818
I n d . Eng. Chem. Res. 1988,27, 818-823
Nishida, N.; Stephanopoulos, G.; Westerberg, A. W. AIChE J. 1978, 27,321. Petlyuk, F. B.; Platonov, V. M.; Slavinskii, D. M. Int. Chem. Eng. 1965, 5,555. Simulation Science Inc., PROCESS, Revision 4,Sept 1985. Tedder, D. W.; Rudd, D. F. AIChE J. 1978,24, 303.
Underwood, A. J. V. Chem. Eng. Prog. 1948,44,603. Westerberg, A. W. Comput. Chem. Eng. 1985, 9,421.
Received for review May 5, 1987 Reuised manuscript received December 14, 1987 Accepted January 21,1988
A Supercritical Carbon Dioxide Extraction from Mixtures of Triglycerides and Higher Fatty Acid Methyl Esters Using a Gas-Effusion-Type System Yutaka Ikushima,* Kiyotaka Hatakeda, Shota Ito, Norio Saito, Takashi Asano, and Tomio Goto Government Industrial Research Institute, Tohoku, Nigatake 4-chome, Sendai 983, Japan
A supercritical carbon dioxide (SC-C02) extraction was performed to selectively separate and concentrate specific components in high purity and a large solubility from mixtures of triglycerides and higher fatty acid methyl esters by using a gas-effusion-type apparatus. The addition of ethyl acetate as an entrainer to the C 0 2 was found to produce a supercritical fluid that had high efficiencies for both the extraction of triolein and the separation of triolein from a mixture of triolein and tristearin. The use of the separation chamber, which was packed with stainless steel Raschig rings, made possible the selective extraction of oleic (ClS-J acid methyl esters from a mixture of stearic (CIH), oleic (ClS1), linoleic (CpJ, and linolenic (Clg3) acid methyl esters. Furthermore, the isolation of Clm and Clg3acid methyl esters from the mixture became feasible by means of both a AgN03 supported on silica gel tube and ethyl acetate. Recently much attention has been drawn to the separation process using a supercritical fluid (SCF). As compared with conventional extraction processes operated at a relatively high temperature, the SCF extraction method has some advantages. For example, it can extract hydrophobic and thermolabile materials without denaturation because of its low operating temperature. Furthermore, it does not need a posttreatment for purification such as the removal of solvents used. Its specific advantages have extensively been described and discussed (Peter and Brunner, 1980; Nagahama, 1981; Randall, 1982; Saito, 1982; Subramanian and McHugh, 1986). In the fields of food and medicinals, the SC-C02 extraction is noticed as a useful extraction of valuable materials contained in natural products (Yamaguchi, 1985). Somes kinds of animals and plants contain significant amounts of valuable glycerides and fatty acids including docosahexanoicacid (DHA), eicosapentanoic acid (EPA), linoleic acid, linolenic acid, etc., which have been noted in terms of their medicinal purposes and their role as precursors of prostaglandins (Lossonczy, 1978; Hirai et al., 1980; Sanders and Younger, 1981). However, when the SC-C02extraction method is actually applied to the isolation of valuable fatty acids contained in natural products, costs for purifying these acids are so high that natural products are difficult to use at the present stage. In addition, it is difficult to separate specific components from the mixture of those acids because these acids are generally liable to be denatured by heat and similar in chemical and physical properties. Therefore, if desired components can be easily separated and concentrated from a mixture, this extraction technique can be applied to isolate useful components from natural resources as they are and would have a great commercial value. We (Ikushima et al., 1986) have studied SC-C02 extraction of mackerel powder to obtain unsaturated fatty acids such as EPA and DHA. It made possible the ex-
traction of such expensive materials without denaturation. Further, Sako et al. (1986) have examined the extraction of the oil containing y-linolenic acid from fungi from SC-C02. They found that C02 and C02 with an entrainer extracted only neutral lipids containing y-linolenic acid. However, a selective extraction of specific components from a mixture of such higher fatty acids with SC-C02by using a gas-effusion-type apparatus has been little used so far. It is the first objective of this work to investigate the effects of both pressure and temperature on the extraction of such substances as triolein and tristearin, which are major constituents of fish oil, and their mixtures with SC-C02. Subsequently, we present a method using C02 or C 0 2 with an entrainer to isolate economically specific components in high purity and a large solubility from mixtures of tristearin-triolein, palmitic (CI6+), stearic (CIa4),arachidic (CzW),and behenic (CZ2+)acid methyl esters having the same number of C atoms and ClH, ClS1, C18-2, and C18-3 acid methyl esters differing only by the degree of unsaturation. The first and second figures in the subscripts refer to the number of carbons and the degree of unsaturation in the molecule, respectively. In particular, in the last part of this work, it is demonstrated that the isolation of specific components from a mixture of CIG1,C18-2,and C1a-3acid methyl esters becomes feasible by use of a AgNO, supported on silica gel tube and ethyl acetate as an entrainer in SC-C02 extraction technique. Experimental Section Materials. Triolein, tristearin, and palmitic, arachidic, and behenic acid methyl esters were obtained from Wako Pure Chemical Industry, Ltd., and stearic, oleic, linoleic, and linolenic acid methyl esters from Kanto Chemical Co. Inc. A commercial-grade carbon dioxide was used. All chemicals were used without further purification. AgN03 supported on silica gel was used for the fractionation. It
0888-5885/88/ 2627-0818$01.50/0 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988 819
:I Q
20
i Y 0
U Figure 1. Schematic diagram of supercritical extraction apparatus. (1)C02cylinder, (2) high-pressure pump, (3) high-preasurepump for an entrainer, (4)cooling circulator, (5) preheater, (6) extractor, (7) back-pressure regulator, (8) separaration chamber, (9) heating tape, (10)temperature controller, (11)receiver, (12) thermocouples, (13) pressure gauge, (14)flow totalizer, (15)constant-temperature bath.
was prepared by impregnating the support powder with aqueous silver nitrate solution to 7 % level by weight and drying under vacuum at 413 K for 4 h. Silica gel (Silbead N) was obtained from Mizusawa Industrial Chemicals, Ltd. I t was powdered to 65-100 mesh. The BET surface area was 513 m2/g (Ikushima et al., 1984). Apparatus and Procedures. A schematic of the apparatus used in this study is presented in Figure 1. A gas-effusion-type apparatus was used. With a 0.1-L extractor maintained at 313 and 333 K,a weighed sample was extracted with SC-C02at pressures of 7.8-27.0 MPa. Liquid carbon dioxide and/or an entrainer was charged into a Hitachi Model 6353 high-pressure liquid pump through a 1/16-in.tube and a check valve, compressed to the desired pressure and then fed into the extractor. The concentration of an entrainer in COP was about 4 % by weight. The pump delivered the C02 at a constant flow rate of 1 nL/min (1 nL = 1 L of gas at 293 K and 101.3 kPa). The pressure inside the extractor was controlled with a heated back-pressure regulator. The extraction pressure is measured with a Bourdon-type pressure gauge. Fluctionations in pressure due to the pump were less than f l kg/cm2 below 200 kg/cm2. The temperature was controlled within i O . 1 K. The apparatus was equipped with a chamber between the extractor and the back-pressure regulator for the selective removal of the dissolved material from the loaded supercritical phase. The chamber was a stainless steel tube 80 cm long and 0.5 em i.d. and was filled with glass wool at both the bottom and top outlets. It was packed with stainless steel Raschig rings 0.4 cm long and 0.3 cm i.d. or AgNO, supported on silica gel. The temperature of the tube was held at 330-403 K. The solution extracted was flashed to atmospheric pressure across the heated backpressure regulator, and the component was collected in a receiver. The amount collected was determined by weighing, and the corresponding flow volume integration of C02 used was measured with a wet gas meter. Glycerides extracted were converted into fatty acid methyl esters according to the usual method (CH,OH-CH30Na). The compositions of methyl esters were determined by a Hitachi Model 663-50 gas chromatography.
Results and Discussion Extraction of Triglycerides. The solubility of such model substances as triolein and tristearin, which are constituents of animal oils, and their mixtures in SC-CO, is examined, and particular attention is drawn to the effects of entrainers on the extraction efficiency. Figure 2 indicates the effect of pressure on the extraction efficiency of tristearin at a temperature of 313 K. The
3
1
Tatal a n a m r of
2
3
C02 consumeb,l~r:
Figure 2. Effect of pressure on the extraction efficiency of tristearin at 313 K. The weight of tristearin charged was 10.0 g. Pressures: ( 0 )9.8 MPa, (0) 14.7 MPa, (A)19.6 MPa, ( 0 )27.0 MPa.
30 M
c
I . -
Y
I
D
g
20
0
u a x
-
c
0 i
-
IO
i
0
Totul mount o f C02 consumed,
I, 3
Figure 3. Effect of temperature on the extraction efficiency of tristearin at 19.6 MPa. The weight of tristearin charged was 10.0 g. Temperatures: (A)313 K, (m) 333 K.
solubility of tristearin in SC-C02 increases with the pressure. This effect is considered to be due mainly to interaction forces between molecules of tristearin and C 0 2 induced by the increase of the density of C 0 2 (Brunner et al., 1979). The rate of extraction remains nearly constant for its initial period of time, but then it gradually decreases under all the pressures examined. The influence of temperature on the solubility of tristearin in SC-C02at 19.6 MPa is shown in Figure 3. The solubility at 333 K is remarkably greater, finally by a factor of 3.5, than that at 313 K. In general, a rise in temperature at a constant pressure decreases the density of SC-C02, while it increases the vapor pressure of the low-volatility components (Paul and Wise, 1971; Sagara, 1981). Therefore, the results shown in Figure 3 can be interpreted as follows: an increase of the vapor pressure of the solute contributes more to the increased solubility of tristearin than does a decrease of the density of the solvent. Tsekhanskaya et al. (1964) reported that at high pressure a rise in temperature causes an increase in the solubility of low-volatility components in gaseous phase in a similar manner. Further, the rate of extraction remains nearly constant similarly for its initial period of time, but then it decreases under the temperature examined. The relative volatilities of components to be extracted are influenced little by SCF used. Therefore, the addition of an entrainer to the C02 is expected to increase the volatilities (Peter and Brunner, 1980). Namely, an entrainer is anticipated to raise affinities between low-vola-
820 Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988
/ I
/
Toto. m o u r t cf
cd2 - 0
cuiiea,
4l
I
10 12 14 IC a r r e s s ~ r e l,l D a Figure 6. Effect of pressure on the extraction efficiency of a mixture of ClW, ClW, Cm, and CZz4acid methyl esters with SC-C02 a t 313 K. The weight of the mixture charged was 10.2 g.
8
J
Figure 4. Effects of the addition of entrainers on the extraction efficiency of triolein a t 19.6 MPa and 313 K. The weight of triolein ether, (+) acetone, (--) charged was 10.0 g. (---) Ethanol, methylene chloride, (-) ethyl acetate, (---I COPalone. (-a*-)
--
A n
o o o o o o o
2
''oc+ion
-
1
1
2
3
.
i
r
6
:
8
F-CCti31 1 . 1 ~ .
Figure 5. Separation efficiency of triolein with the CO,-ethyl acetate for a mixture of triolein and tristearin at 19.6 MPa and 313 K. The weight of the mixture charged was 10.0 g.
tility components and the gas. Figure 4 shows the extraction efficiency of triolein on the addition of some solvents as an entrainer to the C 0 2a t 19.6 MPa and 313 K. Ethanol, acetone, ether, methylene chloride, and ethyl acetate were used. The addition of ethanol, acetone, and ether decreases the extraction efficiency of triolein, but the addition of ethyl acetate greatly enhances it. Further, the addition of methylene chloride similarly increases the extraction efficiency of triolein, but as the extraction process progresses, the extraction efficiency becomes lower than that with C 0 2 alone. The triolein/tristearin mole ratios in the gaseous phase on the extraction of a mixture of triolein and tristearin with the C02 and ethyl acetate a t 19.6 MPa and 313 K are indicated in Figure 5. In this study, the separation factor a is evaluated as follows: a =
~ ~ 0 2 / ~ S 2 ~ / ~ ~ 0 1 / ~ S l (1) ~
where MO2/ Ma2and Mol/MSlare the trioleinltristearin mole ratios in the gaseous phase on the extraction with and without an entrainer, respectively. Evidently some fractionation takes place with the addition of ethyl acetate in the initial fractions. The separation factors of triolein on the addition of ethyl acetate are larger than 1.5 over all fractions, especially 3.6 for fraction 3. Then, the concentration of triolein is maximally 90% at fraction 3. We have found that the addition of ethyl acetate to the CO, prpduces a SCF that has an enhanced selectivity of triolein
" " " " '
" " " " '
L
40
4
Ffocti
c I
2
4c
4 -WL
t I
1
Figure 7. Effect of pressure on the separation efficiency of a mixture of CIW, CIW, CzW, and CPZ4acid methyl esters with SC-C02 a t 313 K and pressures of (a) 7.8, (b) 10.8, and (c) 14.7 MPa. The weight of the mixture charged was 10.2 g. (0) CI6+ ( 0 )CIw, (A) C z M , ( 0 )CPP4acids.
from a mixture of triolein and tristearin. The amount of tristearin-triolein mixture extracted with the C02-ethyl acetate from fraction 1 to 8 is 5.1 g. The solubility of fractions 2 and 3 is about 1.39 mg/nL. This value is approximately doubled compared with the average solubility of triglycerides extracted with the C02alone (0.64 mg/nL). These results indicate that the SCF of COz with ethyl acetate as the entrainer can enhance both the extraction efficiency of triolein and the capacity for the fractionation of triolein from the mixture of triolein and tristearin. In addition, the fluid may make possible some fractionation for fatty acids of different degrees of unsaturation with the large number of C atoms. Extraction of Fatty Acid Methyl Esters from C16 to CZ2.A mixture of C1@, ClH, Cm, and CZ2+acid methyl esters was extracted with SC-C02. Figure 6 shows the effect of pressure on the extraction efficiency of the mixture at 313 K. The solubility of these methyl esters in SC-C02increases greatly with the pressure. At 7.8 MPa, the solubility in SC-C02is very small, ca. 1.4 mg/nL, but as the pressure increases to 10.8 and 14.7 MPa, the values reach about 28 and 39 times, respectively, that at 7.8 MPa. Slightly above the critical pressure of the C 0 2 at a moderate temperature, an increase of pressure causes a remarkable increase of the solubility. The density of the C02 steeply increases above the critical pressure, and it must be an important factor for the increase of the solubility of the mixture of the methyl esters in gaseous phase and closely related to the solute holding power. Figure 7 shows the effect of pressure at 313 K on the and separation efficiency of a mixture of CIw, Clm, C,
Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988 821 Table 1. Properties of F a t t y Acid Methyl Esters Differing Only by t h e Degree of Unsaturation fatty molecular bP, mass df4 K/mmHg acid Clw 298.5 0.8496 (40) 488/15 Cis1 296.5 0.8596 (40) 485-486/15 C1,+2 294.5 0.8720 (40) 485/16 C18-3 292.5 0.8834 (40)
mP,
K n'D 312.6 1.4328 (50) 293.1 1.4522 (50) 1.4594 (25) 1.4711 (20)
CZz4 acid methyl esters of which initial concentrations are 27.9%, 25.1%, 28.8%, and 18.2%, respectively. As shown in Figure 7a, at 7.8 MPa, c16-0 acid methyl ester can be selectively separated in a purity above 60% over the whole extraction process, especially 85% for fraction 5. As the pressure increases from 7.8 to 10.8 MPa, the separation factors of ClW acid methyl ester decrease. Although the separation efficiency over the extraction process a t 10.8 MPa falls, as shown in Figure 6, the solubility is remarkably increased. That is, the selectivity gets worse with increasing concentration of the fatty acid methyl esters in the gas phase (Brunner et al., 1979; Brunner, 1983). Further, a t 14.7 MPa, the concentration of C22-a acid methyl ester gradually increases in each fraction. It could be thought that c16-0,ClW, and C2(M acid methyl esters are selectively extracted in both of the extraction processes a t 7.8 and 10.8 MPa; concequently, C22-0content in the mixture increases, and the concentration reaches ca. 40%. These results indicate that the smaller ester in molecular weight has a tendency to be extracted in preference to the others in the mixture. Extraction of a Mixture of Clm Clbl, Clb2, and Cis3 Acids. A mixture of stearic, oleic, linoleic, and linolenic acid methyl esters, which have the same number of C atoms and differ only by the degree of unsaturation, was extracted with SC-C02. Their properties are indicated in Table I (Handbook of Oil Chemistry, 1971;Markley, 1960). These acids are similar in chemical and physical properties, and it is difficult to separate desired components from the mixture. As shown in Figure 1, our extraction apparatus was equipped with the chamber for the purpose of selectively removing the dissolved materials from the loaded supercritical phase. Eisenbach (1984) indicates that a decrease of the density of the supercritical phase loaded causes condensation of the less volatile components in the same way as in a conventional distillation. That is, when the chamber is held a t a temperature higher than that of the rest of the apparatus, the selective separation may become feasible in the chamber due to differences in volatility among components of the mixture. Figure 8 indicates the effects of both temperatures of the separation tube and the addition of entrainers to the C 0 2 on the extraction and separation efficiencies at 8.8 MPa and 313 K. In this case, the tube is packed with stainless steel Raschig rings. The front and the rear parts of the tube are maintained at temperatures T1 and T2,respectively. A t T I = T 2 = 367 K, the separation factor of Clsl acid methyl ester is above unity in almost all fractions, but those of ClS2 and C18-3 acid methyl esters are below unity over the extraction process. The concentrations of Clgz and Cis3 acid methyl esters are almost identical with those in the original sample. The average solubility over this extraction process is 6.5 mg/nL, but after fraction 6, it reaches 10.0 mg/nL. When the temperature of the separation tube is increased to T I = 373 and T2 = 413 K, the tendency as shown in Figure 8a is enhanced, and the separation factor of ClS1 acid methyl ester increases to ca. 1.5 and that of CIs3 acid methyl ester decreases to below ca. 0.8. This is because the higher temperature of the tube further increases dif-
r
20 Gi i 01 0 Gi
0
1110
300
200
T o t a l amount o f C3* Cmsunec, i1L
Figure 8. (a-d) Effects of both temperatures of the separation tube and entrainers for the separation of a mixture of CIW, Clg,, CIS-*, and Cig3 acid methyl esters at 8.8 MPa. The weight of the mixture CIW, ( 0 )Clgl, (A)Clgp, (0) Cis3 acids. (e) charged was 10.0 g. (0) Effect of the addition of entrainers on the extraction efficiency using the separation tube at 8.8 MPa. (-) COPalone, -) ethanol, -) ethyl acetate. (-a
(-.e
ferences in volatility among the components. It is possible to extract selectively the specific component from the mixture by using the differences in volatility among them due to the control of the tube temperature. In Figure 8b, the degree of extraction up to fraction 14 based on the original sample is 12%. The solubility over the extraction process is 3.8 mg/nL and decreases by about one-half that a t T , = T2 = 367 K. A low solubility may be combined with a reasonable selectivity due to an increase in the temperature of the tube. Thus, ethyl acetate and ethanol as entrainers are used for the purpose of enhancing not only the solubility but the selectivity. As shown in Figure 8e, on the addition of ethyl acetate and ethanol, the solubility remarkably increases to 17.7 and 16.8 mg/nL, respectively. Sako et al. (1986) demonstrated that ethanol in addition to ethyl acetate can enhance the extraction efficiency of a mixture of fatty acid methyl esters. In parts c and d of Figure 8, the separation factors of Clw and Clsl acid methyl esters increase to 1.2 by the use of ethanol. However, reasonable selectivity cannot be obtained as indicated in parts c and d of Figure 8. It is difficult to enhance both the extraction and separation efficiencies of the mixture of fatty acid methyl esters as they are. We have found that a mixture of higher fatty acid methyl esters that differ only by the degree of unsaturation cannot be more selectively isolated with only changes in the temperature inside the chamber, which is packed with Raschig rings. Next, AgN03 supported on silica gel was used as a packing for the separation chamber. Figure 9 shows compositional changes of fractions extracted with SC-C02from a mixture of Clw, Clsl, Cls2, and Cis3 acid methyl esters at T1 = T2 = 373 K and 8.8 MPa. It is found that the concentration of Clw acid methyl ester is increased while that of C18-3 acid methyl ester is decreased over the whole extraction process. Especially, the separation factors of ClW and ClS3acid methyl esters are 1.8 and 0.6 at fraction 9, respectively. The average solubility over this process
822 Ind. Eng. Chem. Res., Vol. 27, No. 5 , 1988
0
0
lo
- r G c T i o n ,ic
Figure 9. Separation efficiency on the extraction of a mixture of CIH, Clsl, CIk2, and Cis3 acid methyl esters with SC-C02a t T1 = T2= 373 K and 8.8 MPa using a AgNO, supported on silica gel tube. ClB4, ( 0 ) The weight of the mixture charged was 10.0 g. (0) (A) Clg2, (0) Clgg acids.
Figure 11. Effect of the addition of ethyl acetate as an entrainer on the separation efficiency of Cis3 acid methyl ester from a mixture of C1m, Clsl, Cls2, and C1g3 acid methyl esters at 10.8 MPa and 313 K. Composition of starting material: (0) Cl8+,,, 28.95%; ( 0 )Cl,_,, 19.45%; (A)C18-2, 28.19%; (0)C1,+ 23.41%.
ducts with AgNO,. Further, it is appropriate to use AgNO, as a separation enhancer because AgNO, is not dissolved in the SC-C02 phase. This result indicates that it is possible to isolate Cia-3 acid methyl ester in high purity from the mixture of esters by a subsequent treatment. Unless the AgNO, supported on silica gel is used, the composition of the mixture of fatty acid methyl esters extracted is almost identical with that of an original sample. As shown in Figure lob, the average solubility in the range of extraction 10-20% is 5.8 mg/nL, the largest of all the fractions. The value is about 5 times the average solubility over the extraction process at 8.8 MPa (1.1 mg/nL). As the pressure increases to 10.8 MPa, the concentration of (218-3 acid methyl ester gradually increases and the solubility is reenhanced to 3.0 mg/nL. These are considered to be associated with increased solute holding power by the increased pressure (Brunner et al., 1979; Brunner, 1983). Figure 11 shows the effect of the addition of ethyl acetate as an entrainer to the CO, for the separation of CIG3 acid methyl ester held in the tube after the treatment shown in Figure 10. The use of ethyl acetate as an entrainer was found to enhance the solubility of higher fatty acid methyl esters and glycerides in the supercritical phase. Evidently, a fractionation takes place with ethyl acetate after fraction 3, and c19-3 acid methyl ester can be separated in a purity above 50%,especially 7070 for fraction 5 . The concentration of C18-3 acid methyl ester reaches about 3 times that of the original sample. It seems that C1g3 acid methyl ester is displaced from AgNO, supported on the silica gel with ethyl acetate, which has the polar nature and is homologous in structure with the fatty acid methyl ester. The solubility of fractions 5 and 6 is about 4.3 mg/nL. This value reaches approximately 4 times that of the average solubility over the whole extraction process (1.0 mg/nL). This indicates that, in the fractions where the concentration of C1g3 acid methyl ester is the highest, the solubility is the largest. Thus, our method may be suited for both the extraction and separation process for a mixture of fatty acids that differ only by the degree of unsaturation.
=-j 88MPa
lOEiMPa
Figure 10. (a) Compositional changes of fractions extracted with SC-C02 from a mixture of ClW, Clsl, Cls2, and Cis3 acid methyl esters using a AgN03 supported on silica gel tube a t T , = T , = 330 K. The weight of the mixture charged was 8.0 g. Composition of starting material: (0) elm, 28.95%; ( 0 )elel, 19.45%; (A) c1s-2, 28.19%; (0)CIG3,23.41%. (b) Solubility changes of the fractions. (a)At 8.8 MPa, (a) a t 10.8 MPa.
is 2.5 mg/nL and becomes smaller compared to that without the AgNO, supported on silica gel. Differences in volatility among the components of the mixture increase, but the esters having the higher degree of unsaturation have difficulty forming adducts with AgN0, because of the higher temperature inside the separation tube, and thus an isolation of specific components from the mixture is considered not to be achieved adequately. Therefore, a good control of the temperature inside the tube may make possible the selective extraction of a mixture of ClW, Clgl, Clg2, and Cia-3 acid methyl esters. Figure 10a shows the utility of AgNO, supported on silica gel as an enhancement agent for the fractionation on the extraction of a mixture of ClW, Clsl, ClG2,and Clg3 acid methyl esters with SC-COPat T, = T2= 330 K. The compositions of ClM, C18-1, C18-2,and Cia-3 acid methyl esters before extraction are 28.95%, 19.45%, 28.1970, and 23.41%, respectively. It is found that Clw acid methyl ester can be selectively separated from the mixture after the degree of extraction goes up to about 10%. Especially, a t the degree of extraction near lo%, the concentration of C18+ acid methyl ester has a maximum above 9070. However, Clgz and CIS-, acid methyl esters can scarcely be extracted. In particular, Cis3 acid methyl ester cannot be obtained at all until the degree of extraction reaches up to ca. 40%. It is thought that, when the loaded SC-C02 passes through the AgNO, supported on silica gel, fatty acid methyl esters having the higher degree of unsaturation, such as and C18-3 acid methyl esters, form ad-
Conclusions The addition of ethyl acetate as an entrainer to the COP enhances the extraction efficiency of triolein and further produces a supercritical fluid that demonstrates good separation efficiency of triolein from a mixture of triolein and tristearin. A decrease in pressure allows one to separate specific components from a mixture of the saturated fatty acids
Ind. Eng. Chem. Res. 1988,27, 823-830
(C1-, Clm, C20, and C22-0acid methyl esters). But the overall solubility is decreased with a decrease of the density of c02. The apparatus is equipped with the chamber for the selective removal of the dissolved materials from the loaded supercritical phase. Clel acid methyl ester is separated to a certain degree from a mixture of ClW, Clel, Cle2, and Cle3acid methyl esters by holding a higher temperature inside the chamber, which is packed with Rashchig rings. The utility of the separation chamber, which is packed with AgN03 supported on silica gel as an enhancement agent for the fractionation, is confirmed on the extraction of a mixture of C18+ C18-1, CIg2, and ClS3 acid methyl esters. It is found that ClW acid methyl ester can be selectively separated from the mixture, and the concentration is maximally above 90%. Further, ethyl acetate is used as an entrainer for the separation of C18-3 acid methyl ester held in the chamber after the treatment described above. A fractionation takes place with the addition, and the concentration reaches ca. 70%. Thus, these results demonstrate that, when one makes the SC-C02extraction by using a proper entrainer and a gas-effusion-type apparatus, it is possible to isolate specific components in a desired purity and a large solubility from a mixture of higher fatty acid methyl esters. We think that this technique can be easily applied to isolate useful components from the natural products which contain valuable acids. Registry No. AgN03, 7761-88-8; COz, 124-38-9; tristearin, 555-43-1; triolein, 122-32-7; ethanol, 64-17-5; ethyl ether, 60-29-7; acetone, 67-64-1; methylene chloride, 75-09-2; ethyl acetate, 141-78-6; methyl hexadecanoate, 112-39-0; methyl stearate, 11261-8; methyl eicosanoate, 1120-28-1; methyl docmanoate, 929-77-1;
823
methyl oleate, 112-62-9; methyl linoleate, 112-63-0; methyl linolenate, 301-00-8.
Literature Cited Brunner, G . Fluid Phase Equilib. 1983, 10, 289. Brunner, G.; Peter, S.; Retzlaff, B.; Riha, R. "High Pressure Science and Technology". Sixth AIRART Conference 1979, Vol. 1, p 565. Eisenbach, W. Ber. Bunsenges. Phys. Chem. 1984, 88, 882. Handbook of Oil Chemistry;The Japan Oil Chemists' Society: Tokyo, 1971; Chapter 1. Hirai, A.; Hamazaki, T.; Terano, T.; Nishikawa, T.; Tamura, A.; Kumagai, A.; Sajiki, J. Lacent 1980, 11, 1132. Ikushima, Y.; Arai, M.; Nishiyama, Y. Appl. Catal. 1984, 11, 305. Ikushima, Y.; Saito, N.; Hatakeda, K.; Ito, S.; Asano, T.; Goto, T. Bull. Chem. SOC.Jpn. 1986,59, 3709. Lossonczy, T. 0. Am. J . Clin. Nutr. 1978, 31, 1340. Markley, K. S. Fatty Acids Part I,2nd ed.; Interscience: New York, 1960; Chapter 6. Nagahama, K. Bunrigizyutsu 1981, 11, 23. Paul, P. F.; Wise, W. S. The Principles of Gas Extraction; Mills and Boor: London, 1971. Peter, S.; Brunner, G. Extraction with Supercritical Gases; Verlag Chemie: Weinheim, 1980. Randall, L. G. Sep. Sci. Technol. 1982, 17, 1. Sagara, H. Kemikaru Enginiyaringu 1981, 461. Saito, S. Petrotech 1982, 5, 115. Sako, T.; Yokochi, T.; Sugeta, T.; Nakazawa, N.; Hakuta, T.; Suzuki, 0.;Sato, S.; Yoshitome, H. J. Jpn. Oil Chem. SOC.1986,35, 463. Sanders, T. A. B.; Younger, K. M. Br. J . Nutr. 1981, 45, 613. Subramanian, B.; McHugh, M. A. Ind. Eng. Process Des. Dev. 1986, 25, 1.
Tsekhanskava, Yu.: Jomter, M. B.: Mashkiya, E. V. 2.Fiz. Chin. 1964, 38,2166. Yamaguchi. K. Eighth Technical Seminar. The Societv of Chemical Eniineering of-Japan, Nagoya, 1985. ' Received for review May 12, 1987 Revised manuscript received December 11, 1987 Accepted December 21, 1987
Mathematical Analysis of Crossflow Magnetically Stabilized Fluidized Bed Chromatography J. Carl Pirkle, Jr.,* and Jeffrey H. Siegell Corporate Research Laboratories, Exxon Research and Engineering Company, Annandale, New Jersey 08801
A mathematical model was developed for a crossflow magnetically stabilized fluidized bed (MSB) chromatograph with the pertinent continuity equations derived in partial differential equation form. In the case of linear adsorption isotherms, Fourier transform analysis yielded an expression for the resolution of the elution curves in terms of system parameters and operating conditions. Calculated predictions of the first moment and the variance of the elution curves were in good agreement with experimental results. A systematic parametric study conducted using the model showed that two factors, resistance to mass transfer and the width of the feed zone, have dominant effects on the size of the chromatographic bed. If smaller particles are utilized, which can be accomplished without excessive pressure drop by using an MSB, the mass-transfer resistance is reduced significantly. This allows an increase in the fraction of the bed width devoted to the feed zone and, thus, results in a lower desorbent (carrier fluid) to feed ratio and more concentrated product solutions. Recently, a crossflow magnetically stabilized fluidized bed (MSB) has been proposed for the continuous chromatographic separation of multicomponent feeds (Siegell et al., 1985, 1986). As shown in Figure 1,the magnetically stabilized fluidized solids, which have adsorbent properties, move horizontally, while the fluidizing fluid moves through the bed with a large vertical and small horizontal velocity. The mixture to be separated is introduced into the bed at the bottom, near the solids entrance. In the fluid phase, adsorbable components of the feed move largely in a vertical direction and only slightly in a horizontal direction. 0888-5885/88/2627-0823$01.50/0
They move significant horizontal distances in the solid phase. When the adsorbable components elute from the top surface of the bed, they are located at different positions, depending on their adsorption equilibrium characteristics, the system parameters, and the operating conditions. These same factors determine the shapes and degrees of overlap of the outflow concentration profiles. The MSB is ideally suited for chromatographic operation. Previous studies have shown the absence of significant fluid and solids backmixing (Rosensweig, 1979; Rosensweig et d., 1981; Siegell, 1982; Siegell and Coulaloglou, 0 1988 American Chemical Society
