Ind. Eng. Chem. Res. 2007, 46, 4621-4625
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Application of MOSCED and UNIFAC to Screen Hydrophobic Solvents for Extraction of Hydrogen-Bonding Organics from Aqueous Solution Timothy C. Frank,*,† John J. Anderson,† and James D. Olson‡ The Dow Chemical Company, Engineering & Process Sciences Laboratory, 1319 Building, Michigan Operations, Midland, Michigan 48667, and The Dow Chemical Company, Analytical Sciences Laboratory, 770 Building, 3200 Kanawha Turnpike Technical Park, South Charleston, West Virginia 25303
Charles A. Eckert School of Chemical & Biomolecular Engineering and Specialty Separations Center, Georgia Institute of Technology, Atlanta, Georgia 30332-0100
Liquid-liquid extraction using hydrophobic extraction solvents is a technology well-suited to the removal of certain hydrogen-bonding organics from water or brine. Even when distillation is technically feasible, extraction might allow a significant reduction in energy consumption depending on the specific application. Various methods are available for estimating the partition ratio (K) to assess technical feasibility; however, these methods often provide only rough approximations because of the complexity of hydrogen-bonding interactions in solution. To better understand the application of the MOSCED and standard UNIFAC activity-coefficient prediction methods for screening extraction solvents, calculations were compared with K data for the extraction of propylene glycol n-propyl ether (PnP), a model hydrogen-bonding compound. Estimates of limiting activity coefficients for PnP dissolved in the organic phase (γ∞PnP,organic) obtained using MOSCED and UNIFAC are shown to be highly correlated with K data for a variety of hydrophobic organic solvents, including various alcohols, ketones, ethers, chlorinated hydrocarbons, aromatics, and aliphatic hydrocarbons. This example demonstrates how MOSCED or UNIFAC can be used to quickly rank candidate solvents for this class of compounds. The methodology facilitates process synthesis and design efforts by reducing the number of experiments required to identify suitable solvents. Introduction The ability to separate organic compounds from water is critical to many industrial manufacturing processes. Organics need to be removed from aqueous streams to recover the organic content for recycle back to the main production process, thus saving valuable resources, and to reduce the organic content prior to biotreatment and discharge of clean water to the environment. Improved methods that facilitate efforts to identify and design suitable processes are desired by industry because they can help enlarge the scope of process schemes considered for a given application and reduce process development costs. Predicting the processing behavior of organics that form hydrogen bonds with water is particularly difficult because of the complexity of organic-water interactions in solution. Furthermore, for many of these compounds, the use of standard distillation is difficult or impossible because of insufficient relative volatility with respect to water; relatively volatility is low because of the hydrophilic nature of the organic solute or because of low or moderate pure-component vapor pressure relative to water. Thus, additional process options often are needed. Liquid-liquid extraction using a hydrophobic extraction solvent can be an attractive alternative to distillation that can reduce energy consumption even when distillation is technically feasible and relative volatility is on the order of R ) 10 or even higher.1 Extraction processes of this type are useful for recovering glycol ethers, polyglycols, and nonionic surfactants * To whom correspondence should be addressed. E-mail: tcfrank@ dow.com. Tel.: 989-636-4310. Fax: 989-636-4616. † The Dow Chemical Company, Engineering & Process Sciences Laboratory. ‡ The Dow Chemical Company, Analytical Sciences Laboratory.
from various aqueous streams produced in bioprocessing applications,2-5 as well as in other applications where hydrogenbonding organic compounds are mixed with water. This study evaluates the use of the MOSCED6,7 and standard UNIFAC8-10 activity-coefficient prediction methods to screen hydrophobic solvents for extracting organics from aqueous solution as an aid to process development. Screening calculations are evaluated by comparison with data1 for the extraction of propylene glycol n-propyl ether (PnP), a model hydrogenbonding compound with the structure C3H7-O-CH2-CH(CH3)-OH. Although the example described here specifically focuses on evaluating the use of MOSCED and UNIFAC to predict the behavior of hydrogen-bonding organic solutes, the methodology is applicable to other types of organic solutes, as well. General Concepts It is well-known that the partition ratio for a given solute of interest can be calculated directly from the ratio of activity coefficients
K (mole fraction basis) )
yi γi,aqueous ) xi γi,organic
(1)
where yi is the mole fraction of solute in the organic solvent phase, xi is the mole fraction of solute in the aqueous phase, and γi is the activity coefficient for the solute dissolved in the indicated phase. A variety of methods are available for estimating values of γi for each phase6-14 or for directly estimating the value of K.15 For example, the use of the standard UNIFAC activity-coefficient prediction method for estimating liquid-
10.1021/ie070010+ CCC: $37.00 © 2007 American Chemical Society Published on Web 05/19/2007
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liquid equilibrium (LLE) is discussed by Gupte and Danner16 and by Hooper, Michel, and Prausnitz.17 An application of the MOSCED activity-coefficient prediction method is described by Escudero, Cabezas, and Coca.18 The well-known UNIFAC group-contribution method calculates the activity coefficient from pure-component properties plus contributions from interactions between the structural groups of each component. MOSCED is a modified regularsolution model that characterizes molecular interactions in terms of pure-component properties alone, including parameters taking into account nonpolar dispersion forces, molecular polarity and polarizability, hydrogen-bonding proton-donor capability (acidity), and hydrogen-bonding proton-acceptor capability (basicity). It is generally recognized that the uncertainty in the calculation of K is very high for most applications involving hydrogenbonding aqueous solutions, regardless of the method used to estimate the activity coefficients. Given that K values generally are difficult to calculate with high numerical accuracy, the present authors have found it useful instead to rank candidate extraction solvents simply according to estimated values of γ∞i,organic given that this term is the main factor differentiating the various solvents; that is, this term represents the specific interactions between solute and solvent in the organic phase. This approach to ranking candidate solvents assumes that any effect due to differences in the solubility of water in the solvent phase can be neglected and that the activity coefficient for solute dissolved in the aqueous phase is essentially constant as a function of extraction solvent. In many cases, these assumptions are expected to yield valid first-order approximations that allow rapid assessments. The present study was done to probe the validity of the method and its assumptions by comparison with data for PnP, a representative hydrogen-bonding compound. The effect of temperature on limiting activity coefficients,19 and thus on K, also is difficult to predict with high accuracy, particularly for systems with strong specific molecular interactions. In many cases, the limiting activity coefficient for the organic dissolved in aqueous solution (γ∞i,aqueous) increases significantly with increasing temperature over a wide temperature range. This can be explained as being due to disruption of hydrogen bonding between the organic and water, allowing hydrophobic interactions to become more significant with increasing temperature.1 For a number of these compounds, a lower critical solution temperature (LCST) is seen in the temperature/composition phase diagram. These are solvated solutions, where significant hydrogen bonding between the organic component and the water render them completely miscible at temperatures below the LCST. Because hydrogen bonding is an exothermic process,1 as the temperature rises, the equilibrium constant for hydrogen bonding diminishes. As a result, as the temperature rises above the LCST, there is less solvation, and the organic and water become increasingly immiscible. The family of glycol ethers is representative of this class of hydrogen-bonding compounds.1 Typical phase diagrams are shown in Figure 1 for ethylene glycol n-butyl ether + water and in Figure 2 for propylene glycol n-propyl ether + water. Other phase diagrams are available elsewhere.20,21 Although the magnitude of the temperature dependence of K is difficult to predict, the presence of an LCST in the organic + water phase diagram is an indicator that K is likely to increase with increasing temperature.1 For PnP (LCST ) 32 °C), it has been shown that changes in the molecular interactions in both phases contribute to the increase in K; the aqueous phase becomes more hostile to the PnP solute (γPnP,aqueous increases as a result of the disruption of hydrogen bonding), and the
Figure 1. Temperature-composition diagram for ethylene glycol n-butyl ether + water, taken from ref 20.
Figure 2. Temperature-composition diagram for propylene glycol n-propyl ether + water, taken from ref 20.
organic phase becomes more attractive to the PnP solute (γPnP,organic decreases) as temperature increases.1 The organic phase behaves more like a regular solution where RT ln γi is constant over a moderate temperature range. Results and Discussion Partition ratio data for the extraction of PnP are summarized in Table 1. The experimental apparatus and procedures, as well as the sources and purities of the solvents, are reported elsewhere.1 The data indicate that aliphatic alcohols and aliphatic ketones are among the better solvents for extraction of this etheric alcohol. Estimated values of the infinite-dilution activity coefficients for PnP dissolved in these solvents, obtained using MOSCED and standard UNIFAC, are reported in Table 2. It is interesting to note that the calculated values obtained using MOSCED and standard UNIFAC differ significantly. This likely is a consequence of the independent development of these models using somewhat different data sets, particularly in this case involving glycol ethers for which data are scarce. However, the two sets of calculated values show similar trends in the ordering of solvents and the qualitative effect of temperature. Figures 3 and 4 compare measured K values with the estimated activity coefficients expressed as 1/γ∞i,organic. Note that it is important to express K on a mole fraction basis as indicated in eq 1; the ordering of K (weight percentage basis) is somewhat different because the molecular weights of the extraction solvents vary. The results are plotted for two narrow temperature ranges to minimize uncertainties in the effect of temperature on the calculated results. In these figures, K values are plotted versus 1/γ∞i,organic instead of γ∞i,organic because the better solvents are indicated by increasing values of 1/γ∞i,organic and because the results then fall on a scale from near 0 to something on the
Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007 4623 Table 1. Partition Ratios for Extraction of PnP from Water1 partition ratio at lower temperature temp (°C)
extraction solvent 2-ethylhexanol 1-octanol methyl isobutyl ketone (MIBK) methyl heptyl ketone (MHK) diisobutyl ketone (DIBK) dichloromethane methyl t-butyl ether (MTBE) toluene 1,2-dichlorobenzene di-n-butyl ether Aromatic 200 cyclohexane kerosene Isopar Mc
K (mole fraction basis)a
partition ratio at higher temperature
K (wt % basis)b
temp (°C)
K (mole fraction basis)a
K (wt % basis)b
40 50 40
66 ( 7 62 ( 7 33 ( 4
11.3 ( 1.2 10.9 ( 1.2 6.6 ( 0.8
95 95 95
210 ( 30 106 ( 12 74 ( 8
37.5 ( 5 19.8 ( 2 15.3 ( 2
40
28 ( 3
4.0 ( 0.4
95
80 ( 9
11.7 ( 1
20
14 ( 2
1.9 ( 0.3
95
76 ( 8
10.9 ( 1
25 30
34 ( 4 23 ( 3
7.4 ( 0.9 5.1 ( 0.6
35 45
48 ( 5 42 ( 5
10.2 ( 1 9.3 ( 1
50 50 30 40 40 40 40
23 ( 3 26 ( 3 10 ( 1 14 ( 2 8(1 8(1 6 ( 0.7
4.5 ( 0.6 3.4 ( 0.4 1.5 ( 0.2 1.7 ( 0.2 1.7 ( 0.2 0.88 ( 0.1 0.65 ( 0.07
95 95 95 95 67 95 95
45 ( 5 51 ( 5 42 ( 5 40 ( 4 17 ( 2 27 ( 3 22 ( 2
8.9 ( 1 6.6 ( 0.7 6.2 ( 0.6 5.0 ( 0.5 3.8 ( 0.4 3.2 ( 0.3 2.2 ( 0.2
a Partition ratio ) (mole fraction of PnP in organic layer)/(mole fraction of PnP in aqueous layer). b Partition ratio ) (weight percentage of PnP in organic layer)/(weight percentage of PnP in aqueous layer). c Isopar is a trademark of Exxon Mobil Corp.
Table 2. Calculated Values of Infinite-Dilution Activity Coefficients estimates of γPnP,organic extraction solvent 2-ethylhexanol 1-octanol methyl isobutyl ketone (MIBK) methyl heptyl ketone (MHK) diisobutyl ketone (DIBK) dichloromethane methyl t-butyl ether (MTBE) toluene 1,2-dichlorobenzene di-n-butyl ether naphthaleneb cyclohexane n-dodecanec n-tetradecaned
temp (°C)
MOSCED
UNIFAC
40 95 50 95 40
a a 1.41 1.32 2.84
1.07 1.12 1.08 1.12 1.82
95 40
1.98 a
1.63 1.76
95 20
a 5.83
1.58 1.96
95 25 35 30
2.53 1.31 1.26 3.06
1.65 0.688 0.686 1.23
45 50 95 50 95 30 95 40 95 40 67 40 95 40 95
2.67 5.56 3.29 a a 6.34 2.99 7.39 3.75 35.1 17.5 31.8 9.31 30.5 8.92
1.25 1.95 1.74 3.19 2.71 1.73 1.67 2.57 2.28 5.14 4.35 4.48 3.29 4.21 3.09
a Model parameters unavailable. b Representing Aromatic 200. resenting kerosene. d Representing Isopar M.
c
Figure 3. Comparison of K from Table 1 (mole fraction basis) with standard UNIFAC estimates of 1/γ∞PnP,organic.
Rep-
order of 1.0, effectively normalizing the results to highlight the better solvents. The identity of the solvent corresponding to a particular point can be determined by comparison with the results in Tables 1 and 2. The calculations were carried out using published UNIFAC group values10 from 2003 and published MOSCED parameter values7 from 2005 for the extraction solvents. Several of the solvents listed in Table 1 are distillation cuts. To make estimates using UNIFAC and MOSCED, these materials were represented
Figure 4. Comparison of K from Table 1 (mole fraction basis) with MOSCED estimates of 1/γ∞PnP,organic. For these calculations, MOSCED parameters for PnP are λ ) 15.94, τ ) 3.97, R ) 4.65, and β ) 11.07, all in units of (J/cm3)1/2, and q ) 1 (dimensionless). They were taken from The Dow Chemical Company database.
using model compounds: Aromatic 200 was equated with naphthalene, kerosene with n-dodecane, and Isopar M (trademark of Exxon Mobil) with n-tetradecane (Table 2). To represent the glycol ether structures, the UNIFAC calculations utilized structural group O-CH2CH2-OH (group 47) to take into account proximity effects between the ether linkage and
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the hydroxyl group at the end of the molecule. MOSCED parameters for 2-ethylhexanol, methyl heptyl ketone, and 1,2dichlorobenzene were not available, and so these solvents were not included in the calculations. They were excluded from the UNIFAC plot in Figure 3 as well, to allow a direct comparison with the MOSCED results. The MOSCED parameters used to represent PnP are given in Figure 4. The resulting plots show that both UNIFAC and MOSCED estimates of 1/γ∞i,organic can yield reasonable correlations with K and so are suitable for the rapid identification of promising candidate solvents. A linear least-squares analysis indicates that the lower-temperature data are correlated with R2 ) 0.96 or greater as indicated in the figures. The correlation is not as good at the higher temperatures, but it is still acceptable for screening purposes. This likely is due to the approximate nature of the temperature dependence built into these models. These results indicate that the ranking of candidate solvents is not a strong function of temperature. Figures 3 and 4 illustrate how application of UNIFAC or MOSCED can, in general, identify promising candidate solvents for further study in the laboratory. In the case of MOSCED, the model parameter values also provide a means for better understanding how polarity, polarizability, acidity, and basicity affect a given solvent’s ability to interact with the solute of interest, as the model parameter values are related to these molecular properties.6,7 The UNIFAC and MOSCED methods are complementary in the sense that one or the other might be more convenient to apply, depending on the availability of parameter values for the system of interest. In the present application, MOSCED yields somewhat better correlation results compared to standard UNIFAC. The modified UNIFAC (Dortmund) model11 was designed to improve the UNIFAC description of asymmetric systems, so it might improve on the standard UNIFAC results. In our experience, both UNIFAC and MOSCED provide very useful guidance but should not be used for the final selection of an extraction solvent; they cannot be relied on to identify all potentially useful solvents or solvent blends or to provide the correct ranking of the top candidates. For example, in the present application, standard UNIFAC did not identify 2-ethylhexanol as the top candidate; including 2-ethylhexanol in the highertemperature correlation in Figure 3 reduces the linear-regression correlation constant from R2 ) 0.96 to R2 ) 0.92. This might be related to a potentially synergistic effect that dissolved water has on the value of K for this particular solvent,1 an effect that is not included in the present screening calculations because of difficulties in accurately predicting mutual solubilities between organic and aqueous phases. Summary and Conclusions This study outlines a methodology utilizing MOSCED or UNIFAC to screen candidate extraction solvents with respect to partition ratios for the extraction of hydrophilic organics from aqueous solution using hydrophobic extraction solvents. The methodology is demonstrated by comparison with data for propylene glycol n-propyl ether, a representative hydrogenbonding organic. The methodology avoids the difficulties and uncertainties involved in accurately predicting the magnitude of K or the effect of temperature; instead, the approach focuses on determining the relative order of limiting activity coefficients in the organic phase, which the representative example indicates is an excellent indicator of relative K values and is not in itself a strong function of temperature. This example thus demonstrates how MOSCED and UNIFAC can provide valuable guidance for screening candidate solvents for the extraction of
glycol ethers and similar compounds in general. This approach to screening extraction solvents can be used to quickly discard solvents that cannot provide the needed partitioning performance and can help to generate a short list of candidate solvents for further evaluation in the laboratory, thus reducing the experimental effort required to evaluate and develop a suitable process. Other factors affecting the commercial viability of an extraction solvent also must be considered, including the health, safety, and environmental aspects of solvent use.22,23 The methodology described in this work should be relevant to ranking solvents for the removal of many industrially important hydrogenbonding compounds from aqueous solution, including glycol ethers, polyglycols, nonionic surfactants, aliphatic amines, and alkanolamines. Acknowledgment The authors gratefully acknowledge David Bush and Sumnesh Gupta for valuable discussions. Literature Cited (1) Frank, T. C.; Donate, F. A.; Merenov, A. S.; Von Wald, G. A.; Alstad, B. J.; Green, C. W.; Thyne, T. C. Separation of Glycol Ethers and Similar Lower Critical Solution Temperature (LCST)-Type HydrogenBonding Organics from Aqueous Solution Using Distillation or LiquidLiquid Extraction. Ind. Eng. Chem. Res. 2007, 46 (11), 3774-3786. (2) Frank, T. C.; Donate, F. A.; Shields, J. E.; Li, Kai; Allen, J. R. Method for Extraction of Intracellular Proteins from a Fermentation Broth. International Patent Application WO 2005/087791, 2005. (3) Frank, T. C.; Donate, F. A.; Alstad, B. J. Process for Removing Water from Aqueous Solutions of Proteins. International Patent Application WO 2005/092915, 2005. (4) Frank, T. C.; Donate, F. A.; Thyne, T. Process for Recovering Organic Compounds from Aqueous Streams Containing Same. International Patent Application WO 2005/087692, 2005. (5) Pollard, J. M.; Shi, A. J.; Goklen, K. E. Solubility and Partitioning Behavior of Surfactants and Additives Used in Bioprocesses. J. Chem. Eng. Data 2006, 51, 230-236. (6) Thomas, E. R.; Eckert, C. A. Prediction of Limiting Activity Coefficients by a Modified Separation of Cohesive Energy Density Model and UNIFAC. Ind. Eng. Chem. Process Des. DeV. 1984, 23, 194-209. (7) Lazzaroni, M. J.; Bush, D.; Eckert, C. A.; Frank, T. C.; Gupta, S.; Olson, J. D. Revision of MOSCED Parameters and Extension to Solid Solubility Calculations. Ind. Eng. Chem. Res. 2005, 44, 4075-4083. (8) Fredenslund, A.; Gmehling, J.; Michelsen, M. L.; Rasmussen, P.; Prausnitz, J. M. Computerized Design of Multicomponent Distillation Columns Using the UNIFAC Group Contribution Method for Calculation of Activity Coefficients. Ind. Eng. Chem. Process Des. DeV. 1977, 16, 450462. (9) Hansen, H.; Schiller, M.; Gmehling, J. Vapor-Liquid Equilibria by UNIFAC Group Contribution. 5. Revision and Extension. Ind. Eng. Chem. Res. 1991, 30, 2352-2355. (10) Wittig, R.; Lohmann, J.; Gmehling, J. Vapor-Liquid Equilibria by UNIFAC Group Contribution. 6. Revision and Extension. Ind. Eng. Chem. Res. 2003, 42, 183-188. (11) Jakob, A.; Grensemann, H.; Lohmann, H.; Gmehling, J. Further Development of Modified UNIFAC (Dortmund): Revision and Extension 5. Ind. Eng. Chem. Res. 2006, 45, 7924-7933. (12) Chen, C.-C.; Crafts, P. A. Correlation and Predicition of Drug Molecule Solubility in Mixed Solvent Systems with the Nonrandom TwoLiquid Segment Activity Coefficient (NRTL-SAC) Model. Ind. Eng. Chem. Res. 2006, 45, 4816-4824. (13) Eckert, F.; Klamt, A. Fast Solvent Screening via Quantum Chemistry: COSMO-RS Approach. AIChE J. 2002, 48, 369-385. (14) Mullins, E.; Oldland, R.; Liu, Y. A.; Wang, S.; Sandler, S. I.; Chen, C.-C.; Zwolak, M.; Seavey, K. C. Sigma-Profile Database for Using COSMO-Based Thermodynamic Methods. Ind. Eng. Chem. Res. 2006, 45, 4389-4415. (15) Meyer, P.; Maurer, G. Correlation and Prediction of Partition Coefficients of Organic Solutes between Water and an Organic Solvent with a Generalized Form of the Linear Solvation Energy Relationship. Ind. Eng. Chem. Res. 1995, 34, 373-381.
Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007 4625 (16) Gupte, P. A.; Danner, R. P. Prediction of Liquid-Liquid Equilibria with UNIFAC: A Critical Evaluation. Ind. Eng. Chem. Res. 1987, 26, 2036-2042. (17) Hooper, H. H.; Michel, S.; Prausnitz, J. M. Correlation of LiquidLiquid Equilibria for Some Water-Organic Liquid Systems in the Region 20-250 Degrees C. Ind. Eng. Chem. Res. 1988, 27, 2182-2187. (18) Escudero, I.; Cabezas, J. L.; Coca, J. Liquid-Liquid Extraction of 2,3-Butanediol from Dilute Aqueous Solutions with Mixed Solvents. Chem. Eng. Commun. 1999, 173, 135-146. (19) Sherman, S. R.; Suleiman, D.; Hait, M. J.; Schiller, M.; Liotta, C. L.; Eckert, C. A.; Li, J.; Carr, P. W.; Poe, R. B.; Rutan, S. C. Correlation of Partial Molar Heats of Transfer at Infinite Dilution by a Linear Solvation Energy Relationship. J. Phys. Chem. 1995, 99, 11239-11247.
(20) Christensen, S. P.; Donate, F. A.; Frank, T. C.; LaTulip, R. J.; Wilson, L. C. Mutual Solubility and Lower Critical Solution Temperature for Water + Glycol Ether Systems. J. Chem. Eng. Data 2005, 50, 869877. (21) Sorensen, J. M.; Arlt, W. Liquid-Liquid Equilibrium Data Collection; DECHEMA: Frankfurt, Germany, 1979-1980; Vol. V, Parts 1-3. (22) Allen, D. T.; Shonnard, D. R. Green Engineering: EnVironmentally Conscious Design of Chemical Processes; Prentice Hall PTR: Upper Saddle River, NJ, 2002. (23) Crowl, D. A.; Louvar, J. F. Chemical Process Safety: Fundamentals with Applications; Prentice Hall PTR: Upper Saddle River, NJ, 2001.
ReceiVed for reView January 3, 2007 ReVised manuscript receiVed April 12, 2007 Accepted April 16, 2007 IE070010+
