Ind. Eng. Chem. Res. 1994,33, 151-156
151
Liquid-Liquid Equilibria for the Oleic Acid-@-Sitosterol-Water System at Elevated Temperatures and Pressures Jose A. Briones3 Joseph C. Mullins, and Mark C. Thies' Department of Chemical Engineering, Clemson University, Clemson, South Carolina 29634
Compressed liquid water a t elevated temperatures is being evaluated as an extractive solvent for separating mixtures of oleochemicals. In this paper, results are presented for a model of the tall oil-water system, namely, the ternary system oleic acid-&sitosterol-water. A continuous-flow apparatus was used to measure liquid-liquid equilibrium compositions for this system a t 572,579, and 586 K. Selectivities of water for oleic acid relative to ,&sitosterol were found t o be 10-15, and distribution coefficients of oleic acid ranged from 0.0063 to 0.048 for the temperatures investigated. The experimental data were correlated with the NRTL equation. Limited data are also presented for the oleic acid-dehydroabietic acid-water system.
Introduction
C H
12 5
Existing methods for separating the components of tall oil and soybean oil deodorizer distillate result in significant losses due to side reactions and also require the use of hazardous organic solvents. In an earlier paper, we proposed a new process for separating mixtures of oleochemicals: by using liquid water at elevated temperatures and pressures as an extractive solvent (Briones et al., 1990). A southern pine tall oil and a soybean oil deodorizer distillate were extracted with liquid water at temperatures from 571 to 585 K. Results indicated that water can be used to extract fatty and resin acids from these two oleochemical mixtures, and that less pitch byproduct is formed (Thies et al., 1992). In this paper, liquid-liquid equilibrium compositions are presented for the ternary system oleic acid-@-sitosterol-water at elevated temperatures and pressures. This system was chosen as a model of the tall oil-water system, with oleic acid being one of the major components in the acid fraction and @-sitosterolin the neutrals fraction. Our objective was to use these data as input to several standard activity coefficient models, and then to test these models for their ability to simulate our proposed extraction process. Limited results were also obtained for the oleic acid-dehydroabietic acid-water system, with oleic acid being representative of the fatty acid fraction and dehydroabietic acid of the resin acid fraction of tall oil. Structures of @-sitosteroland dehydroabietic acid are shown in Figure 1.
CH3
I
Figure 1. Structures of selected talloil compounds: (a) &sitosterol; (b) dehydroabietic acid. MICROMETERING VALVE
W
Experimental Apparatus A continuous-flow apparatus (Briones et al., 1989)was used to measure the desired phase compositions. This apparatus was designed to minimize the residence times of the components of interest at elevated temperatures, which is an important consideration for thermally sensitive substances such as oleochemicals. A schematic of the apparatus is shown in Figure 2. Unless otherwise noted, all fluid transfer lines are 316stainless steel with an outside diameter (0.d.) of 1.59 and an inside diameter (i.d.1 of 0.762 mm. For an experimental run, an oleic acidj3-sitosterol (or oleic acid-dehydroabietic acid) mixture t Present address: Elf Atochem North America Inc., 900First Ave., King of Prussia, PA 19406.
L
ORGANIC
E
r a BATH
Figure 2. Schematic diagram of continuous-flow apparatus.
and water are delivered as compressed liquids by separate high pressure feed pumps (Milton Roy minipump, Model No. 396, and Isco syringe pump, Model No. LC 5000, respectively). For each measured temperature, at least four different oleic acid-@-sitosterol mixtures were used, with concentrations ranging from 7to 30w t % &sitosterol. Only one oleic acid-dehydroabietic acid mixture was used, containing 16.6 w t % dehydroabietic acid. The combined flow rate from the two pumps was constant for a given
0888-5885/94/2633-0151$04.50/00 1994 American Chemical Society
152 Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994
experimental run and ranged from 200 to 300 mL/h during this study. The solvent-to-feed ratio was maintained at approximately 2:l for all runs. A 150-mL gas sample cylinder serves as a surge tank and dampens pressure fluctuations caused by the Milton Roy pump. The pump feed reservoir for the oleochemical mixture is maintained at 333-343 K to reduce the viscosity for easier pumping and to ensure that the @-sitosterolor dehydroabietic acid remains in solution. A nitrogen blanket is maintained over the contents of the feed reservoir so that no oxidation reactions occur. After leaving the pumps, the two liquids enter the equilibrium coil, which is used for heating the two-phase mixture to the desired operating temperature. The temperature of this mixture was always within 0.5 K of the contents of the view cell. After exiting the coil, the equilibrated, two-phase mixture enters the view cell, which functions as a phase separator. The raffinate phase, which is richer in the organics, exits the top of the cell and is expanded to atmospheric pressure across a micrometering valve (Autoclave Engineers, Model No. 6OVRMM). The extract phase, which is richer in water, exits the bottom of the cell and is similarly expanded through a micrometering valve. (Although both phases are greater than 50 mol % water, for convenience we will refer to the phase containing the higher percentage of organics as the “organic-richphase” and the phase containing more water as the “water-rich phase”.) The micrometering valves and sample collection lines are heated to 323-343 K to reduce sample viscosity and prevent the precipitation of solids in the lines. Five consecutive 10-15-g samples of each phase are collected to ensure representative samples and smooth out scatter due to phase separation in the lines. Temperatures of the feed mixture and of each phase in the cell are measured with type K differential thermocouples referenced to an aluminum block located inside the constant-temperature bath. The absolute temperature of the aluminum block is measured with a secondary standard platinum resistance temperature detector (RTD) (Burns Engineering, Inc.). Operating pressures are measured with a Bourdon-tube type, Heise gauge (Model CM, 0-5000 psi range) that was calibrated against a Budenberg dead-weight gauge (Model 380 H). Additional details of the experimental apparatus and procedure can be found elsewhere (Briones, 1992). Sample Analysis Two analytical techniques were used in this investigation. Karl Fischer titration was used to determine water content in the organic-rich, raffinate samples. Gas chromatography (GC) was used to determine (1)the amount of @-sitosteroland @-sitosterolby-products present in both the raffinate and the water-rich, extract samples and (2) the amount of oleic acid in the extract samples. The amount of @-sitosteroland its by-products in the samples was determined with a silylation technique that we have developed on the basis of the work of two earlier workers. Valdez et al. (1986) developed a method for the silylation of fatty acids in aqueous solutions. Marks (1988) demonstrated that samples of soybean oil deodorizer distillate could be derivatized by silylation and analyzed by GC without previous saponification or separation of the neutral and acid fractions; however, his method is not applicable to systems containing water. By combining the results of these two workers, we have developed a method for the silylation of aqueous solutions containing oleochemicals such as fatty acids, resin acids, and sterols, eliminating the need for tedious saponification, extraction, or dehydration steps.
Typically, the main by-product present in the samples was the product of the esterification reaction between @-sitosteroland oleic acid, 0-sitosteryl oleate (which is one of the main components of tall oil pitch). In addition, apercentage of the @-sitosterolpresentunderwent thermal dehydration (i.e., loss of the hydroxyl group) to form dehydrated @-sitosterol. All samples were analyzed on a Hewlett-Packard 5980A gas chromatograph equipped with a flame ionization detector and a 0.53-mm i.d. X 15-m long X 0.15-pm film methyl silicone column (DB-1, J&W Scientific). A description of the techniques used is given below. Raffinate Phase. The collected samples from the organic-rich, raffinate phase were first homogenized by the addition of 30-40 mL of toluene containing a known amount of cholesteryl hexanoate as the internal standard and 40-50 mL of anhydrous methanol. The samples were derivatized by placing 3 drops of the homogenized sample into a 2-mL vial followed by 200 pL of acetonitrile and 800 pL of the silylation reagent. The reagent used was bis(trimethylsily1)trifluoroacetamide(BSTFA) plus 1% trimethylchlorosilane (TMCS). The silylated sample was then injected into the gas chromatograph. From the chromatographic plots, the @-sitosterollcholesterylhexanoate, dehydrated @-sitosteroVcholestery1 hexanoate, and P-sitosteryl oleate/cholesteryl hexanoate area ratios were calculated for a given sample. These GC area ratios were compared with previously prepared calibration curves to obtain the @-sitosterol/cholesterylhexanoate, dehydrated @-sitosterollcholesterylhexanoate, and j3-sitosteryloleate/ cholesteryl hexanoate mass ratios in the sample. From a knowledge of these mass ratios and the amount of cholesteryl hexanoate added to the original sample, the masses of @-sitosterol,dehydrated @-sitosterol,and @-sitosteryl oleate in the raffinate sample were then calculated. The calibration curves were prepared using 5a-cholestane instead of dehydrated @-sitosterol,and cholesteryl stearate instead of @-sitosteryloleate (neither of the compounds of interest are available in a pure form). Reproducibility of a given sample averaged *2%. Additional details on the silylation of aqueous solutions containing fatty acids and neutrals such as sterols are discussed elsewhere (Briones, 1992). The water content in the raffinate samples was determined by Karl Fischer titration. In the titrations, 0.52-mL portions of the homogeneous solution were analyzed for water using a Metrohm automatic titrator (Model No. E547) and buret (Model No. E535) from Brinkmann Instruments. For a given sample, the water content was always reproducible to better than f0.576 error. The mass of oleic acid was calculated from the difference between the mass of the sample and the masses of water, @-sitosterol, dehydrated @-sitosterol,and @-sitosteryloleate present. Extract Phase. Samples of the water-rich, extract phase were first homogenized by the addition of 50-60 mL of acetone containing a known amount of the internal standard erucic acid and 10-15 mL of toluene containing a known amount of cholesteryl hexanoate as the internal standard. The samples were derivatized by placing 20 drops of the homogenized sample into a 2-mL vial followed by 200 pL of acetonitrile and 800 p L of BSTFA + 1% TMCS. After injection into the gas chromatograph, the mass of @-sitosterolin the samples was calculated in a manner similar to that described above for samples of the raffinate phase. To determine the amount of oleic acid in the extract samples, a methylating reagent, trimethylphenylammonium hydroxide (TMAH), was added to a 2-mL portion of the homogeneous sample before injection
Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994 153 into the gas chromatograph. After injection of a 1-pL portion into the gas chromatograph, complete esterification of the oleic acid occurred at the injection port temperature of 573 K. The oleic acidlerucic acid mass ratio of a given sample was calculated from the resulting GC area ratio by comparison to a previously prepared calibration curve. From a knowledge of this mass ratio and the amount of erucic acid added to the original sample, the mass of oleic acid in an extract sample was then calculated. The mass of water in an extract sample was calculated from the difference between the total mass of the sample and the mass of oleic acid and @-sitosterolpresent. No @-sitosteryl oleate was detected in samples of the extract phase. However, a measurable fraction of the @-sitosterolreacted to form dehydrated @-sitosterol. Reproducibility of concentrations obtained for a given sample averaged f l % . Oleic Acid-Dehydroabietic Acid-Water System. Samples from both phases were homogenized by the addition of 20-50 mL of anhydrous methanol. The water content was determined by Karl Fischer titration. The homogeneous samples were derivatized with the addition of TMAH to form fatty acid methyl esters and rosin acid methyl esters as described above. One-microliter portions of the derivatized samples were injected into the gas chromatograph. The area ratio of oleic acid to dehydroabietic acid was calculated and compared with previously prepared calibration mixtures to obtain the mass ratio of oleic acid to dehydroabietic acid in the experimental sample. Additional details regarding the analysis of these samples can be found elsewhere (Briones, 1992). Materials. BSTFA + 1%TMCS was obtained from Regis Chemical Co. TMAH (0.1 M in methanol) was supplied by Kodak Laboratory and Research Products. HPLC grade acetonitrile, Karl Fischer grade methanol, ACS grade toluene, and ACS grade acetone and were obtained from Fisher Scientific. @-Sitosterolwith a purity of 90% was supplied by DBrivBs RBsiniques et TerpBniques (Dax,France). Cholesteryl hexanoate, cholesterylstearate, and 5a-cholestane were obtained from Sigma Chemical Company. Oleic acid with a purity of 93 % was supplied by Gallard-Schlesinger Inc. The major impurities were stearic and linoleic acid. The purity of oleic acid was established by analysis with a Hewlett-Packard 5840A gas chromatograph equipped with a flame ionization detector and a 3.18-mm i.d. X 3.05-m long stainless steel column packed with 10% DEGS-PS on a 100-120 mesh Chromosorb W AW support. Distilled and deionized water was used for all experiments.
Table 1. Liquid-Liquid Equilibrium Compositions for Oleic Acid-@-Sitosterol-Water System (mole fraction) organic-rich, raffinate phase’ water-rich, extract phasea oleic @-sitosteryl oleic acid 8-sitosterol oleate acid B-sitosterol T = 572.2 K; P = 102 bar 0.132b 0.001 47 0.1371 0.0047 O.ooO94 0.00162 5.343-06‘ 0.1362 0.0052 O.OOO62 0.00162 4.89346 0.1403 5.783-06 0.0060 0.000 98 0.001 62 0.1405 1.02345 0.0119 0.001 59 0.001 48 0.1456 1.203-05 0.0196 0.003 43 0.001 28 0.1484 1.37345 0.0264 0.005 02 0.001 14 0.1490 1.59345 0.0345 0.00574 O.ooO94 0.1519 1.54345 0.0376 0.005 42 0.001 01 T = 579.1 K; P = 111bar 0.1003b 0.002 56 0.1107 0.0040 0.000 65 0.002 37 7.64346 0.1162 0.0101 0.001 58 0.002 02 1.12345 0.1274 0.0182 0.002 96 0.001 61 1.56345 0.1335 0.0317 0.004 57 0.001 47 2.31345 T = 586.0 K; P = 119 bar 0.0691b 0.005 6 0.0816 0.0025 0.000 24 0.003 89 1.423-05 0.0088 0.0915 2.19345 0.000 96 0.003 45 0.0149 0.001 91 0.002 73 0.1008 3.893-05 0.0248 0.00300 0.00245 0.1069 4.553-05 Mole fraction water is obtained by difference. Determined from binary oleic acid-water data (Briones et al.. 1989). 5.34346 represents 5.34 x lo”, etc.
2
4
6
8
10
12
14
16
Mole Percent Oleic Acid
Results and Discussion
Figure 3. Experimental and calculated tie lines for the oleic acid@-sitosterol-water system at 572.2 K. The water-rich phase is magnified in the inset.
Experimental solubility data for the ternary system oleic acid-@-sitosterol-water at 572.2, 579.1, and 586.0 K are given in Table 1and are plotted on ternary phase diagrams in Figures 3-5. At these temperatures the oleic acid-@sitosterol-water system forms a type I1 system (Robbins, 1984). As expected, the size of the two-phase region decreases with an increase in temperature, and this system would eventually become type I at temperatures higher than the upper critical solution temperature for the oleic acid-water binary (590 K). Mutual solubility data for the oleic acid-water system (Briones et al., 1989) are also shown, and the ternary measurements are consistent with these earlier results. Operating pressures were maintained about 10bar above the three-phase line (which occurs essentially at the vapor pressure of water) to ensure that no vapor phase was present. Because operating pressures were controlled to about f5 bar and the Heise gauge is accurate to f O . l bar,
reported results are accurate to f 5 bar. Some variation of the temperature in the view cell is characteristic of a flow apparatus. On the basis of both the accuracy of our temperature-measurement scheme and also the normal variations during an experimental run, the reported temperatures are believed to be accurate to within f0.3 K. The experimental compositions shown in Table 1 represent the average of five consecutive samples. For the organic-rich, raffinate phase, oleic acid compositions are believed to be accurate to *2% and @-sitosteroland @-sitosteryloleate compositions to within f 5 % of their true values. For the water-rich, extract phase, oleic acid compositions are believed to be accurate to f2% and @-sitosterolcompositions to *lo%. An average of 5% of the @-sitosterolpresent in the organic-rich phase and 15% of the @-sitosterolpresent in the water-rich phase underwent thermal dehydration to
154 Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994 x
lo'>
-1
0
1
- Expcrimcntal
x - Cdculnted
1.5
I
0.5' 13
13.5
14
14.5
15
15.5
16
20 X
1
X
0
x
15
s
u 10
2 -
7 X
" X
X
1 I
13.5
Mole Percent Oleic Acid
Figure 4. Experimental and calculated tie lines for the oleic acid8-sitosterol-water system at 579.1 K. The water-rich phase is magnified in the inset.
14
14.5
15.5
15
16
Mole Percent Oleic Acid
Figure 6. Selectivities and distribution coefficiente for oleic acid at 572.2 K vs mole percent oleic acid in the organic-richphase. 2.5
x 10.l
i
o - experimental
x -
2
x .Cllculatcd
X X
3
1.5-
X
-
I
1-
0'5:0
I1
10.5
11.5
12
12.5
13
,k
13.5
25 20
X
X
I
X X
0.
151 10
i
5. 0' 10
Mole Percent Oleic Acid
Figure 5. Experimental and calculated tie lines for the oleic acidb-sitosterol-water system at 586.0 K. The water-rich phase is magnified in the inset.
form dehydrated /3-sitosterol. In Table 1,the compositions given for 8-sitosterol include the dehydrated sterol present. A significant fraction of the &sitosterol present in the feed reacts with oleic acid to form 8-sitosteryl oleate. For the temperature range investigated the mass ratio of 8-sitosteryl oleate (Le., pitch) to /3-sitosterolin the organicrich phase was found to be essentially constant at 0.25. Because it is essentially insoluble in water, no 8-sitosteryl oleate was detected in the aqueous phase. Experimental selectivities of water for oleic acid over ,&sitosterolare plotted at the three measured temperatures in Figures 6-8. The selectivity of water for oleic acid over &sitosterol is defined as
:
1
,
mole fraction oleic acid mole fraction B-sitosterol water-rich D b e I mole fraction oleic acid mole fraction @-sitosterol oleic acid-richp h e (1) The selectivities found in this work are approximately 2 times greater than those found for the extraction of fatty and rosin acids from neutrals in tall oil (Briones et al., 1990). The lower selectivities seen in the tall oil extraction are probably due to the presence of other neutrals such as long-chain alcohols and hydrocarbons, which may be more soluble in water than sterols. However, the exper-
a=[
11
10.5
11.5
12
12.5
13
I
13.5
14
Mole Percent Oleic Acid
Figure 7. Selectivities and distributioncoefficienta for oleic acid at 579.1 K vs mole percent oleic acid in the organic-rich phase. 8
x 0 .
y"
Experimental
x - Calculated
X
6 41
J
I
7
7.5
8
8.5
9
9.5
10
10.5
11
75
8
85
9
95
10
105
11
20 15
:IO
I'
Mole Percent Oleic Acid
Figure 8. Selectivities and distribution coefficients for oleic acid at 586.0 K vs mole percent oleic acid in the organic-richphase.
imental values are similar to the selectivities found for the extraction of soybean oil deodorizer distillate (8 lo), which were calculated solely on the basis of selectivity for fatty acids over sterols (Briones et al., 1990). These results N
Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994 155 Table 2. Liquid-Liquid Equilibrium Compositions for Oleic Acid-Dehydroabietic Acid-Water System (mole fraction) ~~~
~
~~
organic-rich phase" water-rich phase0 oleic dehydroabietic oleic dehydroabietic acid acid acid acid T = 570.9 K P = 95 bar 0.118 0.020 0.0012 o.Oo0 44 T = 579.1 K;P = 100 bar 0.099 0.017 0.0019 0.OOO62 a
Mole fraction water is obtained by difference.
confirm our previous findings that water a t these conditions can be used to separate a mixture of fatty acids and neutrals such as @-sitosterol. As seen in Figures 6-8, no clear relationship between the selectivityand temperatures was obtained. Experimentally measured distribution coefficients for oleic acid at the different temperatures are also given in Figures 6-8. The distribution coefficient of the oleic acid is defined as mole fraction oleic acid in water-rich phase mole fraction oleic acid in oleic acid-rich phase (2) These results are also consistent with our results for the tall oil-water system (Briones et al., 1990). Limited experimental data were also obtained for the oleic acid-dehydroabietic acid (DAA)-water system (see Table 21, which was used as a model of the fatty and rosin acids present in tall oil. From these results, an average @ of 0.5 can be calculated, indicating that water selectively extracts DAA over oleic acid. We believe that this selectivity is due to the presence of an aromatic ring in the structure of DAA. Most other rosin acids contain a conjugated diene instead of an aromatic ring, so lower selectivities may occur for these compounds. Thus, although compressedliquid water at elevated temperatures and pressures could in principle be used to separate mixtures of fatty and rosin acids, the selectivities are probably too low for a practical process. Data Correlation. Two activity coefficient models, NRTL (Renon and Prausnitz, 1968) and UNIQUAC (Abrams and Prausnitz, 19751,were tested for their ability to fit our experimental data for the oleic acid-p-sitosterolwater system. Model parameters were calculated with a computer program that was developed by Fredenslund and co-workers and is widely used for the correlation of ternary liquid-liquid phase equilibrium data. This program is described in detail elsewhere (Srarensenet al., 1979; Smensen, 1980; Magnussen et al., 1980). For the NRTL equation, the nonrandomness parameter aij was kept constant at a value of 0.2. The UNIQUAC pure-component volume and area parameters for oleic acid and @-sitosterol,ri and qi, were calculated as the sum of the volume and area parameters for all the groups present in the molecule (Reid et al., 1987). Thus, both equations contain two adjustable parameters for each possible binary pair, and for ternary mixtures there are six adjustable parameters. Because @-sitosteryl oleate is present in detectable quantities only in the raffinate phase, for modeling purposes the mole fractions of oleic acid, water, and @-sitosterolwere normalized to sum to unity in the raffinate phase. Sorensen's program is most effectively used if one first minimizes an objective function that is based on the differences in activities for each component. The resulting parameters are then used as initial guesses for the minimization of the objective function F,, which is based on the differences between experimental and calculated k, =
mole fractions:
In this equation, x is the experimental mole fraction, R is the composition of the predicted tie line lying closest to the experimental tie line considered, i denotes component i ( = l , 2, 3), j denotes phase j (=I, 111, and k denotes tie line k (=l,2, ...,M'). The second term on the right-hand side is a penalty term that reduces the risk of multiple solutions associated with high values of model parameters. Q is a constant and Pn is a model parameter value (n = 1,2, ...,6). The value of Q was taken to be 10-lo,the default value recommended by Smensen. The third term in eq 3 ensures that the distribution coefficient of the solute at dilute concentrations is given a sufficient weighting. Accurate representation in the dilute region is impo$ant for the design of our proposed extraction process. 7,-is the predicted activity coefficient of the solute at infinite dilution, and k," is the distribution coefficient of the solute at infinite dilution. Phase I is the organic-rich phase and phase I1 is water-rich. Throughout this data correlation, @-sitosterolis the solute, and the distribution coefficient of @-sitosterolis defined as follows:
k, = mole fraction @-sitosterolin organic-rich phase (4) mole fraction @-sitosterolin water-rich phase To evaluate the goodness of fit of the data, we used the following criteria:
In eq 5, k, and &, are the experimental and predicted distribution coefficients for @-sitosterol. Several different correlation techniques were investigated to obtain the best fit of both the NRTL and UNIQUAC equations to the experimental data shown in Table 1. First, the objective function F, was used to calculate the six model parameters with no constraints placed on k,". Next, the six model parameters were recalculated by minimizing F,, but with k," initially fixed at the value obtained by extrapolating the experimental solute distribution Coefficients ( k , ) to zero solute concentration. k," was subsequently iterated from this initial fixed value, with new model parameters being calculated as before for each new value of ksmuntil Ak, was minimized. During this procedure Az was found to be relatively insensitive to changes in k,", NO Ak, was chosen as the sole criterion to be minimized. The use of Ak, as the minimization criterion has been reported by other workers (Correa et al., 1987; Zhang and Hill, 1991). The procedure described above was then repeated, except that only four of the model parameters were calculated from ternary data; the oleic acid-water binary parameters were fixed to the values obtained from fitting the binary data previously measured by Briones at the same temperature (Briones, 1992). For the NRTL equation, the lowest values of AhBwere obtained by using fixed values of ks" during the minimization of F, and by fixing the oleic acid-water parameters to the values previously calculated by binary data. The
166 Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994 Table 3. Modal Parameters for Oleic Acid (1)-&Sitosterol (2)-Water (3) System NRTL (a = 0.210 OA/@-sit OM320
T (K) 572.4 579.1 586.0
M,
k,"
&idR -2817.8 -3640.8 -3135.2
M211R -102.7 9974.7 261.94
&idRb -2311.8 -2551.1 -2771.8
&dRb 6149.5 6267.0 6295.3
j3-sitlHrO
4?dR 1715.6 3506.0 2006.9
&dR 2743.43 2oooc 58.8 0.083 5 w 3280.83 0.074 14.1 3072.5 46.8 2 w 0.216 a Unita of all parameters are kelvin. a is dimensionless. * Oleic acid-water parameters were calculated from binary data. c k," waa fixed during calculation of F,. Ax
resulting NRTL parameters are shown in Table 3, and the tie lines calculated from these parameters are compared to experimental data in Figures 3-5. Selectivities and distribution coefficientsas calculated from eqs 1and 2 are also compared to experimental data in Figures 6-8. Numerous attempts to correlate our experimental data with the UNIQUAC equation were unsuccessful. The correlations obtained were considerably poorer than the results obtained for NRTL and for this reason are not included here. These results are discussed in detail by Briones (1992).
Conclusions Liquid-liquid equilibrium compositions have been measured for the ternary system oleic acid-&sitosterolwater at 572,579, and 586 K. The measured selectivities and distribution coefficients are consistent with our earlier results for the tall oil-water system, indicating that the measured ternary is a reasonable model for the extraction of tall oil with compressed liquid water at elevated temperatures. Both the NRTL and UNIQUAC equations were investigated for their ability to fit the experimental results. Neither equation is capable of accurately representing all aspects of the experimental data, but NRTL is the clear choice because of its ability to adequately represent the solubility of the organics in the aqueous phase, resulting in reasonable predictions for selectivities and distribution coefficients. To our knowledge, this is the first time that the NRTL equation has been successfullyused to correlate ternary liquid-liquid equilibrium data at temperatures significantly above 100 "C. Acknowledgment This material is based upon work partially supported by the National ScienceFoundation under Grant No. CBT8809422. The Government has certain rights in the
material. The authors wish to thank Aage Fredenslund and co-workers for the computer program used for data correlation, and Ms.Natalie P. Hovsepian for her assistance with the experimental runs and the Karl Fischer titrations.
Literature Cited Abrams, D. S.; Prausnitz, J. M. AIChE J. 1975,21,116. Briones, J. A. Ph.D. Dissertation, Clemson University, 1992. Briones, J. A,; Beaton, T. A.; Mullins, J. C.; Thies, M. C. Fluid Phase Equilib. 1989,53,475. Briones, J. A.; Mullins, J. C.; Thies, M. C. J. Am. Oil Chem. SOC. 1990,67,852. Correa, J. M.; Arce, A.; Blanco, A.; Correa, A. Fluid Phase Equilib. 1987,32, 151. Magnuseen, T.; S~rensen,J. M.; Rasmuseen, P.; Fredenslund, A. Fluid Phase Equilib. 1980,4,151. Marks, C. J. Am. Oil Chem. SOC.1988,65,1936. Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th ed.; McGraw-Hik New York, 1987. Renon, H.; Prausnitz, J. M. AIChE J. 1968,14,135. Robbins, L. A. In Perry's Chemical Engineers' Handbook, 6th ed.; Green, D. W.,Ed.; McGraw-Hik New York, 1984; Section 15. Smensen, J. M. Ph.D. Disseration, The Technical University of Denmark, 1980. Smensen, J. M.; Magnuseen, T.; Rasmussen, P.; Fredenslund, A. Fluid Phase Equilib. 1979,3,47. Thies, M.C.; Mullins, J. C.; Briones, J. A. US. Patent 5 097 012, 1992. Valdez, D.; Iler, H. D. J. Am. Oil Chem. SOC.1986,63,119. Zhang, Z.;Hill, G. A. J. Chem. Eng. Data 1991,36,453.
Received for review May 20, 1993 Accepted September 27, 1993. Abstract published in Advance ACS Abstracts, December 1, 1993.
