions with tertiary amines (Tamada et al., 1990). The effect of temperature on the distribution coefficient also has been studied (Wennersten, 1983; Tamada and King, 1990). Since the extraction equilibrium is dependent on the concentration of undissociated acid in the aqueous phase, pH is also an important variable. The idea that partition coefficients can be manipulated by changes in pH is not new. The separation of weak acids and bases by controlled pH extraction has been done in the pharmaceutical industry (Robinson and Cha, 1985). However, to date, the effect of pH on the extraction of carboxylic acids with aliphatic amines has not been well documented in the literature. Recently, there has been increasing interest in using anaerobic bacteria for organic acid production from biomass. However, the use of these bacteria for acid production is usually limited by the low acid concentration (> K,, KD = K,, and when [H+] 25%) of amine in water and no separation was necessary. A 10-pL portion diluent, a third, emulsion phase was also observed at the interface between the aqueous and organic phases when of the aqueous sample was injected, and the resulting signal the equilibrium pH of the aqueous phase was lower than from the detector was recorded on a chart recorder (Per3.0. kin-Elmer, Model 123). The peak height on the chromatogram was then used to compare with known standards In general, only the aqueous-phase acid concentration to determine the acid concentration in the sample. was analyzed. A material balance was used to determine Acetic and propionic acids were also analyzed by using the resulting organic-phase acid concentration. However, a gas chromatograph (Varian 3300) equipped with a flame the acid concentration in the organic phase was also deionization detector (FID) and a fused silica megabore termined by first stripping the organic phase with a small column (DBWAX, 15 m X 0.534 mm; J & W Scientific). amount of NaOH solution (pH 12). The alkaline solution The carrier gas was N2 at a flow rate of 10 mL/min. The containing the organic salt was then acidified with phostemperatures were as follows: injector, 100 "C; column, phoric acid and analyzed by using a gas chromatograph. 110 "C; and detector, 160 "C. Ethanol was used as the The results from these two methods agreed well, within internal standard in determining the acid concentration 5-10%. The concentration of undissociated acid was in the sample. calculated from the measured, total acid concentration,the pH, and the dissociation constant, K,,of the acid. Results and Discussion Effect of Acid Concentration. The effect of acid concentration on extraction was first studied. The equiEffect of Acid Concentration. It has been reported librium pH was different from the initial pH because of that the distribution coefficients were higher at lower acid the removal of acid and/or salt from the aqueous phase. concentrations when aqueous acid concentrations were The solution pH tended to increase when the initial pH below 10 g/L (Ricker et al., 1979; Wennersten, 1983). was low but tended to decrease when the initial pH was However, it was not clear if this concentration effect was high. These complications make it difficult to determine observed at the same equilibrium pH. Therefore, before the concentration effect without the influence of pH the pH effect can be explored, the concentration effect has change. However, for each extractant studied, extraction to be examined at the same equilibrium pH first. of organic acid solutions of varying pH values and conThe extraction of acetic acid of various concentrations centrations were performed. The samples were then anwas conducted with Aliquat 336 to study the effect of acid
//
1338 Ind. Eng. Chem. Res., Vol. 30, No.6, 1991
centration effect becomes insignificant. On the other hand, at low [TA],, KD > K b and KDincreases with decreasing
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concentration on KD. As shown in Figure 1, at the same equilibrium pH, the total acetic acid concentration in the organic phase is proportional to that in the aqueous phase. The slopes of the constant-pH lines increased with a decrease in pH, indicatingthat extraction was more effective at a lower pH. However, only the lowest pH line passes through the origin, and as the pH increases, the intercept also increases. Since the distribution coefficient, KD, is defined as the ratio of the ordinate to the abscissa, KD values will be larger at lower concentrations. This effect was stronger at higher pH values and lower acid concentrations but would diminish at acid concentrations higher than 10 g/L. This concentration effect will be amplified if the solution pH is not controlled because low-concentration solutions will have higher pH values. This is consistent with the previous reports that showed higher KD values at lower acid concentrations. Thus, the acid concentration in the organic phase can be related to that in the aqueous phase by [TAlorg = CO + K'D[TAlaq
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Efect of Undissociated Acid. Most aliphatic amines extract acids from the aqueous phase by forming an acid-base complex with the undissociated acid (Kertesand King, 1986). Since the concentrationof undissociated acid is a function of the pH, the extraction of organic acids will greatly depend on the pH of the aqueous phase. If only the undissociated acid could be extracted, the total acid concentration in the organic phase would be dependent only on the undissociated acid concentration in the aqueous phase. This is generally true for tertiary amines, such as Alamine 336. However, if the undissociated acid was the only species being extracted by Aliquat 336, no acetic acid would have appeared in the organic phase at high pH values. Since Aliquat 336 is composed of an organic cation associated with a chloride ion, it can function as an anion-exchange reagent under both acidic and basic conditions (Henkel Corp., 1988). Therefore, both undissociated and dissociated forms of the acid can be extracted with Aliquat 336. This is also demonstrated by the plot of [TA], vs [HA] (Figure 2). As shown in Figure 2, distinct constant-pg lines were obtained for data from different equilibrium pHs. Moreover, the slope of these lines increased with the pH, indicating that the dissociated acid was also being extracted. Effect of pH. As already shown in Figures 1and 2,the extraction of acetic acid with aliphatic amines is greatly affected by the pH. This pH effect cannot be explained by the undissociated acid concentration alone. The effect of pH on the KD value is illustrated in Figures 3-5. In general, the KDvalue increased with a decease in the pH value except at extremely high or low pHs, where KDdid not change significantly with pH. The same trend was found for extractions with Aliquat 336 (Figure 3a), Alamine 336 (Figure 3b), and Aliquat 336 in kerosene for all the carboxylic acids studied (Figure 4), and with all amine/ diluent mixtures studied for propionic acid extraction (Figure 5). It should be noted, however, that in certain cases, large data scattering was observed. This was mainly attributed to the errors in measuring the aqueous acid concentration. A small error in this measurement could result in a large variation in the KD value since the organic-phase acid concentration was determined from the material balance based on the aqueous acid concentration. Nevertheless, r 0
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Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 1339 Table I. Values of If,,K,,and K. for the Extraction of Lactic Acid extractant composition K1 K2 K,,OM PK, Aliquat 336 5.50 X 10” 4.26 1.65 0.17 100% (w/w) 0.35 0.057 1.38 X IO4 3.86 50%, in kerosene 7.50 X 10” 4.12 0.55 0.11 50%, in 2-octanol Alamine 336 4.26 0.00 5.50 X lod 100% (w/w) 1.30 50%, in 2-octanol 2.35 0.00 0.50 X 5.30 ’K, = 1.38 X lo4 M, or pK, = 3.86 (25 “C,1 atm). Table 11. Values of K,,K,,and K, for the Extraction of Acetic Acid extractant comDosition K, K, K.,’ M DK. Aliquat 336 0.37 1.74 X 10” 4.76 100% (w/w) 2.17 0.71 X 1od 5.15 0.90 0.31 50%, in kerosene 25%, in kerosene 0.20 0.05 1.74 X 10“ 4.76 4.76 0.17 1.74 X lo4 50%, in 2-octanol 0.78 Alamine 336 4.76 0.00 1.74 X lob 100% (w/w) 0.55 50%, in 2-octanol 2.50 0.00 0.30 X 10” 5.52 ‘K, = 1.74 X 10” M, or pK, = 4.76 (25 O C , 1 atm).
the HPLC-determined acid concentration generally had an error less than 2%. Also, for extractions with high concentrations of amine in diluent, at equilibrium pH < 3, the KDvalues were lower than expected. This was attributed to the formation of emulsion at the interface between solvent and aqueous phases. It is known that if the solvent does not have good solvating capacity, the acid-amine complexes tend to cluster together and away from the bulk solvent (Tamada et al., 1990). As a result, large amounts of acid extracted would stay in the interfacial region and thus affect the equilibrium between the two phases. The formation of emulsion not only will affect the equilibrium but also will cause difficulty in separating the two phases. However, this problem can be prevented by using a proper diluent to provide the solvating power necessary for the amine extractant and by maintaining the equilibrium pH above 3. Nevertheless, it is clear that the effect of pH on KD generally follows the trend described by eq 8. The three constants, K,, K2,and K,, were then determined by fitting the data to eq 8 using computer nonlinear regression. The results are listed in Tables I-IV. As also shown in Figures 3-5, the predicted curves superimpose the data very well. Thus, the model is valid in representing the pH effect on KD for carboxylic acid extraction with amines. As also shown in Tables I-IV, the K, values giving the best fit between the model and the data are not always similar to the K, value for the acid under standard conditions. There is no apparent trend associated with the variaton of K, values for all extractions studied. However, it was noted that, for most cases, the K, values found from model fitting were smaller than those under standard conditions. This might be caused mainly by the amine and/or diluent dissolved in the aqueous phase, which would affect the solvation of water around the acid molecules and thus decrease the dissociation of acid. Also, the dissolved organic solvent in water might interfere with the function of a pH probe although a calomel electrode was used in this study. It should also be noted that the extraction experiments were conducted at room temperature, which might deviate from 25 O C by a few degrees. Comparison of Amine Extractants. It was noted that dissociated acid was extractable with Aliquat 336 but not with Alamine 336, as indicated by KD values at the high
Table 111. Values of K,, ..K,- and K.- for the Extraction of Propionic Acid extractant composition K1 K, &,OM PK, Aliauat 336 4.82 6.98 1.16 1.52 X 1od lbo%(w/w) 5.54 4.24 0.92 0.29 X IO” 75%, in kerosene 5.74 0.18 X 1od 50% 3.16 0.78 5.44 2.10 0.49 0.36 X 10“ 25% 5.43 4.56 1.00 0.37 X IO” 75%, in 2-octanol 5.24 3.15 0.99 0.57 X 10” 50 % 5.26 25% 2.43 0.78 0.55 X lob 5.01 0.98 X IO” 0% 2.29 0.0 Alamine 336 4.85 2.10 0.0 1.40 X 10” 100% (w/w) 4.84 1.94 0.0 1.43 X 10” 75%, in kerosene 4.86 2.19 0.0 1.37 X 10” 50 % 4.67 2.15 X 10” 25 % 2.09 0.0 5.20 0.63 X 75%)in 2-octanol 3.89 0.0 5.15 7.17 0.0 0.71 X 10” 50 % 4.93 9.44 0.0 1.17 X 10” 25% 5.10 2.29 0.0 0.98 X 0% OK,
= 1.32 X 10” M, or pK, = 4.88 (25 O C , 1 atm).
Table IV. Values of K,,K2,and K, for the Extraction of Butyric Acid extractant composition K1 K2 K.: M PK. aliquat 336 0.20 X 10” 5.70 15.50 1.26 100% (w/w) 0.65 X 10” 5.19 0.80 50%, in kerosene 10.50 4.81 0.38 1.54 X 10” 25%, in kerosene 3.31 0.97 0.50 X 10“ 5.30 50%, in 2-octanol 11.40 Alamine 336 0.25 X 10” 5.60 0.00 100% (w/w) 3.30 0.10 X 10” 6.00 8.50 0.00 50%, in 2-octanol OK,
= 1.74 X lod M, or pK, = 4.76 (25 O C , 1 atm).
pH region (Figure 3). Tertiary amines are capable only of extracting undissociated acid and cannot be used under basic conditions. In contrast, quaternary amines can extract acid under both acidic and basic conditions. However, this may become a disadvantage because it makes Aliquat 336 much more difficult to strip. An extractive separation and recovery process normally involves two steps: extraction and solvent regeneration. It will be advantageous to have zero Kz value, which will allow solvent regeneration through back-extraction with an alkaline solution. A swing of pH between the two steps is thus practical when Alamine 336 is used as the extractant. In general, pure Aliquat 336 had much higher values in KDthan Alamine 336. However, both K1 and K2 values for Aliquat 336 decreased dramatically with decreasing amine concentration in a diluent, no matter which diluent was used. This may limit the use of a diluent with Aliquat 336. On the contrary, K1 for Alamine 336 increased several-fold when 2-octanol was used as the diluent. This makes Alamine 336/2-octanol an effective extractant for carboxylic acids under acidic conditions. Comparison of Carboxylic Acids. As indicated by the values of K1 and Kz, the extractability of carboxylic acids with Aliquat 336 is in the order butyric acid > propionic acid > acetic acid > lactic acid, and with Alamine 336, butyric acid > propionic acid > lactic acid > acetic acid. It is apparent that the longer chain carboxylic acid is more hydrophobic and can be extracted better by amine extractants. The hydroxyl group on lactic acid makes the compound more hydrophilic and thus more difficult to extract than propionic acid. Effect of Diluent. It is known that the distribution coefficient,KD, also depends on the type of diluent used and the resulting extractant concentration in the solvent phase (Ricker et al., 1979,1980;Kertes and King, 1986).
1340 Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 N
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A polar diluent will increase the extracting power of nonpolar amines by providing additional solvating power that allows higher levels of polar acid-amine complexes to stay in the organic phase ( K e r b and King, 1986). On the other hand, a nonpolar diluent will not affect nonpolar amines. If a diluent is inactive, the values of K1 and K2would be proportional to the concentration of the extractant. Effect on Aliquat 336. In general, no apparent effects on extraction performance were obtained from the use of kerosene or 2-octanol as a diluent for Aliquat 336. The values of Kl and K2decreased, more or less, proportionally with increasingthe use of a diluent with Aliquat 336. Some deviations were observed in certain specific pH regions for some acids, but no general trend could be observed from the data available. Therefore, both kerwne and 2-octanol can be considered as inactive diluents to Aliquat 336. As shown in Figure 6, the Kl and K2values for propionic acid extractions with Aliquat 336 in either kerosene or 2-octanol were about the same, and they increased with increasing the concentration of Aliquat 336. This again, suggested that the extraction ability of Aliquat 336 is mainly determined by its concentration and not the diluent. The use of a diluent with Aliquat 336 does, however, improve the physical properties of the extractant and make the mixture much easier to handle than the pure amine. The viscosity of Aliquat 336 in any diluent is substantially
lower than that of pure Aliquat 336. Also, the surface tension that arises at the interface of the two phases is decreased when a diluent is used with Aliquat 336, which allows the two phases to separate faster. The improvement of the physical properties with a diluent is necessary to allow Aliquat 336 to be used in any extraction process. Effect on Alamine 336. Alamine 336 is not nearly as viscous as Aliquat 336. No physical complications arose when it was used in pure form. However, the use of 2octanol as a diluent greatly increased the extraction power of Alamine 336 (Figure 7). The extent of improvement in K1 value was dependent on the acid extracted and amounts of 2-octanol used. It is clear that a polar diluent can greatly improve the extracting power of a nonpolar tertiary amine. This is because the polarity of the diluent provides solvating power that enables more acid-amine complexes to stay in the solvent phase. As also shown in Figure 7, the nonpolar diluent, kerosene, did not affect the of Alamine 336; extraction power (K,) Conclusion and Recommendation The proposed three-parameter model is appropriate to describe the pH effect on the distribution coefficient for carboxylic acid extraction by amines. Aliquat 336 extracts both undissociated and dissociated acids and thus can be used under both acidic and basic conditions. However, extractant regeneration by striping may be difficult for
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Znd. Eng. Chem. Res. 1991,30, 1342-1350
1342
Herrero, A. A. End-product inhibition in anaerobic fermentation. Trends Biotechnol. 1983, I , 44-53. Hsu, S. T. Effects of pH on extractive fermentation for propionic acid production from whey lactose. M.S. Thesis, Department of Chemical Enpineering, The Ohio State Univemity, Columbue, OH, 1989. Jagirdar, G. C.; Sharma, M. M. Recovery and separation of mixtures of organic acids from dilute aqueous solutions. J. Sep. Proc. Technol. 1980, 1(2), 40-43. Kertes, A. S.; King, C. J. Extraction chemistry of fermentation product carboxylic acids. Biotechnol. Bioeng. 1986,28,269-282. Ricker, N. L.; Michaels, J. N.; King, C. J. Solvent properties of organic bases for extraction of acetic acid from water. J. Sep. h o c . Technol. 1979, I(l), 36-41. Ricker, N. L.; Pittman, E. F.; King, C. J. Solvent extraction with amines for recovery of acetic acid from dilute aqueous industrial streams. J. Sep. h o c . Tehcnol. 1980, 2(2), 23-30. Robinson, R. G.; Cha, D. Y. Controlled pH extraction in the separation of weak acids and bases. Biotechnol. Prog. 1986, I , 18-25. Tamada, J. A.; Kertes, A. S.; King, C. J. Extraction of carboxylic acids with amine extractants. 1. Equilibria and law of mass action modeling. Znd. Eng. Chem. Res. 1990, 29, 1319-1326. Tamada, J. A.; King, C. J. Extraction of carboxylic acids with amine extractants. 3. Effect of temperature, water coextraction, and proceas considerations. Znd. Eng. Chem. Res. 1990,29,1333-1338. Wardell, J. M.; King, C. J. Solvent equilibria for extraction of carboxylic acids from water. J. Chem. Eng. Data 1978, 23(2), 144-148. Wennersten, R. The extraction of citric acid from fermentation broth using a solution of a tertiary amine. J. Chem. Tech. Biotechnol. 1983,33B, 85-94. Yabannavar, V. M.; Wang, D. I. C. Integration of extraction with fermentation for organic acid production. Annual Meeting of AIChE, Miami, FL, Nov 1986.
extractant-diluent mixture needs to be further evaluated. If the amine extractant is to be used in an extractive fermentation process, the selection of the extractant will be dependent on the pH range for the fermentation. For anaerobic acetic acid fermentation, which usually requires a pH value higher than 6.0, b i n e 336 will not work well, and Aliquat 336 or other extractants that can work at high pH values must be used. Alamine 336, however, will be good for use in propionic acid, lactic acid, and butyric acid fermentations, which can tolerate a pH value as low as 4.0. A pH swing in the aqueous phase will be able to regenerate the amine extractant, making the extraction with Alamine 336 an attractive method for separating and recovering carboxylic acids from dilute, aqueous solutions. Registry No. Lactic acid, 50-21-5; acetic acid, 64-19-7; propionic acid, 79-09-4; butyric acid, 107-92-6; 2-octanol, 123-96-6.
Literature Cited Bar, R.; Gainer, J. L. Acid fermentation in water-organic solvent two-liquid phase systems. Biotechnol. B o g . 1987, 3, 109-114. Bueche, R.M.; Shimshick, E. J.; Yates, R. A. Recovery of acetic acid from dilute acetate solution. Biotechnol. Bioeng. Symp. 1982,22, 249-262. Daugulis, A. J. Integrated reaction and product recovery in bioreactor systems. Biotechnol, Prog. 1988,4,113-122. Dean, J. A., Ed. Lunge's Handbook of Chemistry, 13th ed.; McGraw-Hill: New York, 1985; pp 5-62-5-67. Helsel, R. W. Waste recovery: Removing carboxylic acids from acqueous wastes. Chem. Eng. Prog. 1977, 73(5),55-59. Henkel Corp. Alamine 336. Red Line Technical Bulletin; Henkel Technical Center: Minneapolis, MN, 1988. Henkel Corp. Aliquat 336. Red Line Technical Bulletin; Henkel Technical Center: Minneapolis, MN, 1988.
Received for review August 23, 1990 Accepted December 10, 1990
Copolymer Composition Control of Emulsion Copolymers in Reactors with Limited Capacity for Heat Removal Gurutze Arzamendi and Jos6 M. Asua* Crupo de Zngenier;? Quimica, Departamento de Qulmica Aplicada, Facultad de Ciencias Quimicas, Universidad del Pats Vasco, Apartado 1072, 20080 San Sebastibn, Spain
A method for the determination of the optimal monomer addition strategy to produce a homogeneous copolymer under conditions in which the reactor has a limited capacity for heat removal is presented. The method allows for the calculation of the monomer addition profiles for both constant and time-dependent heat removal rates. The approach was successfully applied to the emulsion copolymerization of vinyl acetate and methyl acrylate carried out in a laboratory reactor that had been transformed to reduce ita capacity for heat removal to the level of a large-scale reactor. Introduction The control of the composition of the copolymers prepared from monomers with widely different reactivity ratios is usually achieved by carrying out the polymerization under starved conditions (Snuparek and Krska, 1977; Basaet and Hoy, 1981; El-Aaeser et al., 1983). In this way, the polymerization becomes controlled by the addition rate and the reaction rate of both monomers is the same as the feed rate. This results in a copolymer of the same composition as the feed. However, under starved conditions, the concentration of the monomers in the polymer particles is low and this leads to long process times. For solution polymerization, Johnson et al. (1982) developed a control Strategy aiming at maintaining a constant
* To whom correspondence should be addressed. O888-5885f 91 f 2630-1342$02.50f 0
molar ratio of coreactants in the reactor by adjusting the addition rate of the more reactive monomer. Analytical solutions for this process have been obtained by Choi (1989). These solutions cannot be applied to emulsion polymerization because, in this multiphase system, the concentration of the monomers in the polymerization loci is different from the average concentration in the reactor. Guyot et al. (1981) controlled the composition of emulsion copolymers using a feedback strategy based on on-line gas chromatographic analysis of the monomer mixture. A more advanced feedback control strategy based on the use of a Kalman filter for state estimation has been presented by Dimitratos et al. (1989). An alternative method is the so-called optimal semistarved monomer addition strategy (Arzamendi and hua, 1989). In this feed-forward strategy, the reactor is initially charged with all of the less reactive monomer plus the amount of the more reactive monomer needed to initially 0 1991 American Chemical Society
