171
Chemical Engineering and Processing, 33 ( 1994) 171- 187
High-pressure phase equilibria and densities of the binary mixtures carbon dioxide-oleic acid, carbon dioxide-methyl myristate, and carbon dioxide-methyl palmitate and of the ternary mixture carbon dioxide-methyl myristate-methyl palmitate C. A. Lockemann Institut
ftir Therm&he
Verfahrenstechnik,
Universitiit Karlsruhe
(TH), D-76128
Karlsruhe,
Germany
(Received March 15, 1994)
Abstract Densities and thermodynamic equilibrium compositions of both the gas and the liquid phases were determined for binary mixtures of carbon dioxide and oleic acid, methyl myristate, and methyl pahnitate, and for the ternary system carbon dioxide-methyl myristate-methyl pahnitate at pressures up to 130 bar. Binary equilibria and densities were measured at 40 “C, 50 “C, and 60 “C, the ternary system was studied at 40 “C and at 50 “C. High-pressure densities were also recorded for all pure components. The apparatus used for the experiments consisted of a high-pressure view cell through which both phases were recirculated. Compositions were determined from samples taken with the help of six-way valves. Densities were measured with a vibrating sensor tube. Solubilities increase with pressure at a given temperature; however, the separation of a mixture of methyl myristate and methyl pahnitate becomes less selective. All numerical values are given in full detail.
Introduction
The extraction of edible oils and the fractionation and purification of oil constituents with the help of supercritical fluids has gained considerable interest. In a
typical purification step, harmful components, such as free fatty acids, are removed from an oil. During fractionation of an oil, specific fatty compounds are obtained for future processing. These compounds are usually separated as esters, not as acids or glycerides, because of their higher stability and volatility. Fatty acid methyl esters are of particular interest, since they are important intermediates for manufacturing detergents or synthesizing oil and fat products with specific properties [ 11. The conventional technique for the separation of oil compounds is by vacuum distillation. However, some of these substances, such as polyunsaturated fatty acids and their esters, are sensitive to heat. In these cases, the use of supercritical fluids provides a means to avoid degradation. Knowledge of phase equilibria, both of pure substances and of mixtures in contact with the supercritical component, is essential to the design of a separation process. Carbon dioxide is most commonly used as supercritical fluid in food or pharmaceutical applications. Besides having a low critical temperature and a moderate critical pressure, it is non-flammable, non-toxic, and inexpensive.
02%2701/94/$7.00 SSDI 0255-2701(94)00528-P
Volumetric properties of equilibrium phases at high pressures have rarely been reported. However, their knowledge is required for detailed thermodynamic analysis, since partial molar volumes play an important role in high-pressure thermodynamics [2]. Owing to a sharp decline of kinematic viscosities in both the supercritical and the liquid phase as pressure is increased, local differences in density may result in strong free convection which again greatly affects mass-transfer rates [3]. The purpose of this paper is to report experimentally determined high-pressure phase equilibria of binary mixtures of carbon dioxide and oleic acid (cis-9-octadecenoic acid), carbon dioxide and methyl myristate (tetradecanoic acid methyl ester), and carbon dioxide and methyl palmitate (hexadecanoic acid methyl ester), as well as of the ternary system carbon dioxide-methyl myristate-methyl palmitate. Densities at high pressures were determined for both the saturated supercritical and liquid phases. The densities of the pure substances were also recorded as a function of pressure. The equilibrium compositions of both the liquid and the supercritical phases in the system CO,-oleic acid were determined by Seekamp [4] and Jakob [S] at 40 “C and 60 “C. Chrastil[6] and King [7] studied the solubility of oleic acid in the supercritical phase. Inomata et al. [8] employed a static method to determine phase equilibria of the systems CO,-methyl myristate and CO,-methyl palmitate. Wu et al. [9] measured the
0 1994 -
Elsevier Science S.A. All rights reserved
172
solubility of both esters in supercritical CO2 by using a dynamic method. Ternary phase equilibria of the compounds studied here are not known to have been published. Densities may generally be calculated from equations of state. However, there are only very few mixtures for which volumetric properties may be predicted with sufficient accuracy from such equations. While cubic equations of state perform quite satisfactorily when used for computing phase equilibria, they usually fail to describe measured molar volumes more than qualitatively. That, in part, is because they rely on critical properties which themselves frequently have to be estimated, as many organic compounds, and also the lipids studied in this work, decompose before actually reaching their critical pressure and temperature. If the densities of the coexisting phases are required, precise measurements are therefore indispensible. The density of pure carbon dioxide was measured in numerous works. From highly accurate experimental data, Span and Wagner [lo] developed a multi-parameter equation of state, the most precise of its kind so far. By contrast, densities of saturated supercritical phases have rarely been recorded. The supercritical-phase density of the system CO,-oleic acid, which has been studied in this work as well, was measured by Jakob [S, 11, 121 at 40 “C and 60 “C, also with a vibrating sensor tube. CO, saturated with oleic acid possesses a significantly higher density than pure COz. However, Jakob also reported densities of subcritical gaseous COz. in which oleic acid is virtually insoluble, to be higher than the data on which the Span and Wagner equation of state is based, so that some of the values given by Jakob seem doubtful. For detailed analysis of the mechanisms involved in supercritical extraction of liquids, the densities of the pure liquids must also be known. Swern [l] listed densities of oleic acid and both esters at ambient pressure. The density of a pure liquid generally increases with pressure. As liquid compressibilities usually are low, this change of density frequently is small. The work of Jakob contains high-pressure densities of pure oleic acid. Banipal et al. [ 131 determined the densities of several fatty acids as a function of pressure and showed that the densities of lipids increase by slightly less than 10 kg m - 3 as pressure is being raised from 1 to 100 bar. The density of oleic acid saturated with CO2 was measured by Jakob at 40 “C and 60 “C as well. In the pressure range studied (up to 130 bar), the densities of the saturated liquid phase are always significantly higher than those of the pure liquid. By contrast, Jakob observed decreasing densities with rising pressures and increasing solubilities in the system ethane-oleic acid, where the dissolved gas clearly leads to a less dense
structure of the liquid. The increase in density is obviously specific to systems containing COz. Densities of the systems CO,-methyl myristate and CO,-methyl palmitate have not been published previously.
Experimental
The experimental set-up used is shown in Fig. 1. The equilibrium chamber was a self-designed high-pressure view cell made of stainless steel with MAXOS windows and with an inner volume of 123 ml. The interior gaskets between the main frame and the windows consisted of indium. The exterior gaskets, made of polyaramide-reinforced elastomer, served as cushions between the steel covers and the windows. The covers, the windows, and the main frame were bolted together by 18 stainless steel screws. The cell was held in a 40 1 water bath made of glass and kept at constant temperature with a thermostat (Lauda CSG). The water bath was surrounded by transparent polycarbonate sheets of 10 mm thickness. Liquid CO, was charged into a three-piston membrane pump (LEWA Ek 16 / M3) and compressed to experimental pressures. It was cooled with the help of a cryostat (Lauda UK40-SD) to prevent evaporation inside the pump. A throttle reduced the pulsations caused by the pump. At the end of each experiment, CO1 was released through a shut valve and a needle valve, which allowed for controlled pressure reduction. The COz was heated electrically to prevent freezing of the needle valve. Two magnetic gear pumps (Micropump 1805 C pump heads with Verder 2030 drives, O-3000 rpm) were used to recirculate the liquid and the supercritical phases. Sampling of both phases was done with six-way valves (Valco 6UW) with isolatable sample loops of 1.215 ml for the liquid phase and 8.609 ml for the
Fig. 1. Schematic
diagram
of the experimental
set-up.
173
supercritical phase. Filters trapped graphite particles used to lubricate the gear pumps. Densities were determined with a vibrating tube sensor (Micro Motion D6) built into the circulation lines in place of the six-way valves. The sensor was connected to an electronics unit (Micro Motion, with IS. Barrier MMB-DMS), a density monitor (Micro Motion DMS, with an accuracy of 2 kg md3), and a mass flow meter (Micro Motion DlO-RT, 0.4% accurate). The sensor was frequently recalibrated with air, acetone, and degassed deionized water. It was mounted on a 14 kg steel plate to protect it from shock and thermostated electrically, since it could not be immersed in the water bath. All tubing consisted of stainless steel with Swagelok fittings and Whitey valves. Temperatures were recorded with NiCr/Ni thermocouples. Pressure inside the view cell was measured with a transducer (Burster 8210 D, O-200 bar, accuracy 0.5%). Other pressure measurements were made with WIKA and Bourdon gauges. In order to determine phase equilibria, the view cell was filled with the pure liquid or the liquid mixture and thermostated. Carbon dioxide was then pumped into the cell until the set pressure was reached. Equilibrium was attained by recirculating both phases. The liquid phase was withdrawn from the lower end of the cell and fed again from above, where it formed a falling film in contact with the supercritical phase. The supercritical fluid was removed from the top of the cell, then pumped to its bottom, from where it bubbled through the liquid. Equilibrium thus was rapidly attained, and both phases were thoroughly mixed. When the position of the interface in the view cell, which was observed with a cathetometer, remained constant, equilibrium was considered to have been reached, and the sample loops were isolated. The contents of the sample loops were analyzed by the following method. After switching the loops out of the system, the valves at the end of the transfer lines were carefully opened to expand the CO, to atmospheric pressure. The formerly dissolved liquid components precipitated in the sample loops and then were flushed with acetone into vials. The acetone was allowed to evaporate, and the remaining liquid was weighed. In the case of liquid mixtures of methyl myristate and methyl palmitate, some of the liquid was redissolved in acetone. The amount of each of the liquid components was determined with a gas chromatograph by the method described below. As the sample-loop volumes and the densities of both phases had been determined before, the total mass of the phase trapped in each loop was known. COr mass was obtained by subtracting the mass of the liquid component(s) from the total mass. Mole fractions of all
components could be calculated with the help of their molecular weights. Extreme care had to be taken to avoid excess pressures during pressure build-up. Pressure reduction caused solute to precipitate in the gas-phase sample loop where it could not be dissolved again as the recirculated CO* was always saturated. Excess solute flushed out after depressurization of the loop distorted measurements. The compositions of the mixtures were determined by using an HP 5890 gas chromatograph with a weakly polar packed column (Perkin-Elmer Dexsil 300 GC Chrom) and an FID detector. Helium was used as a carrier gas. The oven temperature, initially held at 100 “C, was heated to 140 “C at a rate of 15 K min -I, then to 200 “C at 2 K min -I, and finally to 220 “C at 10 K min-‘, where it was held for 1 min. Retention times thus obtained were less than 2 min for acetone, 10 min for methyl myristate, 17 min for methyl palmitate, and 19 min for oleic acid. The chromatographic response factors were determined by analyzing standard mixtures obtained gravimetrically. The mole fractions of methyl myristate were related to the total amount of matter, including the trace impurities. The accuracy of this calibration was better than + 0.001. When measuring densities, both phases were recirculated as above to attain equilibrium. The density monitor displayed the exact value directly. However, only one phase could be studied at a time. In order to determine the density of the other phase, pressure had to be released, the sensor had to be built into the other circulation line, and the experiment had to be repeated. Densities of the pure liquids at high pressures were determined by charging helium, which did not measurably dissolve in the liquids during the experiments, into the view cell. Methyl myristate was obtained from MerckSchuchardt at > 99% purity, with traces of methyl laurate and methyl palmitate. Methyl palmitate (Roth, 95% pure) was found to contain about 4% methyl myristate and about 1% methyl stearate. Both substances were used without further treatment, as the mixtures contained a maximum of 1% of substances other than methyl myristate and methyl palmitate. Impurities of such low concentration were not expected to affect the results beyond the limits of experimental accuracy. The purity of the oleic acid supplied by Roth was raised from 70% to 90% by freezing out most of the impurities, mainly stearic acid. For all experiments, bone-dry carbon dioxide of grade 4.5 (MesserGriesheim) was employed. The acetone (Roth) used to flush the sample loops and to clean the apparatus was of 99.5% purity. Helium of grade 4.6 (MesserGriesheim) was used to transmit pressure to the pure liquids when measuring their densities.
174
TABLE 2. Binary phase equilibrium of the system CO*-oleic acid at 50 “C
High-pressure phase equilibria High-pressure phase equilib$a of the binary system CO,-oleic acid were measured at 40 “C and 50 “C and at pressures up to 80 bar. The binary systems COzmethyl myristate and CO,-methyl palmitate were studied at 40 “C, 50 “C, and 60 “C, and at pressures up to the consolute point of the mixture. The ternary system CO,-methyl myristate-methyl pahnitate was investigated at 40 “C and 50 “C and between 70 bar and 130 bar. At pressures below 70 bar, solubilities of the liquid component in the supercritical phase are so low that they could not be accurately determined with the method described above. Only the composition of the liquid phase is therefore reported for low pressures. The amount of liquid trapped in the gas-phase sample loop at 70 bar and 80 bar was so minute that errors due to flushing and weighing reduced accuracy. The composition of the liquid phase was highly reproducible in all cases. The mole fractions phases of the binary systems
of CO2 obtained in both at a given pressure and
temperature are listed in full detail in Tables 1 to 8. Figure 2 shows system pressure as a function of CO* mole fraction in the binary mixture CO,-oleic acid. The data on the left-hand side are saturation pressures as a function of liquid-phase composition, those on the righthand side represent saturation pressures at given vapor composition. The diagram also contains data of Seekamp [4] and Jakob [5]. Agreement between the different sets of data is good. Chrastil [6] employed a static method in order to measure gas-phase solubilities; his data are significantly lower than those of Seekamp and Jakob and therefore not included in the diagram. King et al. [7] measured solubilities of oleic acid in COz only at 35 “C and 60 “C. In Figs. 3-5, the phase equilibria of CO,-methyl myristate and CO,-methyl palmitate, both at 50 “C, are shown. Data of Inomata et al. [8] are also included. Saturation pressures belonging to a certain composition both in the liquid and the supercritical phases as measured by Inomata et al. are always slightly higher than TABLE 1. Binary phase equilibrium of the system CO,-oleic acid at 40 “C T=40
“C
9.4 19.5 25.4 39.7
0.1253 0.2763 0.3387 0.4587
_ _ -
41.1 50.0 70.0
0.4759 0.5308 0.6340
_ -
T=50”C
10.7 19.9 26.7 31.8 32.7 50.7 60.6 70.1 80.1
0.1038 0.2214 0.3128 0.3747 0.3827 0.5683 0.6268 0.6942 0.7605
TABLE 3. Binary phase equilibrium myristate T=40 P
_ _ _ _ _
of the system CO-methyl
at 40 “C “C
Ml
XCOl
lco,
8.8
0.1381 0.3363 0.4450 0.4505 0.4590 0.5429 0.6353 0.6983 0.7429 0.7796
_ _ _ _ _ _ _ _ _ _
0.7725 0.8541 0.9427
0.9999 0.9989 0.9862
18.8 27.3 28.0 29.1 39.9 49.1 59.5 68.8 69.1 70.0 80.0 90.0
TABLE 4. Binary phase equilibrium myristate T=50
9.2 9.7 18.9 25.7 30.8 31.7 49.7 59.1 69.1 70.0 79.1 80.0 90.0 100.0 110.0
of the system CO,-methyl
at 50 “C “C
0.1004 0.1145 0.2608 0.3339 0.3819 0.3882 0.5413 0.6163
_ _ _
0.6784 0.7154 0.7431
0.9998 _
0.7587 0.8262 0.8633 0.9120
0.9996 0.9989 0.9973 0.9908
175 TABLE 5. Binary phase equilibrium of the system CO,-methyl myristate at 60 “C
TABLE 8. Binary phase equilibrium of the system CO,-methyl palmitate at 60 “C
T=6O”C
T=6o”C
P
ParI
10.0 19.4 29.5 41.2 49.5 50.0 60.5 70.0 70.7 80.0 81.1 90.0 100.0 110.0 120.0
. -Qo~
h2
0.1387 0.2341 0.3504 0.4521 0.5041 0.5035
_ _ _ _
0.5674 0.6540 0.6329 0.6944 0.6812 0.7411 0.7675 0.8115 0.8434
0.9995 0.9992 _ 0.9989 0.9986 0.9985 0.9976
P
[bar1 0.2269 0.3740 0.4410 0.5045 0.5670 0.6066 0.6620 0.6990 0.7401 0.7656 0.8110 0.8512
9.8 22.8 31.7 41.1 53.3 60.6 70.7 90.0 100.0 110.0 120.0 130.0
_ _ _ _ _ _ _ 0.9994 0.9992 0.9990 0.9980 0.9969
TABLE 6. Binary phase equilibrium of the system CO,-methyl palmitate at 40 “C T=40 P
“C
[bar1
12.2 24.2 30.5 41.1 52.5 62.1 70.0 73.3 80.0 90.0 100.0
XC02
YC,
0.1977 0.3917 0.4625
_ _ _
0.5528 0.6613 0.7041 0.7989 0.7876 0.8296 0.8661 0.9062
_ _ 0.9999 0.9999 0.9964 0.9861
,i
;
0.2
a
‘7’, ,
;.,a, 0.4
0.6
0.6
Fig. 2. Binary phase diagram of the system CO,-oleic 40 “C.
1 1
acid at
TABLE 7. Binary phase equilibrium of the system CO,-methyl palmitate at 50 “C T=50
“C
lnomata et al.
10.6 22.7 27.0 30.2 41.3 52.5 62.1 70.0 71.6 80.0 90.0 100.0 110.0 120.0 130.0
0.2091 0.3519 0.4043 0.4271 0.5318 0.6404 0.7005 0.7507 0.7365 0.7957 0.8389 0.8732 0.8932 0.9088 0.9336
_ 0.9996 0.9997 0.9992 0.9980 0.9930 0.9880 0.9817
.
‘ho
L
00
0.2
0.4
0.6
0.8
Fig. 3. Binary phase diagram of the system CO,-methyl at 50 “C.
1
my&ate
177 TABLE 10. Equilibrium mole fractions and separation factors of the ternary system CO,-methyl myristate-methyl palmitate at 40 “C and 70 bar
TABLE 11. (continued)
T=40°C,p=70bar
MM: MP
k-o, GW.l XMP
Yco, Yh4M .klP
4:4
0.84364 0.09182 0.06454
0.999709 0.000181 0.000110
1.157
0.84706 0.05735 0.09559
0.999520 0.000214 0.000266
1.341
0.80561 0.05398 0.14042
0.999300 0.000270 0.000430
1.633
0.79584 0.03596 0.17455
0.998362 0.000276 0.000362
4.494
0.82844 _
0.999866 _
0.17156
0.000134
MM: MP
P,,, &ml
%Ip
LO*
0.77251 0.22749
0.999856 0.000144 _
7:1
0.73974 0.23529 0.02497
0.999070 0.000893 0.000037
2.582
0.78606 0.16653 0.04741
0.999898 0.000069 0.000033
0.595
0.80289 0.12743 0.06968
0.997929 0.001450 0.000621
1.277
0.80832 0.11208 0.07960
0.999753 0.000145 0.000102
1.010
0.78130 0.08305 0.13566
0.998124 0.000796 0.001080
1.204
0.73802 0.07264 0.18934
0.999413 0.000233 0.000354
1.716
0.73763 0.03596 0.22641
0.998365 0.000948 0.000687
8.687
0.79887 _
0.999886 _
0.20113
0.000114
5:3
44
3:5
2:6
I:7
0:s
“C,p =80 bar
GI
YMM A4P
8:O
612
T=40
3:5
2:6
1:7
0:8
G(
_
TABLE 12. Equilibrium mole fractions and separation factors of the ternary system CO,-methyl myristate-methyl palmitate at 40 “C and 90 bar T=40°C,p=90bar
8:0
0.94270 0.05730 -
0.986230 0.013770 _
_
7:1
0.92888 0.06405 0.00707
0.992351 0.007026 0.000624
1.244
0.93568 0.05015 0.01417
0.986430 0.011103 0.002467
1.271
_
TABLE I 1. Equilibrium mole fractions and separation factors of the ternary system CO,-methyl my&ate-methyl palmitate at 40 “C and 80 bar
6:2
T=40
5:3
_
_
_
414
0.91207 0.05161 0.03633
0.994484 0.003605 0.001911
1.328
0.89608 0.03901 0.06491
0.994437 0.00245 1 0.003112
1.310
0.87509 0.0343 1 0.09059
0.997430 0.000908 0.001663’
1.441
0.86722 0.01928 0.11350
0.998095 0.000539 0.001367
2.320
0.86611
0.996420 -
0.13389
0.003580
‘C,p
=80 bar
MM: MP
&oz _ Gn”l XMP
Fco, YMM YMP
8:0
0.85405 0.14595 _
0.998900 0.001100 _
-
0.83046 0.15357 0.01597
0.999189 0.000752 0.000059
1.327
0.85567 0.11331 0.03103
0.999386 0.00048 1 0.000133
0.990
0.87941 0.07704 0.04355
0.998235 0.001150 0.000615
1.057
7:l
612
5:3
a
3:5
216
1:7
0~8
_
178 TABLE 13. Equilibrium mole fractions and separation factors of the ternary system CO,-methyl my&ate-methyl palmitate at 40 “C and 100 bar T=40
“C,p
= 100 bar
MM: MP
Z,,, &lM
XMP
&02
TABLE T=50
14. (continued) “C,p=70
8:0
_
_
7:1
_
_
.Qo2 &4M XMP
Yc,, &lM kvlP
2:6
0.65995 0.09116 0.24889
0.998427 0.000563 0.001010
1.522
0.64960 0.05267 0.29774
0.998803 0.000368 0.000829
2.509
0.75068 _
0.999596 _
0.24132
0.000404
1:7
6~2 _
5:3
_ 0:8
414 3:s
_
_
_
2:6
0.91223 0.02394 0.06383
0.984390 0.004855 0.010755
1.204
0.90277 0.01455 0.08268
0.988968 0.001837 0.009194
1.135
0.90616
0.986120 _
T=50°C,p=80bar
0.09384
0.013880
MM: MP
1:7
0:8
TABLE 15. Equilibrium mole fractions and separation factors of the ternary system CO,-methyl my&ate-methyl palmitate at 50 “C and 80 bar
Zoo2 XMM XMP
8:0 TABLE 14. Equilibrium mole fractions and separation factors of the ternary system CO,-methyl my&ate-methyl palmitate at 50 “C and 70 bar ~~50
“C,p
MM: MP
=70
7:1
6:2
5:3
4:4
3:s
fC0,
a
XMP
YMM YMP
0.71535 0.28465 _
0.999765 0.000235 _
0.70021 0.26923 0.03056
0.998587 0.001300 0.000113
1.306
0.69067 0.23714 0.07219
0.999967 0.000011 0.000022
0.154
0.71436 0.18235 0.10329
0.999094 0.000774 0.000132
3.321
0.67656 0.13501 0.18843
0.999382 0.000387 0.00023 1
2.338
0.72106 0.10877 0.17017
0.999178 0.000354 0.000468
1.183
XMM
8:0
7:1
bar I&,,
a
MM: MP a
YMM JLlP
bar
6:2
5:3 4:4
3:5
2:6
1:7
0:8
Pco, BklM YMP
r
0.75869 0.24131
0.999588 0.000412 -
-
0.77700 0.20074 0.02226
0.999178 0.000751 0.000071
1.180
0.75586 0.18999 0.05416
0.999886 0.000083 0.00003 1
0.763
0.76353 0.15139 0.08508
0.999112 0.000700 0.000188
2.092
0.73933 0.15150 0.10916
0.999489 0.000319 0.000191
1.201
0.76133 0.09258 0.14610
0.999663 0.000160 0.000177
1.429
0.70770 0.08037 0.21193
0.999334 0.000254 0.000412
1.623
0.70175 0.04380 0.25444
0.999457 0.000196 0.000347
3.286
0.79570 -
0.999690 -
_
0.20430
0.000310
179 TABLE 16. Equilibrium mole fractions and separation factors of the ternary system CO,-methyl myristate-methyl palmitate at 50 “C and 90 bar T=50
“C,p
MM: MP
8:O
7:l
6~2
5:3
44
3:5
2:6
1:7
0:8
=90
bar ZCco,
TABLE 17. (continued) T=50”C,p=lOObar .Go, hm.4 XMP
&o, km4 6.w
4:4
0.82971 0.09878 0.07151
0.996901 0.002024 0.001075
1.364
0.85217 0.06052 0.0873 1
0.997841 0.000998 0.001161
1.240
0.81401 0.05111 0.13487
0.997639 0.000748 0.001613
1.222
0.81433 0.02804 0.15763
0.998832 0.000278 0.000890
1.755
0.87343
0.997990
0.12657
0.002010
a
%.V.l XMP
YCO2 YMM PrvlP
0.82620 0.17380 _
0.998940 0.001060 _
-
0.83210 0.15119 0.01671
0.998332 0.001514 0.000154
1.084
0.80505 0.15203 0.04292
0.999650 0.000275 0.000075
1.035
0.81001 0.12249 0.06750
0.9983 14 0.001200 0.000486
1.361
0.80089 0.11773 0.08138
0.999144 0.000546 0.0003 10
1.220
0.81480 0.07282 0.11238
0.999317 0.000317 0.000366
1.340
0.76913 0.06314 0.16774
0.998876 0.000369 0.000755
1.299
0.76046 0.03604 0.20350
0.999254 0.000289 0.000457
3.569
0.83892 _
0.999188
0.16108
0.000812
3:5
2:6
1:7
0:8
TABLE 18. Equilibrium mole fractions and separation factors of the ternary system CO,-methyl myristate-methyl palmitate at 50 “C and 110 bar
MM: MP
&ml XMP
Yco, Pm YMP
8:0
7:l
a
62
44
3:5
0.86333 0.13667
0.997250 0.002750 _
7:l
0.88144 0.10626 0.01230
0.997210 0.002568 0.000221
1.342
0.85756 0.11135 0.03109
0.998406 0.001309 0.000285
1.281
0.87334 0.08118 0.04548
0.997628 0.001661 0.000711
1.308
5:3
a
0.91198 0.08802
0.9908 10 0.009190
0.92926 0.06322 0.00752
0.984275 0.014429 0.001295
1.325
0.89521 0.08079 0.02400
0.993400 0.005392 0.001206
1.328
0.91890 0.05171 0.02939
0.988777 0.007686 0.003538
1.235
0.86664 0.07206 0.06129
0.994141 0.003856 0.002004
1.637
0.88106 0.04764 0.07130
0.994248 0.002680 0.003072
1.306
0.86066 0.03821 0.10113
0.996546 0.001240 0.002215
1.481
0.84529 0.02258 0.13213
0.996081 0.000730 0.003189
1.339
0.89315
0.993040 _
0.10685
0.006960
-
_
8:O
62
PC,, XMM * XMP
5:3
i,,,
-
T=5O”C,p=llObar
TABLE 17. Equilibrium mole fractions and separation factors of the ternary system CO,-methyl myristate-methyl palm&ate at 50 “C and 100 bar
MM: MP
a
MM: MP
26
1:7
018
-
180 TABLE 19. Equilibrium mole fractions and separation factors of the ternary system CO,-methyl myristate-methyl palmitate at 50 “C and 120 bar T = 50 “C, p = 120 bax MM: MP
Rc,,
GO,
%M
Am
+.w
YMP
pressures and temperatures. In the row “MM:MP”, the approximate ratios of methyl myristate and methyl pahnitate in the liquid mixture before adding CO, are tabulated. Figures 6 to 8 show isobaric ternary phase diagrams at 40 “C.
G(
0.4
I
8:0 0.3
7:1
1
(
_
6:2
Liquid phase a SWrcriyl
phase
I 1
5:3 4:4
35
2:6
1:7
0:8
0.91925 0.04627 0.03448
0.982723 0.020635 0.006642
1.193
0.92028 0.03064 0.04909
0.978168 0.008932 0.012900
1.109
0.88315 0.03167 0.08518
0.991318 0.002794 0.005887
1.277
0.87197 0.01911 0.10892
0.991005 0.001651 0.007344
1.282
0.90881
0.988040 _ 0.011960
0.09119
:L 0
0.2 0.3 0.4 “xl.% Fig. 6. Ternary phase diagram of the system CO,-methyl tate-methyl palmitate at 40 “C and 70 bar. 0.1
0.2 Liquid phase Supercritical phase
I
0.15 -
/
TABLE 20. Equilibrium mole fractions and separation factors of the ternary system CO,-methyl myristate-methyl palmitate at 50 “C and 130 bar
MM: MP
myris-
/’
’ ‘Single phase
//
fcol
_ X&ml
Fig. 7. Ternary phase diagram of the system CO,-methyl tate-methyl palmitate at 40 “C and 90 bar.
XMP
myris-
8:O 0.2
_
7:1 612 5:3
0.15
4:4
_
3:5 216
1:7
0:8
mp 0.1 I
0.92650 0.01952 0.05398
0.006844 0.017626
0.91400 0.01263 0.07337
0.981486 0.003073 0.015441
1.156
0.93358
0.981700 _ 0.018300
_
0.06642
0.975529
b
1.074 0.05 i
J
Of 0
0.05
0.1
0.15
0.2
‘j;wA
Fig. 8. Ternary phase diagram of the system CO,-methyl tate-methyl palmitate at 40 “C and 100 bar.
myris-
181
At 70 bar, the system behaves in an interesting way, which is presented in Fig. 6. If the liquid mixture contains mostly methyl myristate or methyl palmitate and only small amounts of the other component, CO2 solubility in the liquid phase is lower than in the binary system. At intermediate concentrations, more CO2 dissolves in the liquid phase than in the binary system, and the total of both liquid components in the supercritical phase is larger than the amount of each of them in the supercritical phase of the binary systems. The two-phase region thus becomes highly concave, an effect known as cosolvency. Methyl myristate and methyl palmitate obviously act as entrainers upon each other and thus enhance mixture solubility. At 90 bar, the binodal band has separated into two separate binodal curves. A mixture of ~/(~MM+~.MP) = 0.375 saturated with CO, does not separate into two phases. This is represented by the “single phase” line in Fig. 7. A pressure of 100 bar is higher than the critical pressure of the binary system CO,-methyl myristate at 40 “C (Fig. 8). The two-phase region has disappeared, and a binodal curve only exists for mixtures of high methyl pahnitate content. As long as methyl myristate content in these mixtures is small, mutual solubilities of the liquid components and CO, are smaller than in the case of pure methyl palmitate. With rising methyl myristate concentration, the solubility increases until complete miscibility is reached. At 40 “C and 110 bar, the binodal curve finally has also disappeared on the methyl palmitate side. Thermodynamic separation factors have been plotted in Figs. 9 and 10. The dependence of LYon pressure for a liquid mixture of constant composition is presented in Fig. 9. As pressure rises, the separation factors decrease at both 40 “C and 50 “C. High pressure leads to enhanced solubilities in both phases. Typically, higher capacity also means reduced selectivity, as the effect of pressure clearly demonstrates. So does the effect of temperature: at 40 “C, solubility at a given pressure is higher than at 50 “C. Again, enhanced solubilities reduce selectivity and result in separation factors closer to unity. Figure 10 shows how the separation factor depends on the composition of the CO,-free liquid mixture. At 100 bar, solubilities in both phases are quite high, and separation factors accordingly are smaller than at lower pressures. At 40 “C, two-phase behavior is only observed for mixtures of high methyl palm&ate content. Generally, the effect of liquid-mixture composition on the separation factor is not very large. Separation factors are slightly higher only in the case of low methyl myristate and low methyl palm&ate concentrations. As the ternary phase diagrams showed, tie-lines are longer for such mixtures, and solubilities are reduced, resulting in higher selectivities.
0
0
0
0
.
70
80
90
100
0
110
120
P WI
Fig. 9. Thermodynamic separation factor in the system CO,methyl myristate-methyl palmitate as a function of pressure for a rmxture &,,/Q,, + &) = 0.625 at 40 “C and 50 “C.
0’ 0
0.2
0.4 L
0.6 I(;.u+ks
0.6
1
)
Fig. 10. Thermodynamic separation factor in the system CO*methyl myristate-methyl . _ ture composltion x,,/(Z&, of 40 “C and 50 “C.
palmitate as a function of liquid mix+ EM,) at 100 bar and temperatures
Tiegs and Peter [ 141 demonstrated that in the system CO,-oleic acid-stearic acid, separation was possible at small mole fractions of either of the lipids. The separation factor was smaller than unity if the oleic acid concentration was small. For large oleic acid concentrations, it was greater than unity. In the case of intermediate compositions, the extraction of the mixture at equilibrium conditions was completely unselective.
High-pressure
densities
Densities were measured as a function of pressure at 40 “C, 50 “C, and 60 “C. Both the gas and the liquid phases of the binary systems CO,-oleic acid, CO,methyl myristate, and CO,-methyl palmitate were studied. In addition, densities of the coexisting phases
182
of the ternary system CO,-methyl myristate-methyl palmitate with a composition &&&, + &) = 0.5 were determined. Finally, the pure liquids oleic acid, methyl myristate, and methyl palmitate were also investigated in order to show how saturation with CO, affected liquid densities at high pressures. The experimental results of test runs with pure CO* agreed with the Span and Wagner equation of state [lo] within F 2 kg m-3 and thus proved that the density sensor operated accurately as specified. Tables 21 to 23 list the densities of the liquid and the gas phases of the system CO,-oleic acid at thermodynamic equilibrium and 40 “C, 50 “C, and 60 “C. In the saturated liquid phase, density increases as pressure and CO2 concentration rise. Agreement with the data of Jakob [S] is good. Densities in the saturated gas or supercritical phase are virtually the same -as those of pure CO,, since the solubility of oleic acid in COz is small. Only at 40 “C and 125 bar, where the CO, mole fraction reaches j+,,+ = 10m3, is the density of the supercritical phase above that of the pure supercritical fluid. By contrast, Jakob recorded densities higher than those of pure CO, even at subcritical pressures, presumably because of experimental errors. Solubilities of oleic acid in subcritical gaseous CO, are virtually immeasurable TABLE saturated CO,-OA
21. Densities of the liquid and the gas phases system CO,-oleic acid at 40 “C
of the
(40 “C)
[kg m-7
23. Densities of the liquid and the gas phases system CO,-oleic acid at 60 “C
CO,-OA
pI tkgm-7
pB[kg m-7
1 10 25 50
860 862 865 871
_
75 90 100 110 125
876
and are not expected to affect densities,. The values given in this work are therefore considered more accurate. The densities of pure oleic acid at atmospheric pressure and at 96 bar are tabulated in Tables 24 to 26. Densities increase with pressure and decrease as temperatures rise. At equal pressures and temperatures, the density of pure oleic acid is lower than that of the saturated liquid. The change of density of pure oleic acid measured here is greater than with Jakob. As Jakob gave no description of how high pressures were set for the pure 24. Density
pg [kg m-7
OA (pure)
_ _ -
P
902 _ 908
231 454 626 688 740
P
1 96
TABLE
CO,-OA P
ParI
1 25 50 75 90 100 110 125
22. Densities of the liquid and the gas phases system CO,-oleic acid at 50 “C
of the
P
PI
[kg m-‘I
[kgm-‘l
879 893
25. Density
of pure oleic acid at 50 “C
(50 “C)
[bar1
P
1 96
(50 “C)
of pure oleic acid at 40 “C
(40 “C)
[bad
OA (pure) TABLE saturated
173 234 287 353 470
880 884
879 887 895
912
[kg m-‘I
869 882
pa [kgm+l
869 875 881
_ _
887 _ 892 _ 897
194 279 370 492 613
of the
(60 “C)
[bar1
P
TABLE pI
1 25 50 75 90 100 110 125
TABLE saturated
TABLE
26. Density
OA (pure)
1 96
of pure oleic acid at 60 “C
(60 “C)
860 872
183
liquid, different experimental techniques may account for the discrepancies. However, Banipal et af. [ 131 measured an average increase of ca. 7 kg m -3 for saturated fatty acids when pressure was raised from 1 bar to 90 bar. While their increase is smaller than those measured in this work, it was recorded in a different temperature range. The densities of the equilibrium phases of the mixture CO,-methyl myristate are given in Tables 27 to 29. As with CO,-oleic acid, the density of the saturated gas
TABLE 27. Densities of the liquid and the gas phases saturated system CO,-methyl myristate at 40 “C COZ-MM P
(40 “C)
[bar1
1 10 20 30 40 50 60 70 80 82 84 86 88 90
TABLE saturated CO,-MM P
[bar1
1 10 20 30 40 50 60 70 80 90 94 100 102 104 106 108 110 112 114 116
of the
pI [kg m-?
ps [kg m-‘I
853
_
856 861 866 871
_ _ _
875 880
_
883 885 884 883 882 880 869
198 278 300 331 380 476 648
28. Densities of the liquid and the gas phases system CO,-methyl myristate at 50 “C (50 “C)
pi [kg m-‘I 845 846 849 852 856 861 865 867 869 870 872 863 862 860 858 856 855 852 847 840
ps [kg m-‘I
_ 172 218 283 393 422 457 496 534 573 607 642 674
of the
TABLE saturated CO,-MM
29. Densities of the liquid and the gas phases system CO,-methyl myristate at 60 “C (60 “C)
pI [kg0
pg [kg m-‘I
1 10
853 839
_ _
20 30 40 50 60
841 845 848 851 853
_ _ _ _
70 80 90 100 110 120 130
855 857 859 860 862 862 860
156 192 235 291 364 462 565
P
[bar1
of the
phase is the same as the density of pure CO2 until the methyl myristate mole fraction reaches jMM = 10m3. The liquid phase behaves in a remarkable way: as pressure is being raised, its density first increases, passes through a maximum, and then decreases as the critical pressure of the mixture at that temperature is approached. At the consolute point, the densities of both phases finally must become equal. By contrast, the molar density of the liquid phase rises monotonously as pressure and CO, solubility are raised. Prausnitz et al. [2] showed that such an increase is only possible if the partial molar volume of the liquid component changes. Close to the consolute point, it may even become negative: in spite of the compression caused by high pressures, the liquid stucture becomes less dense. Because of the low molecular weight of CO,, this loose structure finally leads to a decrease of the liquid massspecific density. In the experiments performed with the system CO,-oleic acid, pressures were yet too far from the consolute point for a density maximum to be observed. The densities of pure methyl myristate tabulated in Tables 30 to 32 rise linearly with pressure: compressibilities of lipids in this pressure region are virtually constant. Near the critical pressure of the mixture CO,-methyl myristate at a given temperature, the densities of the pure liquid may be higher than those of the CO,-saturated liquid phase. The density changes of pure methyl myristate and pure oleic acid are roughly equal. Tables 33 to 35 contain the densities of the saturated liquid and supercritical phases of the system CO,methyl palmitate. As with CO,-methyl myristate, the density of the supercritical phase is higher than that of pure CO, for mole fractions larger than j&, = 10 -3; in
184 TABLE
30. Density
MM (pure)
of pure methyl
myristate
at 40 “C
TABLE saturated
[bar1
P
853 855 856 857 858 859 861 862 863
31. Density
MM (pure) P
[bar1 1
10 20 30 40 50 60 70 79.1
at 50 “C
32. Density
of pure methyl
pI km-‘1
pg km-‘1
1
851 855 859
_ _ -
865 868 873 878
_ _
882 884 881 873 864
198 276 540 720 744
20 30 40 50 60 70 80 90 100 107
TABLE saturated CO,-MP
845 846 848 849 850 852 853 854 855
MM (pure) P
myristate
P [kg m-7
1 10 20 30 40 50 60 70 79.5
TABLE
of pure methyl
[bar1 10
(50 “C)
[bar1
(40 “C)
[kg m-7 P
1 10 20 30 40 50 60 70 79.9
TABLE
of the
(40 ‘C) CO,-MP
P
33. Densities of the liquid and the gas phases system CO,-methyl myristate at 40 “C
P
my&ate
at 60 “C
(60 “C)
P [kg m-7 838 839 841 842 844 845 846 847 849
the liquid phase, an increase in density, its passing through a maximum, and its decline as the critical point of the mixture is approached are dbserved again. The densities of pure methyl palmitate at 40 “C, 50 “C, and 60 “C are given in Tables 36 to 38. Within the limits of experimental accuracy, the compressibilities of both esters are equal. The density of pure methyl palmitate is slightly lower than that of pure methyl myristate in all cases.
34. Densities of the liquid and the gas phases system CO,-methyl myristate at 50 “C (50 “C)
kl
1 10 20 30 40 50 60 70 80 90 100 110 120 130
TABLE saturated CO,-MM
pI [kgm-3l
ps [kg m-7
844 846 849 852 856 859 862
_ _ _ _
867 869 871 871 870 866 857
172 221 288 402 551 662 738
35. Densities of the liquid and the gas phases system CO,-methyl myristate at 60 “C (60 “C)
p8 [kg m-7 1 10 20 30 40 50 60 70 80 90 100 110 120 130
of the
837 840 843
_ _ _
846 849 852 855
_ _ -
857 859 861 863 863 863 861
156 193 239 292 366 453 544
of the
185
TABLE
36. Density
MP (pure) P
P
1 10 20 30 40 50 60 70 78.0
37. Density
at 50 “C
11 20
852 855 859
_ _
30 40
863 867
_
50 60 70 80 90 100
872 876 880 885 874 859
198 278 599 659
TABLE 40. Densities of the liquid and the gas phases saturated system CO,-methyl myristate-methyl palmitate . . ^ composrtion xTulM/(ZtMM+ i,,) at 50 “C
844 845 846 847 849 850 851 852 852
38. Density
MP (pure)
pg [kg m-7
[bar1
1
palmitate
(40 “C) pI [kg m-‘I
P
P [kg m-7
1 10 20 30 40 50 60 70 76.7
of pure methyl
CO,-MM-MP P
pahnitate
at 60 “C
(60 “C) P
1 10 20 30 40 50 60 70 75.7
CO,-MM-MP
[kg m-‘I
of pure methyl
TABLE 39. Densities of the liquid and the gas phases of the saturated system CO,-methyl myristate-methyl palmitate with a w composition xMM/(B,, + &,,r) at 40 “C (no misprint “11 bar”!)
(50 “C)
tbarl
TABLE
at 40 “C
851 852 854 855 856 857 859 860 861
MP (pure) P
palmitate
(40 “C)
[bar1
TABLE
of pure methyl
[kg m-7
837 838 839 840 842 843 844 845 845
The densities of the coexisting phases of the system CO,-methyl myristate-methyl palmitate with a composition ~.MM/(RMVIM + &) = 0.5 are listed in Tables 39 and 40. Densities essentially lie between those of the two binary systemsIf ( j&, + pm) is sufficiently large, the saturated supercritical fluid possesses a higher density than pure COz. Liquid-phase density again has a maximum. Figure 11 shows how the density of the gas phase changes with pressure. The experiments conducted with pure CO2 agree well with the data predicted by the
Bar1
1 10 20 30 40 50 60 70 80 90 100 110 120 130
of the with a
(50 “C) pI [kg m-7
ps [kg m-7
845 846 851 854 859
_ _ _
862 865
_
869 870 873 873 866 861 849
172 223 290 400 563 682 708
Spn and Wagner EOS
Saturated CO*, this work
P
vJ=l
Fig. 11. Density of pure CO, and of CO, saturated myristate as a function of pressure at 50 “C.
with methyl
186
Boo-
.
. a
0
.
l* %
a E 0 r.
.
0
0
0
1
0
q
20
40
60
80
.
l
.
.
99
Liquid phase .I!:
100
8.e5
0.9
_
0.95
1
Xm,
Fig. 12. Density of pure methyl my&ate and of methyl myristate saturated with CO, as a function of pressure at 40 “C.
.
90 bar
Supercritical phase
p WI
. mo-
86
80 bar
mo-
Q s400 0
. 0
@04
.
.
. . ...* .
600-
Fig. 14. Densities of the saturated coexisting supercritical and liquid phases of the system CO,-methyl myristate as a function of CO, mole fraction at equilibrium at 40 “C. Isobars connect values of coexisting phases
the pure fluid, free convection may result, especially if kinematic viscosities are low, thus enhancing transport processes significantly. In both phases of the systems studied here, free convection is likely to occur.
* 400-
.
*
AA
*
m0’
0
20
40
60
80
P BarI Fig. 13. Densities of the saturated coexisting supercritical and liquid phases of the system CO,-methyl myristate as a function of pressure at 40 “C.
Span and Wagner equation of state [lo]. The density of CO* saturated with methyl myristate is significantly higher than that of pure CO1 as solubility in the supercritical phase becomes large. The behavior of liquid-phase densities is presented in Fig. 12. The density maximum of the CO*-saturated liquid phase is particularly striking for the system COzmethyl myristate at 40 “C. In Fig. 13, the densities of both the liquid and the supercritical phase at thermodynamic equilibrium are plotted. The diagram clearly shows how the densities of both phases approach each other to become equal at the critical point. Figure 14 finally shows densities as a function of composition. The values in the coexisting phases belonging together are connected by isobars. In separation processes, density gradients within a phase may strongly affect mass transfer. If the density of the saturated phase-at the interface-differs from
Acknowledgments The author is obliged to Susana Mufioz de SotoSolii, Jorg Dietzel, Thomas Langner, Jiirg Meyer, and Daniel Wuppermann for helping perform the experiments, and to Ms. Zimmermann for her analytical work. He would like to thank Mr. Schon and Dr. Dahmen from the Institut fur Heifle Chemie, Kernforschungszentrum Karlsruhe, for providing the density sensor. Financial support of the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
Nomenclature P
2-i Yi
u P
Pressure, bar Liquid-phase mole fraction of component i Gas-phase mole fraction of component i Thermodynamic separation factor Density, kg mm3
Subscripts g
: MM MP OA
Gas-phase or supercritical-phase Component i Liquid-phase property Methyl myristate Methyl pahnitate Oleic acid
property
187
References D. Swern (ed.), Bailey’s Industrial Oil and Fat Products, 4th edn., Wiley, New York, 1979. .I. M Prausnitz, R. N. Lichtenthaler and E. Gomez de Azevedo, Molecular Thermodynamics ofFluid-Phase Eyuilibria, PrenticeHall, Englewood Cliffs, NJ, 1986. G. Knaff and E.-U. Schhinder, Mass transfer for dissolving solids in supercritical carbon dioxide, I. Resistance of the boundary layer, Chem. Eng. Process., 21 (1987) 151-163; G. Knaff and E.-U. Schltinder, Mass transfer for dissolving solids in supercritical carbon dioxide, II. Resistance in the porous layer, Chem. Eng. Process., 21 (1987) 193-197 M. Seekamp, The rate of dissolution of oleic acid in compressed carbon dioxide and ethane, Doctoral dissertation, Universitgt Erlangen-Niirnberg, 1987 (in German). H. Jakob, The flow behaviour of coexisting gas and liquid phases at high pressures, Doctoral dissertation, Universitiit Erlangen-Niirnberg, 1988 (in German). J. Chrastil, Solubility of solids and liquids in supercritical gases, J. Phys. Chem., 86 (1982) 30163021. M. B. King, D. A. Alderson, F. H. Fallah, D. M. Kassim, K. M. Kassim, J. R. Sheldon and R. S. Mahmud, Some vapour/ liquid and vapour/solid equilibrium measurements of relevance for supercritical extraction operations, and their correlation, in M. E. Paulaitis, J. M. L. Penninger, R. D. Gray, Jr. and P. Davidson (eds.), ChemicaI Engineering at Supercritical
8
9
10
11
12
13
14
Fluid Conditions, Ann Arbor Science, Ann Arbor, MI, 1983, pp. 31-80. H. Inomata, T. Kondo, S. Hirohama, K. Arai, Y. Suzuki and M. Konno, Vapour-liquid equilibria for binary mixtures of carbon dioxide and fatty acid methyl esters, Fluid Phase Eyuilib., 46 (1989) 41-52. A. H. Wu, A. Stammer and J. M. Prausnitz, Extraction of fatty acid methyl esters with supercritical carbon dioxide, Znt. Symp. on Supercritical Fluids, Nice, France, 1988, Vol. 1, pp. 107-I 14. R. Span and W. Wagner, A new equation of state for carbon dioxide covering the fluid range for temperatures from 216.6 K to 1100 K at pressures up to 800 MPa, J. Phys. Chem. ReJ Data, Submitted. S. Peter and H. Jakob, Viscosity of coexisting phases at supercritical fluid extraction, ht. Symp. on Supercritical Flui&, Nice, France, 1988 Vol. 1, pp. 303-310. S. Peter and H. Jakob, The rheological behavior of coexisting phases in systems containing fatty acids and dense gases, J.Supercrit.Fluids, 4 (1991) 166- 172. T. S. Banipal, S. K. Garg and J. C. Ahluwalia, Densities of some higher alkan-1-oic acids at temperatures from 343.15 K to 373.15 K and at pressures up to 9 MPa, J. Chem. Thermodynamics, 24 (1992) 729-735. C. Tiegs and S. Peter, On the separation of mixtures of oleic acid and stearic acid by extraction with a supercritical solvent, Fette, Se&n, Anstrichmittel 87 (1985) 231-235 (in German).