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Supercritical-fluid extraction of oil-palm components
The Journal of Supercritical Fluids, 1995,8,46-50
46
Supercritical-Fluid Extraction of Oil-Palm Components Andreas
Birtigh,
Monika Johannsen,
and Gerd Brunner*
Arbeitsbereich Thermische Verfahrenstechnik, Technische Universitit Hamburg-Harburg, Eil3endorfer Strai3e 38, D-21073 Hamburg, Germany Narendran
Nair
Centre for Drug Research, Universiti Sains Malaysia, Minden, Penang I 1800, Malaysia Received February 24, 1994; accepted in revised form July 8, 1994
Residue from the mechanical processing of palm oil was treated with supercritical-fluid extraction (SFE) to extract carotenes. This was done at different extraction conditions. Also, leaves of palm oil trees were extracted using supercritical fluid to gain high concentrations of tocopherols in the extract. The aim of these SFE experiments was to evaluate whether either the carotene or tocopherol contents of the extracts were sufficiently high enough to allow downstream processing of these waste products. In addition, solubility data for a-tocopherol in supercritical carbon dioxide in a density range of 220-930 kg m-3 were measured. Keywords:
supercritical-fluid
extraction, palm oil, tocopherols, carotenes, solubility
INTRODUCTION Palm oil is a main export product from Malaysia which has about 1.9 million hectares of plantations. The palm fruits are first treated with steam and thereafter the oil-containing parts are separated from the bunch. By mechanical extraction of these parts, crude palm oil results. The residue still contains carotenes, that is 36.4% acarotene and 54.4% p-carotene. Carotene is a lipid-soluble orange-red substance present in flowers, fruit, and roots of plants and belongs to the group of carotenoids, which also includes the isomers a-carotene, &carotene, y carotene, and cryptoxanthin. Carotene is a precursor of vitamin A in human and animal metabolism and it is used in the food processing industry for coloring purposes.‘** This large aliphatic molecule has a molecular weight of 536.9 g mol-l and one of its isomers, @carotene, is illustrated in Figure 1. The solubilities of pure pcarotene in supercritical CO2 have been reported by several authors1*3 (Figure 2). The residue has a yellow-reddish color and it looks like grass. The fibres have a length of about 20 mm and are 2-3 mm wide. Until now this residue is a waste product which is burned.
0896-8446/95/0801-0046$7.50/O
Figure
1.
Molecular structure of pcarotene.
Another by-product obtained by harvesting palm oil fruits are palm leaves. It is known, that they contain a-, p-, and y-tocopherol.4 Tocopherols are contained in plants and oil from plants and they are better known as vitamin E. The isomer with the highest vitamin E activity is a-tocopherol and this has become an important additive to all kinds of food products. The chemical structure of a-tocopherol is shown in Figure 3. EXPERIMENTAL APPARATUS AND PROCEDURE The residue was taken directly out of a production line from the M. P. Mathew palm oil mill. The experi-
0 1995 PRA Presti
The Journal of Super-critical Fluids, Vol. 8, No. I, 1995
Extraction of Oil-Palm Components
47
Buffer Pump Figure
Figure
2.
3.
Solubilities of pcarotene in CO,.
Molecular structure of o-tocopherol.
ments took place in the laboratories of the Universiti Sains Malaysia in Penang, Malaysia. The carotene fraction of the dried residue was estimated by analyzing the extract of a Soxhlet extraction and yielded 0.39 x 10m3kg carotene per kg residue. For this work, a closed loop apparatus as shown in Figure 4 was used. The loaded solvent was regenerated and recycled into the extraction for reuse. The extractor had a volume of 2 L in which a gasket with a volume of 1.3 L was inserted. The loaded solvent was depressurized to 5 MPa and re-heated to 3 13 K before entering the separator. The thermodynamic conditions in the separator resulted in a lower solvent density which subsequently lead to a lower solubility of all solutes in the solvent. A metal basket was inserted into the separator to collect the complete extract phase. After the experiments the basket was taken out and the difference in weight was determined precisely. In the case of residue extraction, the dried feed (dried at 248 K; the evaporated water content was 45 mass %) was filled into the extractor and thermostated at 343 K. Unloaded CO2 was liquefied and pumped into the cycle stream until the extraction conditions were reached. These conditions were as follows: Extraction Extraction Separation Separation
pressure temperature pressure temperature
30,40, and 50 MPa 343 K
5MPa 313 K
The extraction cycle was started and the regenerated CO* from the separator was liquefied and recycled into the
Figure
4.
Flowsheet of the apparatus.
extractor. The total mass of residue per experiment was 120 g and the mass flow of the supercritical solvent was 6 kg h-l as monitored by a Rheonik mass flow meter. This results in a feed to solvent ratio of 50 (kg solvent per hour and kg feed). In a first attempt, we tried to record the amount of total extract over time, but this was given up due to problems with extract material sticking in the tubings. As a consequence, an inlay was inserted. After a total mass of g-kg solvent has flown through the extractor the experiment was stopped and the inlay was taken out of the separator and weighed to get the net weight of the extract. Then the inlay was heated until all the extracted material was liquid. The liquefied extract was filled in a small glass beaker. Using a micro balance, a known amount of extract (0.1 g) was dissolved in 16.5 g of Rhexane (p.a.) and analyzed using a Hitachi U-2000 UVspectrophotometer at 445 nm wavelength. The reported amounts of carotenes are the sums of a-, p-, and ycarotene. Additional experiments were undertaken to test whether increasing the amount of COZ would increase the amount of carotene extracted. This was done by restarting the extraction using the already extracted residue material but an empty inlay in the separator. However, only negligible parts were found. The extract samples were also analyzed for free fatty acid content (FFA). In the case of palm leaves, leaves where chopped into small pieces (c2 mm*) and were dried. 0.16 kg of dried feed were filled into the extractor. The experimental conditions were the same as for the residue. The extract was analyzed in the same manner as for the residue using the spectrophotometer with a wavelength of 293 nm. Again, the reported concentrations are the sum of the different tocopherol isomers. RESULTS AND DISCUSSION Residue Experiments. The total amount of carotenes gained from the residue extraction is listed in Table I. Also listed in Table I is the FFA content of the samples. It can be assumed that the rest of the samples mainly consists of triglycerides. For each pressure level,
48 Birtigh et al.
The Journal of Supercritical Fluids, Vol. 8, No. I, 1995
TABLE Results
50
Extractor plessure MPa
Total amount carotenes mg
30 30 40 40 50 50
28.1 30.6 37.2 35.9 46.9 46.0
g4t 15;
:
t e! 0 $j
10 0 Ext%or
Figure
5.
PresZre [MPa]
kg m-3 788 788 856 856 904 904
=I sY
e 3 20 ; 5
Density of CO2
3.09 3.93 5.15 5.13 4.88 4.55
8
% $2 = 30
Extraction
Concentration carotenes in extract 1O-3kg kg-’
100 % Soxhlet-
&I
I
of the Carotene
50
Concentration FFA in extract % 25.1 27.3 12.0 11.1 4.1 2.9
6 0
5-
: .
3
0
* 1i
o 01
30 40 50 Extractor Pressure [MPa]
Mass of extracted carotenes vs. extraction
pressure. two experiments were performed. The maximum deviation between two results at same conditions is 8.8% for the total amount of carotene at an extraction pressure of 30 MPa. This is mostly due to the inhomogenity of the residue material. The same results are shown in the following two graphs. In Figure 5, the total amount of carotenes gained by soxhlet extraction is documented as well. In Figure 6 the concentration of carotene in the extract is shown. The total amount of extracted carotenes shows that only for an extraction pressure of 50 MPa the carotene concentration that is found with soxhlet extraction can be reached by WE. This is not due to a limit in solubility since 9 kg of solvent can dissolve at least 100 mg of carotenes for all tested pressures. Present in the residue were only 47 mg of carotenes. The reason is that the mass transport in the matrix of the residue material is strongly influenced by the presence of triglycerides. Carotenes are well dissolved in triglycerides which show a strong pressure dependence for solubility in the tested pressure range.5 It is assumed that carotenes are transported with triglycerides. While the total amount of carotenes is rising with increasing pressure, the concentration of carotenes in the
Figure 6. Concentration of carotenes in the extract vs. extraction pressure.
extract is decreasing. This is due to higher solubility of other materials especially triglycerides in the solvent. The concentration of the FFA in the extract is decreasing with rising pressure. This is also because of the higher solubility of the triglycerides in the solvent at higher pressures while for the FFA the pressure of 30 MPa is high enough to extract most of the FFA in the residue material. Palm Leaves Experiments. The second series of experiments focused on the extraction of tocopherols from palm leaves. To extract it with the small scale closed-loop apparatus available, palm leaves were collected fresh from a palm tree, and were chopped and dried the same day. These mature leaves had a total length of about 4 m. The central branch was not used. The water content before drying was 51.4 mass %. The analysis of the extract from soxhlet extraction yielded a tocopherol content of 1.71 10m3kg tocopherol per kg dried palm leaves. Solubility data of a-tocopherol in supercritical carbon dioxide found in the literature are listed in Table II. In our laboratories at Technical University Hamburg-Harburg, the solubilities of a-tocopherol were measured by using a static analytical method with direct
The Journal of Supercritical Fluids, Vol. 8, No. I, 1995
TABLE
II
300 _
Synopsis of Solubility Measurement copherol in Supercritical Carbon
Pressure (MPa) 8-20
1-18 15-35
10-35 9-26
Extraction of Oil-Palm Components
Temperature (K)
of a-ToDioxide
Chrastil ( 1982)6 Ohgalci et al. (1989)’ Zehnder (1992)* Meier ( 1992)9 Pereira et al. (1993)lO
313-353 298, 313 327-353 303-353 292-333
100 % Soxhlet
l250: d 2200: aI rp150 : 8 pl0:
Author
s3
49
0
0 0
50: I 30 Extractor
0’
, 40 Pressure
I 50 [MPa]
Data Johrnnrsn
8.
Figure
Mass of tocopherol vs. extraction pressure. 0
I -
a
0
200 300 400 500 600 700 600 9001000 Density Solvent [kg/m al Figure
7.
Solubility of a-tocopherol
in supercritical
co*.
30 Extractor
coupling of an equilibrium cell to a supercritical-fluid chromatographic system. A detailed description of this apparatus for solubility measurement was reported by Johannsen.” Figure 7 summarizes our results. They are in good agreement with data from the literature. Only the values of Chrast@ are remarkably lower than our results. For the extraction of tocopherols from oil palm leaves the results are listed in Table III. In Figure 8. the total amount of tocopherols gained by soxhlet extraction is shown. In Figure 9, the concentration of tocopherols as extracted by SCF is given. It is obvious, that the concentration is declining with extraction pressure. The maximum amount of extracted tocopherol is already reached with 30 MPa, but again with higher extraction pressures
Figure
of Tocopherol
[MPa]
Concentration of tocopherol vs. extraction
the concentration is decreasing because of higher solubility for other diluting materials. CONCLUSION Waste products of the palm oil production process were treated with supercritical-fluid extraction using carbon dioxide. Carotenes from the residue and tocopherols from the palm leaves were extracted. In general, the extracted amounts of carotenes and tocopherols from the residues and palm leaves were found to be not high Ill
Extraction
from
Palm
Extractor pressure (MPa)
Concentration tocopherols (kg kg-‘)
Total mass tocopherols (mg)
30 40
0.113 0.077 0.072
233 233 201
50
50
pressure.
TABLE Results
9.
40 Pressure
Leaves
Density of COz (kg m-3) 788 856 904
50
Birtigh et al.
The Journal
enough to allow an economic industrial size extraction. However, extraction of tocopherols from leaves taken off from palm trees in order to collect the palm fruits may be a route to be evaluated further. ACKNOWLEDGMENTS We thank the Deutsche Forschungsgesellschaft for their financial support under grant No. BR 846/7-2. We also thank Mr. Lim Thuan Swee from M.P. Mathew palm oil mill Sdn. Bhd. REEERENCES
(1)
Cygnarowicz, M.; Maxwell, R.; Seider, W. Fluid Phase Equil.
(2) (3)
1990,59,
57.
Coenen, H.; Kriegel E. Chem.-Ing.-Tech. 1983, 55, 890. Gerard, D. MS. Thesis, University Saarbrticken, 1980.
(4) (5) (6) (7)
of Super-critical
Fluids,
Vol. 8, No. I, 1995
Gapor; A. B.; Fujimoto, K.; Shishikura, A.; Kaneda, T.; Arai, K.; Saito, S. ELAEIS 1990,2, 167. Stahl, E.; Quirin, K. W.; Gerard, D. Dense Gases for Extraction and Refining; Springer-Verlag; Bsrlin, 1988. Chrastil, J. J. Phys. Chem. 1982, 86, 3016, Ohgaki, K.; Tsukahara, I.; Semba, K.; Katayama, T. Int. Chem. Eng. 1989, 29, 302.
Zehnder, B. H. Ph.D. Dissertation, ETH Zurich, Switzerland, 1992. (9) Meier, U. Ph.D. Dissertation, ETH Zurich, Switzerland, 1992. (10) Pereira, P. I.; Goncalves, M.; Coto, B.; Gomes de Azevedo, E.; Nunes da Ponte, M. Fluid Phase Equil. 1993,91, 133. (11) Johannsen, M.; Brunner, G. Fluid Phase Equil. 1994, 95, 215. (8)
46
Supercritical-Fluid Extraction of Oil-Palm Components Andreas
Birtigh,
Monika Johannsen,
and Gerd Brunner*
Arbeitsbereich Thermische Verfahrenstechnik, Technische Universitit Hamburg-Harburg, Eil3endorfer Strai3e 38, D-21073 Hamburg, Germany Narendran
Nair
Centre for Drug Research, Universiti Sains Malaysia, Minden, Penang I 1800, Malaysia Received February 24, 1994; accepted in revised form July 8, 1994
Residue from the mechanical processing of palm oil was treated with supercritical-fluid extraction (SFE) to extract carotenes. This was done at different extraction conditions. Also, leaves of palm oil trees were extracted using supercritical fluid to gain high concentrations of tocopherols in the extract. The aim of these SFE experiments was to evaluate whether either the carotene or tocopherol contents of the extracts were sufficiently high enough to allow downstream processing of these waste products. In addition, solubility data for a-tocopherol in supercritical carbon dioxide in a density range of 220-930 kg m-3 were measured. Keywords:
supercritical-fluid
extraction, palm oil, tocopherols, carotenes, solubility
INTRODUCTION Palm oil is a main export product from Malaysia which has about 1.9 million hectares of plantations. The palm fruits are first treated with steam and thereafter the oil-containing parts are separated from the bunch. By mechanical extraction of these parts, crude palm oil results. The residue still contains carotenes, that is 36.4% acarotene and 54.4% p-carotene. Carotene is a lipid-soluble orange-red substance present in flowers, fruit, and roots of plants and belongs to the group of carotenoids, which also includes the isomers a-carotene, &carotene, y carotene, and cryptoxanthin. Carotene is a precursor of vitamin A in human and animal metabolism and it is used in the food processing industry for coloring purposes.‘** This large aliphatic molecule has a molecular weight of 536.9 g mol-l and one of its isomers, @carotene, is illustrated in Figure 1. The solubilities of pure pcarotene in supercritical CO2 have been reported by several authors1*3 (Figure 2). The residue has a yellow-reddish color and it looks like grass. The fibres have a length of about 20 mm and are 2-3 mm wide. Until now this residue is a waste product which is burned.
0896-8446/95/0801-0046$7.50/O
Figure
1.
Molecular structure of pcarotene.
Another by-product obtained by harvesting palm oil fruits are palm leaves. It is known, that they contain a-, p-, and y-tocopherol.4 Tocopherols are contained in plants and oil from plants and they are better known as vitamin E. The isomer with the highest vitamin E activity is a-tocopherol and this has become an important additive to all kinds of food products. The chemical structure of a-tocopherol is shown in Figure 3. EXPERIMENTAL APPARATUS AND PROCEDURE The residue was taken directly out of a production line from the M. P. Mathew palm oil mill. The experi-
0 1995 PRA Presti
The Journal of Super-critical Fluids, Vol. 8, No. I, 1995
Extraction of Oil-Palm Components
47
Buffer Pump Figure
Figure
2.
3.
Solubilities of pcarotene in CO,.
Molecular structure of o-tocopherol.
ments took place in the laboratories of the Universiti Sains Malaysia in Penang, Malaysia. The carotene fraction of the dried residue was estimated by analyzing the extract of a Soxhlet extraction and yielded 0.39 x 10m3kg carotene per kg residue. For this work, a closed loop apparatus as shown in Figure 4 was used. The loaded solvent was regenerated and recycled into the extraction for reuse. The extractor had a volume of 2 L in which a gasket with a volume of 1.3 L was inserted. The loaded solvent was depressurized to 5 MPa and re-heated to 3 13 K before entering the separator. The thermodynamic conditions in the separator resulted in a lower solvent density which subsequently lead to a lower solubility of all solutes in the solvent. A metal basket was inserted into the separator to collect the complete extract phase. After the experiments the basket was taken out and the difference in weight was determined precisely. In the case of residue extraction, the dried feed (dried at 248 K; the evaporated water content was 45 mass %) was filled into the extractor and thermostated at 343 K. Unloaded CO2 was liquefied and pumped into the cycle stream until the extraction conditions were reached. These conditions were as follows: Extraction Extraction Separation Separation
pressure temperature pressure temperature
30,40, and 50 MPa 343 K
5MPa 313 K
The extraction cycle was started and the regenerated CO* from the separator was liquefied and recycled into the
Figure
4.
Flowsheet of the apparatus.
extractor. The total mass of residue per experiment was 120 g and the mass flow of the supercritical solvent was 6 kg h-l as monitored by a Rheonik mass flow meter. This results in a feed to solvent ratio of 50 (kg solvent per hour and kg feed). In a first attempt, we tried to record the amount of total extract over time, but this was given up due to problems with extract material sticking in the tubings. As a consequence, an inlay was inserted. After a total mass of g-kg solvent has flown through the extractor the experiment was stopped and the inlay was taken out of the separator and weighed to get the net weight of the extract. Then the inlay was heated until all the extracted material was liquid. The liquefied extract was filled in a small glass beaker. Using a micro balance, a known amount of extract (0.1 g) was dissolved in 16.5 g of Rhexane (p.a.) and analyzed using a Hitachi U-2000 UVspectrophotometer at 445 nm wavelength. The reported amounts of carotenes are the sums of a-, p-, and ycarotene. Additional experiments were undertaken to test whether increasing the amount of COZ would increase the amount of carotene extracted. This was done by restarting the extraction using the already extracted residue material but an empty inlay in the separator. However, only negligible parts were found. The extract samples were also analyzed for free fatty acid content (FFA). In the case of palm leaves, leaves where chopped into small pieces (c2 mm*) and were dried. 0.16 kg of dried feed were filled into the extractor. The experimental conditions were the same as for the residue. The extract was analyzed in the same manner as for the residue using the spectrophotometer with a wavelength of 293 nm. Again, the reported concentrations are the sum of the different tocopherol isomers. RESULTS AND DISCUSSION Residue Experiments. The total amount of carotenes gained from the residue extraction is listed in Table I. Also listed in Table I is the FFA content of the samples. It can be assumed that the rest of the samples mainly consists of triglycerides. For each pressure level,
48 Birtigh et al.
The Journal of Supercritical Fluids, Vol. 8, No. I, 1995
TABLE Results
50
Extractor plessure MPa
Total amount carotenes mg
30 30 40 40 50 50
28.1 30.6 37.2 35.9 46.9 46.0
g4t 15;
:
t e! 0 $j
10 0 Ext%or
Figure
5.
PresZre [MPa]
kg m-3 788 788 856 856 904 904
=I sY
e 3 20 ; 5
Density of CO2
3.09 3.93 5.15 5.13 4.88 4.55
8
% $2 = 30
Extraction
Concentration carotenes in extract 1O-3kg kg-’
100 % Soxhlet-
&I
I
of the Carotene
50
Concentration FFA in extract % 25.1 27.3 12.0 11.1 4.1 2.9
6 0
5-
: .
3
0
* 1i
o 01
30 40 50 Extractor Pressure [MPa]
Mass of extracted carotenes vs. extraction
pressure. two experiments were performed. The maximum deviation between two results at same conditions is 8.8% for the total amount of carotene at an extraction pressure of 30 MPa. This is mostly due to the inhomogenity of the residue material. The same results are shown in the following two graphs. In Figure 5, the total amount of carotenes gained by soxhlet extraction is documented as well. In Figure 6 the concentration of carotene in the extract is shown. The total amount of extracted carotenes shows that only for an extraction pressure of 50 MPa the carotene concentration that is found with soxhlet extraction can be reached by WE. This is not due to a limit in solubility since 9 kg of solvent can dissolve at least 100 mg of carotenes for all tested pressures. Present in the residue were only 47 mg of carotenes. The reason is that the mass transport in the matrix of the residue material is strongly influenced by the presence of triglycerides. Carotenes are well dissolved in triglycerides which show a strong pressure dependence for solubility in the tested pressure range.5 It is assumed that carotenes are transported with triglycerides. While the total amount of carotenes is rising with increasing pressure, the concentration of carotenes in the
Figure 6. Concentration of carotenes in the extract vs. extraction pressure.
extract is decreasing. This is due to higher solubility of other materials especially triglycerides in the solvent. The concentration of the FFA in the extract is decreasing with rising pressure. This is also because of the higher solubility of the triglycerides in the solvent at higher pressures while for the FFA the pressure of 30 MPa is high enough to extract most of the FFA in the residue material. Palm Leaves Experiments. The second series of experiments focused on the extraction of tocopherols from palm leaves. To extract it with the small scale closed-loop apparatus available, palm leaves were collected fresh from a palm tree, and were chopped and dried the same day. These mature leaves had a total length of about 4 m. The central branch was not used. The water content before drying was 51.4 mass %. The analysis of the extract from soxhlet extraction yielded a tocopherol content of 1.71 10m3kg tocopherol per kg dried palm leaves. Solubility data of a-tocopherol in supercritical carbon dioxide found in the literature are listed in Table II. In our laboratories at Technical University Hamburg-Harburg, the solubilities of a-tocopherol were measured by using a static analytical method with direct
The Journal of Supercritical Fluids, Vol. 8, No. I, 1995
TABLE
II
300 _
Synopsis of Solubility Measurement copherol in Supercritical Carbon
Pressure (MPa) 8-20
1-18 15-35
10-35 9-26
Extraction of Oil-Palm Components
Temperature (K)
of a-ToDioxide
Chrastil ( 1982)6 Ohgalci et al. (1989)’ Zehnder (1992)* Meier ( 1992)9 Pereira et al. (1993)lO
313-353 298, 313 327-353 303-353 292-333
100 % Soxhlet
l250: d 2200: aI rp150 : 8 pl0:
Author
s3
49
0
0 0
50: I 30 Extractor
0’
, 40 Pressure
I 50 [MPa]
Data Johrnnrsn
8.
Figure
Mass of tocopherol vs. extraction pressure. 0
I -
a
0
200 300 400 500 600 700 600 9001000 Density Solvent [kg/m al Figure
7.
Solubility of a-tocopherol
in supercritical
co*.
30 Extractor
coupling of an equilibrium cell to a supercritical-fluid chromatographic system. A detailed description of this apparatus for solubility measurement was reported by Johannsen.” Figure 7 summarizes our results. They are in good agreement with data from the literature. Only the values of Chrast@ are remarkably lower than our results. For the extraction of tocopherols from oil palm leaves the results are listed in Table III. In Figure 8. the total amount of tocopherols gained by soxhlet extraction is shown. In Figure 9, the concentration of tocopherols as extracted by SCF is given. It is obvious, that the concentration is declining with extraction pressure. The maximum amount of extracted tocopherol is already reached with 30 MPa, but again with higher extraction pressures
Figure
of Tocopherol
[MPa]
Concentration of tocopherol vs. extraction
the concentration is decreasing because of higher solubility for other diluting materials. CONCLUSION Waste products of the palm oil production process were treated with supercritical-fluid extraction using carbon dioxide. Carotenes from the residue and tocopherols from the palm leaves were extracted. In general, the extracted amounts of carotenes and tocopherols from the residues and palm leaves were found to be not high Ill
Extraction
from
Palm
Extractor pressure (MPa)
Concentration tocopherols (kg kg-‘)
Total mass tocopherols (mg)
30 40
0.113 0.077 0.072
233 233 201
50
50
pressure.
TABLE Results
9.
40 Pressure
Leaves
Density of COz (kg m-3) 788 856 904
50
Birtigh et al.
The Journal
enough to allow an economic industrial size extraction. However, extraction of tocopherols from leaves taken off from palm trees in order to collect the palm fruits may be a route to be evaluated further. ACKNOWLEDGMENTS We thank the Deutsche Forschungsgesellschaft for their financial support under grant No. BR 846/7-2. We also thank Mr. Lim Thuan Swee from M.P. Mathew palm oil mill Sdn. Bhd. REEERENCES
(1)
Cygnarowicz, M.; Maxwell, R.; Seider, W. Fluid Phase Equil.
(2) (3)
1990,59,
57.
Coenen, H.; Kriegel E. Chem.-Ing.-Tech. 1983, 55, 890. Gerard, D. MS. Thesis, University Saarbrticken, 1980.
(4) (5) (6) (7)
of Super-critical
Fluids,
Vol. 8, No. I, 1995
Gapor; A. B.; Fujimoto, K.; Shishikura, A.; Kaneda, T.; Arai, K.; Saito, S. ELAEIS 1990,2, 167. Stahl, E.; Quirin, K. W.; Gerard, D. Dense Gases for Extraction and Refining; Springer-Verlag; Bsrlin, 1988. Chrastil, J. J. Phys. Chem. 1982, 86, 3016, Ohgaki, K.; Tsukahara, I.; Semba, K.; Katayama, T. Int. Chem. Eng. 1989, 29, 302.
Zehnder, B. H. Ph.D. Dissertation, ETH Zurich, Switzerland, 1992. (9) Meier, U. Ph.D. Dissertation, ETH Zurich, Switzerland, 1992. (10) Pereira, P. I.; Goncalves, M.; Coto, B.; Gomes de Azevedo, E.; Nunes da Ponte, M. Fluid Phase Equil. 1993,91, 133. (11) Johannsen, M.; Brunner, G. Fluid Phase Equil. 1994, 95, 215. (8)