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Rate of the vegetable oil extraction with supercritical CO2—III. Extraction from sea buckthorn
Pergamon
Chemical Engineering; Science, Vol, 51, No. 18, pp. 4347 4352, 1996
PII: S0009-2509(96)00263-17
Copyright i ; 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved
ooo9 2509"96$15.00+ 0.00
RATE OF THE VEGETABLE OIL EXTRACTION WITH SUPERCRITICAL CO2--III. EXTRACTION FROM SEA BUCKTHORN J. Sq'ASTOVA, J. JEZ, M. BARTLOVA and H. SOVOVA* Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, 165 02 Prague 6, Czech Republic (First received 9 August 1995; accepted 19 December 1995)
Al~tract--Oil from the seed and pulp of sea buckthorn berries (Hippopha~ rhamnoides L.) was extracted with carbon dioxide at pressures 9.(~27 MPa and temperatures 25-60°C. Influenceof extraction conditions on solubility and mass transfer rate was studied. No marked changes in composition of extracted oil in the course of extraction were observed. Experimental extraction curves were evaluated using a model with grinding efficiency,volume mass transfer coefficients in solid and fluid phases, and parameter of flow asymmetry as adjustable parameters. The solid-phase mass transfer coefficientwas increasing with rising temperature. Copyright © 1996 Elsevier Science Ltd INTRODUCTION Sea buckthorn is a bush that grows wildly in the mountain regions of the middle and southeastern Asia and Europe, as well as in Scandinavia and on the west European coast. Its fruit are berries of orange to red colour and have an acid, lightly bitter taste. They contain many vitamins (B, C, E, K, provitamin A) and other biologically active substances. The main products obtained from the fruit are juice rich in vitamin C and oil rich in unsaturated fatty acids. The sea buckthorn oil is used in medicine (Hlava and Vali~ek, 1989; S{astovfi, 1994). It finds application in healing ointments applied to wounds and burns, in medicines for treating cancer, in vitamin preparations. It is also uscd in cosmctics e.g. as natural plasticizer and emulsifier, and in skin-regenerating compositions and UV filters. Content of oil in fruit varies from 2 to 17 wt% on dry basis. It is interesting that the oil occurs not only in seeds but also in the pulp of the fruit. While the seed oil contains mainly fatty acids with 18 carbon atoms in molecule (linoleic and linolenic acids represent up to 60%), palmitic and palmitooleic acids with 16 carbon atoms in molecule predominate in the oil from the pulp (Zhamyansan, 1978). The sea buckthorn oil is extracted from the fruit after separation of juice and after drying. Three types of solvents are used: organic solvents, which are later separated from the oil by distillation, or pure vegetable oils--usually sunflower oil, which remain in the product (Bukshtynov et at., 1978; Golubyev et al., 1990), or dense gases like carbon dioxide (Shaftan et al., 1986). The advantage of the last process is that the product is natural, free from the solvent and without any products of thermal degradation, because the extraction with dense gases can be carried out at relatively low temperatures. *Corresponding author.
In Russia, sea buckthorn oil was extracted with liquid carbon dioxide both in the laboratory apparatus and in the large-scale equipment described by Kasyanov et al. (1978). Berries of plants growing wild in Siberia and Caucasus were used. Air-dry pulp and seed of the berries in mass ratio approximately 45:55 were first ground to particles of 0.3~0.4 mm size and then flaked to a thickness of 0.12~0.20mm. Countercurrent extraction at 20°C and 5.7 MPa was performed for 3-3.5 h. Yield of oil was 4-8wt% according to the composition of the plant material (Shaftan et al., 1986). In Germany, the firm FLAVEX extracted sea buckthorn oil from Lithuanian plant material with supercritical carbon dioxide. Extraction conditions were close to 35 MPa and 40°C, the yield was 16.5 wt%, the extract contained 29% palmitooleic acid (Flavex, 1992). The aim of this study is to investigate the effect of extraction conditions on the rate of supercritical fluid extraction from buckthorn seed and pulp. EXPERIMENTAL Material. Three varieties of sea buckthorn berries cultivated in Czech Republic were obtained from the University of Agriculture in Prague. Most of the juice was separated from the defrosted berries by pressing. The pressed cake was then air-dried, the seed was mechanically separated from the pulp, and the material was ground. Moisture levels 4-5 wt% in dry pulp and 8-9wt% in dry seed were determined gravimetrically; density of the solid phase was measured using a pycnometer. The values are listed in Table 1 together with void fraction of lightly pressed bed of ground material, and with oil content determined by Soxhlet extraction with n-hexane. Purity of carbon dioxide supplied by Technoplyn Litvinov was better than 99.9 wt%.
4347
J. STASTOVAe t al.
4348 Table 1. Properties of ground material Variety
Ps (kg/m~) e
solvent flow distributors. Feed of the plant material ranged from 3 to 40 g. Extraction pressure was 27 MPa in most experiments, extraction temperature was adjusted to 25, 40 or 60°C. Additional experiments were conducted with the pressure decreased to 17.4 or 9.6 MPa at 40°C. Flow rate of carbon dioxide was adjusted to 1 1/min (measured at atmospheric pressure). Solubility of sea buckthorn oil in dense carbon dioxide was measured by the dynamic method using the same apparatus. Glass wool in the 12 ml extractor was thoroughly wetted by 1-2ml of oil extracted previously from the seed or pulp. Flow rate of the solvent was adjusted to 0.1-0.2 1/min. It was checked by preliminary experiments that the extraction rate was independent of the solvent flow rate at these conditions. The extracts were analysed by HPLC. The apparatus was a Hewlett Packard 1090M chromatograph equipped with diode array UV-VIS detector and a column 25 cm x 4 mm i.d. packed with Nucleosil C 18, 5 lam. The conditions were: mobile phase methanol, flow rate 0.4 ml/min, detection at wavelength 220 nm.
x0 (mg oil/g material)
Seed A B C
1200 1270 1240
0.347 0.334 0.364
8.1 8.0 13.4
Pulp A B C
1190 1250 1130
0.520 0.522 0.460
12.0 9.7 11.0
Extraction experiments were performed using the semicontinuous extraction apparatus described previously (Sovovfi et al., 1994). Carbon dioxide from a pressure cylinder was charged to a compressor and controlling valve maintaining the extraction pressure within + 0.05 MPa. It then passed a preheating coil immersed together with the extractor in a water bath with temperature controlled within +_ 0.1°C, and entered the top of the extractor packed with the extracted material. Upon leaving the extractor, the supercritical CO2 stream containing the dissolved oil passed through a heated micrometering valve that allowed its expansion to ambient pressure. The extracted oil was separated from the CO2 and was collected in a glass U-tube. The depressurized gas flowed through the separator to flow totalizer before being vented. U-tubes were exchanged in the course of the extraction and the increase of extract amount was determined gravimetrically. Total number of extraction experiments was 35. Two extractors were used alternatively: 12 ml extractor 8 mm i.d., and 150 ml extractor 33 mm i.d. The extractor was packed with the ground seed or pulp placed between two layers of glass beads serving as
RESULTS
The extracts obtained from seed and pulp differed in colour and consistency. Seed oil was a yellow or light orange viscous liquid, while the pulp oil was of pasty consistency and dark red colour, which liquidized at approximately 50°C. Chromatographic peaks of extract sample (see Fig. 1) represent groups of chemically similar substances as free fatty acids, mono-, di- and triglycerides. There were no systematic differences observed either between the chromatograms
o6 100
80
60 < ¢q
4C
20
I 2
I 4
I 6
I 8
10
12
14
I 16
--
Time (min) Fig. 1. HPLC chromatogram of buckthorn oil extract (mAU = milliampereunits). Identified peaks: (1) free fatty acids, (2) triglycerides.
Rate of the vegetable oil extraction
4349
Table 2. Oil solubility, mg oil/g COz: comparison with the correlation of del Valle and Aguilera (1988) Extraction conditions: pressure (MPa)/temperature (~C) Variety
27/25
27/40
27/60
17.4/40
A B C A B C
6.5 7.1 6.9 7.7 8.2 8.2
6.5 7.4 6.8 7.9 8.6 8.4
5.8 6.8 6.6 7.2 7.9 8.2
3.0 4.0
5.8±2.8
6.2±3.0
6.2±3.3
2.5±3.3
Seed
Pulp
Correlation
of extracts obtained from the individual varieties, or between the samples of extracts collected in the course of extraction. The main extract components, triglycerides, represent 76-92% of the total peak area, while the free fatty acids only 1~1%. There were differences in CO2 solubility of oil extracted from the individual sea buckthorn varieties, as is shown in Table 2. Pulp oil was always more soluble than seed oil, most probably due to its shorter molecules. Almost all solubility values were higher than those calculated as mean values from the correlation derived by del Valle and Aguilera (1988) for the solubility of vegetable oils in supercritical CO2. However, all experimental solubilities remain inside the wide confidence limits of the correlation. Extraction model with four adjustable parameters, which is described in the appendix, was fitted to experimental extraction curves. This version of the model differs from that derived in the first paper of the series (Sovovfi, 1994) by several simplifications. Namely, parameter of grinding efficiency G has been introduced as well as dimensionless time ~,; solvent flow inhomogeneity is simulated by dividing the bed of extracted material into parallel halves, instead of the six parallel sections used previously (Sovovfi et al., 1994). Examples of the evaluation of experimental results are shown in Figs 2-5, where the symbols
0.3
0.2
0.1
3.8 4.3
/f ~/o
8'0
t2o
16o
Qt, g
Fig. 3. Effect of extraction temperature on the course of extraction. 12ml extractor, feed 5.1 g seed of variety A, solvent flow rate 1.75gCO2/min, grade of grinding 3, extraction pressure 27 MPa. Extraction temperature: (11) 25c'C, ( + ) 60°C.
0.06
0.04
z
m 0.02
0.3-
0
10
20
30
40
GUN Fig. 4. Effect of pressure on the course of extraction. 12 ml extractor, seed of variety A, grade of grinding 3, extraction temperature 40°C. Extraction pressure: (11) 27 MPa, ( + ) 17.4 MPa, (Fq) 9.6 MPa.
0.1
0
40
8'0
1½0
160
Qt, g Fig. 2. Effect of grade of grinding on the extraction rate. 12 ml extractor, feed 5 g seed of variety A, solvent flow rate 1.78 g CO2/min, extraction conditions 27 MPa/40°C. Grade of grinding: (11) 1, ( + ) 2, ([3) 3.
represent experimental points and the curves are calculated using the model. Model parameters resulting from the fitting procedure based on the least-squares method are listed in Tables 3-6. Confidence limits of the regressed parameters shown in Tables 4-6 were evaluated from repeated experiments. To compare the
J. S1"ASTOVAet al.
4350 0.09j f 0.06
0.03
i0
20 Qt/N
30
4o
Fig 5. Comparison of extraction from seed and pulp. 12 ml extractor, variety A, grade of grinding 3, extraction conditions 27 MPa/40°C. Material: (m) seed, ( + ) pulp.
Table 3. Effect of grade of grinding on model parameters: extraction from seed of variety A, 12ml extractor, 27 MPa/40°C Grade 1 2 3
G 0.38 0.60 0.67
ASM 0.55 0.33 0.08
kya (min- 1) k~a (min- 1) 0.34 0.50 0.56
0.00037 0.00055 0.00060
Table 4. Grinding efficiency G at grade of grinding 3 Variety
A
Seed Pulp
B
0.67 + 0.02 0.65 ± 0.04 0.68 + 0.03 0.74 + 0.04
C 0.68 _+ 0.02 0.69 + 0.02
Table 5. Volume mass transfer coefficient in solvent phase, ksa, min-1: pressure 27 MPa, grade of grinding 3. (Recalculated to the solvent interstitial velocity v = 1 mm/s according to ksa.~v°'54.) Temperature (°C) Seed Pulp
25
40
60
0.40 -I- 0.04 0.40 ± 0.03 0.32 _ 0.03 0.64 + 0.06 0.72 ___0.07 0.53 + 0.05
solvent-phase mass transfer coefficients measured at different interstitial velocities, their values were recalculated to one reference velocity using eq. (A.13) from the appendix.
Grinding efficiency, G, and solvent flow asymmetry, ASM: parameter of grinding efficiency G is closely related to the grade of grinding. Drop size distribution
of the particles of ground material was bimodal with one peak close to 0.5 mm and the other peak close to 0.01 mm. Grinding at grade 1 produced a mixture of 2 8 w t % small particles and 72wt% large particles, while the portion of small particles in the material obtained by grinding at grade 3 was 50 wt%. Small particles (0.01 mm) stuck together forming larger conglomerates. These conglomerates disintegrated, after the oil had been extracted from the surface of the particles. Table 3 shows the increase of grinding efficiency and of both volume mass transfer coefficients, which are proportional to the surface area, with the grade of grinding. Parameter of flow asymmetry, ASM, diminishes gradually with increasing grade of grinding. As it was confirmed in the previous work (Sovovfi et al., 1994) that carbon dioxide flows down through the bed of extracted particles practically in plug flow, with ASM = 0, the reason for the high values of ASM evaluated from the experiments with roughly ground sea buckthorn must be different. Probably, it is the bimodal particle size distribution, which deforms the measured extraction curve in a similar way, as flow asymmetry does in the monodisperse extraction model we have used. Most of the experiments were carried out at the grade of grinding equal to 3. As it is seen from Table 4, the parameter of grinding efficiency at constant grade of grinding had a similar value for all varieties. Its value for pulp was somewhat higher than for seed, because the pulp was drier and therefore more brittle. Volume mass transfer coefficient in the solvent, k fa, is given in Table 5. It has been recalculated to the interstitial velocity of carbon dioxide 1 mm/s in order to join together the results obtained from experiments carried out using extractors of various diameters. The interstitial velocity in the 12 ml extractor ranged from 1.8 to 2.1 mm/s for the seed, and from 1.2 to 1.4 mm/s for the pulp; in the 150 ml extractor it was 17 times less.
Volume mass transfer coefficient in the solid phase, ksa, which controls the second period of extraction, was by three orders of magnitude smaller than the coefficient in the solvent. It was increasing substantially with rise in temperature (see Table 6). CONCLUSIONS Extraction with dense carbon dioxide is an appropriate method for the separation of oil from sea buckthorn fruit. With the downflow of CO2, two periods of extraction are sharply distinguished. The a m o u n t of oil extracted in the first, fast extraction period depends on the grade of grinding. Rate of extraction in the second period is by several orders of magnitude
Table 6. Volume mass transfer coefficient in solid phase, k~a, min- 1: grade of grinding 3 Temperature (°C) Seed Pulp
25
40
60
0.0003 ___0.0001 0.0006 _+0.0002
0.0006 + 0.0001 0.0012 _+0.0002
0.0014 + 0.0003 0.0018 ± 0.0002
4351
Rate of the vegetable oil extraction smaller a n d rises with increasing temperature. The studied varieties of sea b u c k t h o r n varied n o t only in the oil content, but also in its composition, because certain differences in the oil solubility in c a r b o n dioxide were found (less t h a n 14% b o t h for pulp a n d seed oil). Solubility of the pulp oil was by 19% higher t h a n that of the seed oil, o n average. A new version of the previously derived extraction model could be used to describe the b u c k t h o r n extraction curves a n d to evaluate grinding efficiency, mass transfer coefficients, a n d flow asymmetry, which are listed in Tables 3-6. The mass transfer coefficient in the solvent was approximately p r o p o r t i o n a l to the square root of solvent interstitial velocity. The mass transfer coefficient in the solid phase was t e m p e r a t u r e dependent. The value of the last parameter, ASM, was r a t h e r a measure of size i n h o m o g e n e i t y of the extracted particles, t h a n a measure of deviation of the solvent flow t h r o u g h the bed of particles from plug flow, as it is defined in the model.
Acknowledgement- The authors acknowledge the financial support from the Grant Agency of the Czech Republic. The work has been carried under Grant No. 510/93/2283.
a
ASM E G h J k I N
Q t U t~ X
Y Yr
Y Z
NOTATION specific interfacial area, 1parameter of flow asymmetry (0 ~< A S M < 1) mass of extract, M grinding efficiency (0 ~< G ~< 1) dimensionless axial co-ordinate (0 ~< h ~< 1) mass transfer rate, M 1 3 T mass transfer coefficient, 1T axial co-ordinate, 1 solid feed, M mass flow rate of solvent, M T - 1 time, T superficial solvent velocity, I T interstitial solvent velocity, I T solid-phase concentration, M M - 1 solvent-phase concentration, M M solubility, M M p a r a m e t e r of the second extraction period defined by eq. (A12) p a r a m e t e r of the first extraction period defined by eq. (AI2)
Greek letters ~void fraction p density, M1 3 dimensionless time defined by eq. (A7)
REFERENCES
Bukshtynov, A. D. et al., 1978, Oblepikha (sea buckthorn), pp. 173-181. Lesnaja promyshlennost', Moskva. del Valle, J. M. and Aguilera, J. M., 1988, An improved equation for predicting the solubility of vegetable oils in supercritical CO 2. Ind. Engng Chem. Res. 27, 1551 1553. Flavex, 1992, CO2-extracts. Prospectus of FLAVEX Naturextrakte, Rehlingen. Golubyev, V. N., Kolesnik, A. A. and Ismailov, T. K., 1990, Kompleksnaja pererabotka oblepikhi s pomoshchyu membran. (Complex processing of sea buckthorn using membranes.) Pishtshevaja promyshlennost' (11), 32--35. Hlava B. and Vali6ek P., 1989, Rostlinnb harmonizhtory (Vegetable harmonizers) University of Agriculture, Praha. Kasyanov, G. 1., Pekhov, A. V. and Taran, A. A., 1978, Naturarnye pishchevye aromatizatory CO 2 ekstrakty. (Natural Food Aromatizers CO 2 Extracts.) Pischchevaja promyshlennost', Moskva. Shaftan, E. A., Mikhailova, N. S., Pekhov, A. V. and Dyubankova, N. F., 1986, Sravnitelnoe izuchenie khladonovogo i uglekislotnogo ekstraktov iz vozdushnosukhogo zhoma plodov Hippophag rhamnoides L. kavkazskogo i sibirskogo proiskhozhdenia. (Comparative study of khladon and carbon dioxide extracts from air-dry pulp of fruit of Hippophai; rhamnoides L. of Caucasian and Siberian origin.) Rastiteln)je resursy 22, 6(~66. Sovov~, H., 1984, Rate of the vegetable oil extraction with supercritical CO 2 I. Modelling of extraction curves. Chem. Engng Sci. 4 9 , 409M14. Sovova, H., Ku6era, J. and Jet, J., 1994, Rate of the vegetable oil extraction with supercritical COz-ll. Extraction of grape oil. Chem. Enyng Sci. 4 9 , 415~20. gt'astovS. J., 1994, Rakytnik ~e~etl~tkov~, (Sea buckthorn). Report 7/94, Inst. of Chem. Process Fundam., Prague. Zhamyansan, Ja., 1978, Issledovania masel semyan i myagkoti plodov Hippopha~ rhamnoides (Investigation of the oils from seed and pulp of HippophaO rhamnoides). Khimia prirodnykh soedinen(j l, 133 134. APPENDIX Mass balance On the assumptions that a plug flow of the solvent exists in the fixed bed of solid particles, axial dispersion is negligible, and the solute accumulation in the solvent can also be neglected, the material balances for an element of volume are given by ~x = J(x, y) cJt
- p~(1 - - ~ . ) : -
p~(l
O~y
c)~
Subscripts f solvent phase i = 1, 2 simulated halves of the bed k b o u n d a r y between the first a n d second period 0 initial conditions s solid phase
(A2)
If the solvent is solute-free at the entrance of the extractor and if all particles have the same initial solute content x0. the boundary conditions are
x(h, t = O) = Xo,
y(h = 0, t) = 0
(A3)
and the mass of solute extracted from the fixed bed equals to y(h = 1, t)dt.
E= 0
Super script + at interfacial b o u n d a r y
= J(x, v).
(A1)
(A4)
0
Rate of mass transfer As the plant tissue is torn during the grinding, part of the solute is released. Concentration of this easily accessible solute in the solid phase is Gxo at the beginning of extraction. It is extracted in the first period of extraction with a rate controlled by its diffusion and convection in the solvent:
J(x,y)=kfapf()'r--y ) f o r x > ( l - - G ) xo.
(A5)
The second period of extraction starts when the easily accessible solute has been removed. The rate of extraction
4352
J. STASTOVA et al.
depends now on the diffusion of solute from the interior of the plant tissue to the surface. Instead of taking into account the complex nature of the vegetable matrix, we apply a simplified formula
J(x,y)=ksaps(x-x
+) f o r x < ~ ( 1 - G ) x o
Solution Equations (A 1)-(A6) can be integrated numerically to obtain the concentration profiles and the mass of extract. However, an approximate analytical solution exists (SovovL 1994) which can be applied on conditio~ that k~ ,~ k/, so that it holds in the second period y ,~ y,, x - x ÷ - x and eq. (A6) can be arranged as follows: for x ~<(1 - G) Xo.
~PR = ~ +
In {1 -- G[1 -- exp(Y)]}.
(A10)
(A6)
with one solid-phase mass transfer coefficient ks.
J = ksap~x(l - y/y,)
Two regions exist inside the bed in the interval of dimensionless time • from G/Z to ~uk:
(A6a)
While the solute from the interior of particles is already being extracted in the region closer to the solvent entrance, the extraction from particle surface is still taking place closer to the solvent outlet. The dimensionless coordinate of the division between the regions is hk,
hk=lln[l+{exp[Y@Y-G)l-l}/G ] G for ~ ~< ~ < KPk .
(A11)
With the dimensionless time qJ introduced,
tp = t" QyJ(Nxo)
(A7)
the solid-phase concentration profile calculated eqs (A1-A5) and (A6a) equals to 1 - Ztp exp ( - Zh)
from
The quantities Z, Y are proportional to the mass transfer coefficients,
Z
Nklap~
G for W < Z
Nksaxo
(A 12)
0(1 - e)y,
and the solvent-phase mass transfer coefficient k / i s assumed to be dependent on the solvent flow rate according to the relation k/~v TM ~ 0 TM .
1 - G e x p [ - Z(h - hk)] for ~z~hk X
(A13)
Deviation from the solvent plu9 flow
XO
II+{expIY(w-G)~(I-G)-I}exp(-Yh'~
'
The flow of the solvent through the bed is often inhomegenous in the first period of extraction. This is simulated by dividing the bed into two parallel halves with equal solid feeds
G for q ~ > q ~ and f o r - ~ < q ~ < q ~ k , h<~hk Z
N1.2 = N/2 (A8)
and the mass of extract is • [1 - exp( - Z)]
- Gexp[Z(hk - 1)] E E
Y
0(1 -- e)p:
for tP < -
and with the flow rates depending on the parameter of flow asymmetry ASM, 0 , . 2 = (Q/2)(1 + A S M ) .
G Z
G for - ~< te < qJk Z
(A15)
The solid-phase concentration and the a m o u n t of extract are calculated from eqs (A8) and (A9) for each half separately as functions of the quantities ~t/1,2 =
~
(A14)
(1 + ASM)W,
Y1.2 = Y/(1 + ASM),
Nxo
1 - l l n { 1 + [ e x p ( Y ) - 1]exp
Z1,2 = Z/(1 + ASM) °46 .
(AI6)
The mass of extract obtained from the whole bed is for ~ >i qJk(A9)
E = NXo 0.5
..-7"2-{~iZi, Yi, hki, G) . '
i=1Nixo
(A17)
Chemical Engineering; Science, Vol, 51, No. 18, pp. 4347 4352, 1996
PII: S0009-2509(96)00263-17
Copyright i ; 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved
ooo9 2509"96$15.00+ 0.00
RATE OF THE VEGETABLE OIL EXTRACTION WITH SUPERCRITICAL CO2--III. EXTRACTION FROM SEA BUCKTHORN J. Sq'ASTOVA, J. JEZ, M. BARTLOVA and H. SOVOVA* Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, 165 02 Prague 6, Czech Republic (First received 9 August 1995; accepted 19 December 1995)
Al~tract--Oil from the seed and pulp of sea buckthorn berries (Hippopha~ rhamnoides L.) was extracted with carbon dioxide at pressures 9.(~27 MPa and temperatures 25-60°C. Influenceof extraction conditions on solubility and mass transfer rate was studied. No marked changes in composition of extracted oil in the course of extraction were observed. Experimental extraction curves were evaluated using a model with grinding efficiency,volume mass transfer coefficients in solid and fluid phases, and parameter of flow asymmetry as adjustable parameters. The solid-phase mass transfer coefficientwas increasing with rising temperature. Copyright © 1996 Elsevier Science Ltd INTRODUCTION Sea buckthorn is a bush that grows wildly in the mountain regions of the middle and southeastern Asia and Europe, as well as in Scandinavia and on the west European coast. Its fruit are berries of orange to red colour and have an acid, lightly bitter taste. They contain many vitamins (B, C, E, K, provitamin A) and other biologically active substances. The main products obtained from the fruit are juice rich in vitamin C and oil rich in unsaturated fatty acids. The sea buckthorn oil is used in medicine (Hlava and Vali~ek, 1989; S{astovfi, 1994). It finds application in healing ointments applied to wounds and burns, in medicines for treating cancer, in vitamin preparations. It is also uscd in cosmctics e.g. as natural plasticizer and emulsifier, and in skin-regenerating compositions and UV filters. Content of oil in fruit varies from 2 to 17 wt% on dry basis. It is interesting that the oil occurs not only in seeds but also in the pulp of the fruit. While the seed oil contains mainly fatty acids with 18 carbon atoms in molecule (linoleic and linolenic acids represent up to 60%), palmitic and palmitooleic acids with 16 carbon atoms in molecule predominate in the oil from the pulp (Zhamyansan, 1978). The sea buckthorn oil is extracted from the fruit after separation of juice and after drying. Three types of solvents are used: organic solvents, which are later separated from the oil by distillation, or pure vegetable oils--usually sunflower oil, which remain in the product (Bukshtynov et at., 1978; Golubyev et al., 1990), or dense gases like carbon dioxide (Shaftan et al., 1986). The advantage of the last process is that the product is natural, free from the solvent and without any products of thermal degradation, because the extraction with dense gases can be carried out at relatively low temperatures. *Corresponding author.
In Russia, sea buckthorn oil was extracted with liquid carbon dioxide both in the laboratory apparatus and in the large-scale equipment described by Kasyanov et al. (1978). Berries of plants growing wild in Siberia and Caucasus were used. Air-dry pulp and seed of the berries in mass ratio approximately 45:55 were first ground to particles of 0.3~0.4 mm size and then flaked to a thickness of 0.12~0.20mm. Countercurrent extraction at 20°C and 5.7 MPa was performed for 3-3.5 h. Yield of oil was 4-8wt% according to the composition of the plant material (Shaftan et al., 1986). In Germany, the firm FLAVEX extracted sea buckthorn oil from Lithuanian plant material with supercritical carbon dioxide. Extraction conditions were close to 35 MPa and 40°C, the yield was 16.5 wt%, the extract contained 29% palmitooleic acid (Flavex, 1992). The aim of this study is to investigate the effect of extraction conditions on the rate of supercritical fluid extraction from buckthorn seed and pulp. EXPERIMENTAL Material. Three varieties of sea buckthorn berries cultivated in Czech Republic were obtained from the University of Agriculture in Prague. Most of the juice was separated from the defrosted berries by pressing. The pressed cake was then air-dried, the seed was mechanically separated from the pulp, and the material was ground. Moisture levels 4-5 wt% in dry pulp and 8-9wt% in dry seed were determined gravimetrically; density of the solid phase was measured using a pycnometer. The values are listed in Table 1 together with void fraction of lightly pressed bed of ground material, and with oil content determined by Soxhlet extraction with n-hexane. Purity of carbon dioxide supplied by Technoplyn Litvinov was better than 99.9 wt%.
4347
J. STASTOVAe t al.
4348 Table 1. Properties of ground material Variety
Ps (kg/m~) e
solvent flow distributors. Feed of the plant material ranged from 3 to 40 g. Extraction pressure was 27 MPa in most experiments, extraction temperature was adjusted to 25, 40 or 60°C. Additional experiments were conducted with the pressure decreased to 17.4 or 9.6 MPa at 40°C. Flow rate of carbon dioxide was adjusted to 1 1/min (measured at atmospheric pressure). Solubility of sea buckthorn oil in dense carbon dioxide was measured by the dynamic method using the same apparatus. Glass wool in the 12 ml extractor was thoroughly wetted by 1-2ml of oil extracted previously from the seed or pulp. Flow rate of the solvent was adjusted to 0.1-0.2 1/min. It was checked by preliminary experiments that the extraction rate was independent of the solvent flow rate at these conditions. The extracts were analysed by HPLC. The apparatus was a Hewlett Packard 1090M chromatograph equipped with diode array UV-VIS detector and a column 25 cm x 4 mm i.d. packed with Nucleosil C 18, 5 lam. The conditions were: mobile phase methanol, flow rate 0.4 ml/min, detection at wavelength 220 nm.
x0 (mg oil/g material)
Seed A B C
1200 1270 1240
0.347 0.334 0.364
8.1 8.0 13.4
Pulp A B C
1190 1250 1130
0.520 0.522 0.460
12.0 9.7 11.0
Extraction experiments were performed using the semicontinuous extraction apparatus described previously (Sovovfi et al., 1994). Carbon dioxide from a pressure cylinder was charged to a compressor and controlling valve maintaining the extraction pressure within + 0.05 MPa. It then passed a preheating coil immersed together with the extractor in a water bath with temperature controlled within +_ 0.1°C, and entered the top of the extractor packed with the extracted material. Upon leaving the extractor, the supercritical CO2 stream containing the dissolved oil passed through a heated micrometering valve that allowed its expansion to ambient pressure. The extracted oil was separated from the CO2 and was collected in a glass U-tube. The depressurized gas flowed through the separator to flow totalizer before being vented. U-tubes were exchanged in the course of the extraction and the increase of extract amount was determined gravimetrically. Total number of extraction experiments was 35. Two extractors were used alternatively: 12 ml extractor 8 mm i.d., and 150 ml extractor 33 mm i.d. The extractor was packed with the ground seed or pulp placed between two layers of glass beads serving as
RESULTS
The extracts obtained from seed and pulp differed in colour and consistency. Seed oil was a yellow or light orange viscous liquid, while the pulp oil was of pasty consistency and dark red colour, which liquidized at approximately 50°C. Chromatographic peaks of extract sample (see Fig. 1) represent groups of chemically similar substances as free fatty acids, mono-, di- and triglycerides. There were no systematic differences observed either between the chromatograms
o6 100
80
60 < ¢q
4C
20
I 2
I 4
I 6
I 8
10
12
14
I 16
--
Time (min) Fig. 1. HPLC chromatogram of buckthorn oil extract (mAU = milliampereunits). Identified peaks: (1) free fatty acids, (2) triglycerides.
Rate of the vegetable oil extraction
4349
Table 2. Oil solubility, mg oil/g COz: comparison with the correlation of del Valle and Aguilera (1988) Extraction conditions: pressure (MPa)/temperature (~C) Variety
27/25
27/40
27/60
17.4/40
A B C A B C
6.5 7.1 6.9 7.7 8.2 8.2
6.5 7.4 6.8 7.9 8.6 8.4
5.8 6.8 6.6 7.2 7.9 8.2
3.0 4.0
5.8±2.8
6.2±3.0
6.2±3.3
2.5±3.3
Seed
Pulp
Correlation
of extracts obtained from the individual varieties, or between the samples of extracts collected in the course of extraction. The main extract components, triglycerides, represent 76-92% of the total peak area, while the free fatty acids only 1~1%. There were differences in CO2 solubility of oil extracted from the individual sea buckthorn varieties, as is shown in Table 2. Pulp oil was always more soluble than seed oil, most probably due to its shorter molecules. Almost all solubility values were higher than those calculated as mean values from the correlation derived by del Valle and Aguilera (1988) for the solubility of vegetable oils in supercritical CO2. However, all experimental solubilities remain inside the wide confidence limits of the correlation. Extraction model with four adjustable parameters, which is described in the appendix, was fitted to experimental extraction curves. This version of the model differs from that derived in the first paper of the series (Sovovfi, 1994) by several simplifications. Namely, parameter of grinding efficiency G has been introduced as well as dimensionless time ~,; solvent flow inhomogeneity is simulated by dividing the bed of extracted material into parallel halves, instead of the six parallel sections used previously (Sovovfi et al., 1994). Examples of the evaluation of experimental results are shown in Figs 2-5, where the symbols
0.3
0.2
0.1
3.8 4.3
/f ~/o
8'0
t2o
16o
Qt, g
Fig. 3. Effect of extraction temperature on the course of extraction. 12ml extractor, feed 5.1 g seed of variety A, solvent flow rate 1.75gCO2/min, grade of grinding 3, extraction pressure 27 MPa. Extraction temperature: (11) 25c'C, ( + ) 60°C.
0.06
0.04
z
m 0.02
0.3-
0
10
20
30
40
GUN Fig. 4. Effect of pressure on the course of extraction. 12 ml extractor, seed of variety A, grade of grinding 3, extraction temperature 40°C. Extraction pressure: (11) 27 MPa, ( + ) 17.4 MPa, (Fq) 9.6 MPa.
0.1
0
40
8'0
1½0
160
Qt, g Fig. 2. Effect of grade of grinding on the extraction rate. 12 ml extractor, feed 5 g seed of variety A, solvent flow rate 1.78 g CO2/min, extraction conditions 27 MPa/40°C. Grade of grinding: (11) 1, ( + ) 2, ([3) 3.
represent experimental points and the curves are calculated using the model. Model parameters resulting from the fitting procedure based on the least-squares method are listed in Tables 3-6. Confidence limits of the regressed parameters shown in Tables 4-6 were evaluated from repeated experiments. To compare the
J. S1"ASTOVAet al.
4350 0.09j f 0.06
0.03
i0
20 Qt/N
30
4o
Fig 5. Comparison of extraction from seed and pulp. 12 ml extractor, variety A, grade of grinding 3, extraction conditions 27 MPa/40°C. Material: (m) seed, ( + ) pulp.
Table 3. Effect of grade of grinding on model parameters: extraction from seed of variety A, 12ml extractor, 27 MPa/40°C Grade 1 2 3
G 0.38 0.60 0.67
ASM 0.55 0.33 0.08
kya (min- 1) k~a (min- 1) 0.34 0.50 0.56
0.00037 0.00055 0.00060
Table 4. Grinding efficiency G at grade of grinding 3 Variety
A
Seed Pulp
B
0.67 + 0.02 0.65 ± 0.04 0.68 + 0.03 0.74 + 0.04
C 0.68 _+ 0.02 0.69 + 0.02
Table 5. Volume mass transfer coefficient in solvent phase, ksa, min-1: pressure 27 MPa, grade of grinding 3. (Recalculated to the solvent interstitial velocity v = 1 mm/s according to ksa.~v°'54.) Temperature (°C) Seed Pulp
25
40
60
0.40 -I- 0.04 0.40 ± 0.03 0.32 _ 0.03 0.64 + 0.06 0.72 ___0.07 0.53 + 0.05
solvent-phase mass transfer coefficients measured at different interstitial velocities, their values were recalculated to one reference velocity using eq. (A.13) from the appendix.
Grinding efficiency, G, and solvent flow asymmetry, ASM: parameter of grinding efficiency G is closely related to the grade of grinding. Drop size distribution
of the particles of ground material was bimodal with one peak close to 0.5 mm and the other peak close to 0.01 mm. Grinding at grade 1 produced a mixture of 2 8 w t % small particles and 72wt% large particles, while the portion of small particles in the material obtained by grinding at grade 3 was 50 wt%. Small particles (0.01 mm) stuck together forming larger conglomerates. These conglomerates disintegrated, after the oil had been extracted from the surface of the particles. Table 3 shows the increase of grinding efficiency and of both volume mass transfer coefficients, which are proportional to the surface area, with the grade of grinding. Parameter of flow asymmetry, ASM, diminishes gradually with increasing grade of grinding. As it was confirmed in the previous work (Sovovfi et al., 1994) that carbon dioxide flows down through the bed of extracted particles practically in plug flow, with ASM = 0, the reason for the high values of ASM evaluated from the experiments with roughly ground sea buckthorn must be different. Probably, it is the bimodal particle size distribution, which deforms the measured extraction curve in a similar way, as flow asymmetry does in the monodisperse extraction model we have used. Most of the experiments were carried out at the grade of grinding equal to 3. As it is seen from Table 4, the parameter of grinding efficiency at constant grade of grinding had a similar value for all varieties. Its value for pulp was somewhat higher than for seed, because the pulp was drier and therefore more brittle. Volume mass transfer coefficient in the solvent, k fa, is given in Table 5. It has been recalculated to the interstitial velocity of carbon dioxide 1 mm/s in order to join together the results obtained from experiments carried out using extractors of various diameters. The interstitial velocity in the 12 ml extractor ranged from 1.8 to 2.1 mm/s for the seed, and from 1.2 to 1.4 mm/s for the pulp; in the 150 ml extractor it was 17 times less.
Volume mass transfer coefficient in the solid phase, ksa, which controls the second period of extraction, was by three orders of magnitude smaller than the coefficient in the solvent. It was increasing substantially with rise in temperature (see Table 6). CONCLUSIONS Extraction with dense carbon dioxide is an appropriate method for the separation of oil from sea buckthorn fruit. With the downflow of CO2, two periods of extraction are sharply distinguished. The a m o u n t of oil extracted in the first, fast extraction period depends on the grade of grinding. Rate of extraction in the second period is by several orders of magnitude
Table 6. Volume mass transfer coefficient in solid phase, k~a, min- 1: grade of grinding 3 Temperature (°C) Seed Pulp
25
40
60
0.0003 ___0.0001 0.0006 _+0.0002
0.0006 + 0.0001 0.0012 _+0.0002
0.0014 + 0.0003 0.0018 ± 0.0002
4351
Rate of the vegetable oil extraction smaller a n d rises with increasing temperature. The studied varieties of sea b u c k t h o r n varied n o t only in the oil content, but also in its composition, because certain differences in the oil solubility in c a r b o n dioxide were found (less t h a n 14% b o t h for pulp a n d seed oil). Solubility of the pulp oil was by 19% higher t h a n that of the seed oil, o n average. A new version of the previously derived extraction model could be used to describe the b u c k t h o r n extraction curves a n d to evaluate grinding efficiency, mass transfer coefficients, a n d flow asymmetry, which are listed in Tables 3-6. The mass transfer coefficient in the solvent was approximately p r o p o r t i o n a l to the square root of solvent interstitial velocity. The mass transfer coefficient in the solid phase was t e m p e r a t u r e dependent. The value of the last parameter, ASM, was r a t h e r a measure of size i n h o m o g e n e i t y of the extracted particles, t h a n a measure of deviation of the solvent flow t h r o u g h the bed of particles from plug flow, as it is defined in the model.
Acknowledgement- The authors acknowledge the financial support from the Grant Agency of the Czech Republic. The work has been carried under Grant No. 510/93/2283.
a
ASM E G h J k I N
Q t U t~ X
Y Yr
Y Z
NOTATION specific interfacial area, 1parameter of flow asymmetry (0 ~< A S M < 1) mass of extract, M grinding efficiency (0 ~< G ~< 1) dimensionless axial co-ordinate (0 ~< h ~< 1) mass transfer rate, M 1 3 T mass transfer coefficient, 1T axial co-ordinate, 1 solid feed, M mass flow rate of solvent, M T - 1 time, T superficial solvent velocity, I T interstitial solvent velocity, I T solid-phase concentration, M M - 1 solvent-phase concentration, M M solubility, M M p a r a m e t e r of the second extraction period defined by eq. (A12) p a r a m e t e r of the first extraction period defined by eq. (AI2)
Greek letters ~void fraction p density, M1 3 dimensionless time defined by eq. (A7)
REFERENCES
Bukshtynov, A. D. et al., 1978, Oblepikha (sea buckthorn), pp. 173-181. Lesnaja promyshlennost', Moskva. del Valle, J. M. and Aguilera, J. M., 1988, An improved equation for predicting the solubility of vegetable oils in supercritical CO 2. Ind. Engng Chem. Res. 27, 1551 1553. Flavex, 1992, CO2-extracts. Prospectus of FLAVEX Naturextrakte, Rehlingen. Golubyev, V. N., Kolesnik, A. A. and Ismailov, T. K., 1990, Kompleksnaja pererabotka oblepikhi s pomoshchyu membran. (Complex processing of sea buckthorn using membranes.) Pishtshevaja promyshlennost' (11), 32--35. Hlava B. and Vali6ek P., 1989, Rostlinnb harmonizhtory (Vegetable harmonizers) University of Agriculture, Praha. Kasyanov, G. 1., Pekhov, A. V. and Taran, A. A., 1978, Naturarnye pishchevye aromatizatory CO 2 ekstrakty. (Natural Food Aromatizers CO 2 Extracts.) Pischchevaja promyshlennost', Moskva. Shaftan, E. A., Mikhailova, N. S., Pekhov, A. V. and Dyubankova, N. F., 1986, Sravnitelnoe izuchenie khladonovogo i uglekislotnogo ekstraktov iz vozdushnosukhogo zhoma plodov Hippophag rhamnoides L. kavkazskogo i sibirskogo proiskhozhdenia. (Comparative study of khladon and carbon dioxide extracts from air-dry pulp of fruit of Hippophai; rhamnoides L. of Caucasian and Siberian origin.) Rastiteln)je resursy 22, 6(~66. Sovov~, H., 1984, Rate of the vegetable oil extraction with supercritical CO 2 I. Modelling of extraction curves. Chem. Engng Sci. 4 9 , 409M14. Sovova, H., Ku6era, J. and Jet, J., 1994, Rate of the vegetable oil extraction with supercritical COz-ll. Extraction of grape oil. Chem. Enyng Sci. 4 9 , 415~20. gt'astovS. J., 1994, Rakytnik ~e~etl~tkov~, (Sea buckthorn). Report 7/94, Inst. of Chem. Process Fundam., Prague. Zhamyansan, Ja., 1978, Issledovania masel semyan i myagkoti plodov Hippopha~ rhamnoides (Investigation of the oils from seed and pulp of HippophaO rhamnoides). Khimia prirodnykh soedinen(j l, 133 134. APPENDIX Mass balance On the assumptions that a plug flow of the solvent exists in the fixed bed of solid particles, axial dispersion is negligible, and the solute accumulation in the solvent can also be neglected, the material balances for an element of volume are given by ~x = J(x, y) cJt
- p~(1 - - ~ . ) : -
p~(l
O~y
c)~
Subscripts f solvent phase i = 1, 2 simulated halves of the bed k b o u n d a r y between the first a n d second period 0 initial conditions s solid phase
(A2)
If the solvent is solute-free at the entrance of the extractor and if all particles have the same initial solute content x0. the boundary conditions are
x(h, t = O) = Xo,
y(h = 0, t) = 0
(A3)
and the mass of solute extracted from the fixed bed equals to y(h = 1, t)dt.
E= 0
Super script + at interfacial b o u n d a r y
= J(x, v).
(A1)
(A4)
0
Rate of mass transfer As the plant tissue is torn during the grinding, part of the solute is released. Concentration of this easily accessible solute in the solid phase is Gxo at the beginning of extraction. It is extracted in the first period of extraction with a rate controlled by its diffusion and convection in the solvent:
J(x,y)=kfapf()'r--y ) f o r x > ( l - - G ) xo.
(A5)
The second period of extraction starts when the easily accessible solute has been removed. The rate of extraction
4352
J. STASTOVA et al.
depends now on the diffusion of solute from the interior of the plant tissue to the surface. Instead of taking into account the complex nature of the vegetable matrix, we apply a simplified formula
J(x,y)=ksaps(x-x
+) f o r x < ~ ( 1 - G ) x o
Solution Equations (A 1)-(A6) can be integrated numerically to obtain the concentration profiles and the mass of extract. However, an approximate analytical solution exists (SovovL 1994) which can be applied on conditio~ that k~ ,~ k/, so that it holds in the second period y ,~ y,, x - x ÷ - x and eq. (A6) can be arranged as follows: for x ~<(1 - G) Xo.
~PR = ~ +
In {1 -- G[1 -- exp(Y)]}.
(A10)
(A6)
with one solid-phase mass transfer coefficient ks.
J = ksap~x(l - y/y,)
Two regions exist inside the bed in the interval of dimensionless time • from G/Z to ~uk:
(A6a)
While the solute from the interior of particles is already being extracted in the region closer to the solvent entrance, the extraction from particle surface is still taking place closer to the solvent outlet. The dimensionless coordinate of the division between the regions is hk,
hk=lln[l+{exp[Y@Y-G)l-l}/G ] G for ~ ~< ~ < KPk .
(A11)
With the dimensionless time qJ introduced,
tp = t" QyJ(Nxo)
(A7)
the solid-phase concentration profile calculated eqs (A1-A5) and (A6a) equals to 1 - Ztp exp ( - Zh)
from
The quantities Z, Y are proportional to the mass transfer coefficients,
Z
Nklap~
G for W < Z
Nksaxo
(A 12)
0(1 - e)y,
and the solvent-phase mass transfer coefficient k / i s assumed to be dependent on the solvent flow rate according to the relation k/~v TM ~ 0 TM .
1 - G e x p [ - Z(h - hk)] for ~z~
(A13)
Deviation from the solvent plu9 flow
XO
II+{expIY(w-G)~(I-G)-I}exp(-Yh'~
'
The flow of the solvent through the bed is often inhomegenous in the first period of extraction. This is simulated by dividing the bed into two parallel halves with equal solid feeds
G for q ~ > q ~ and f o r - ~ < q ~ < q ~ k , h<~hk Z
N1.2 = N/2 (A8)
and the mass of extract is • [1 - exp( - Z)]
- Gexp[Z(hk - 1)] E E
Y
0(1 -- e)p:
for tP < -
and with the flow rates depending on the parameter of flow asymmetry ASM, 0 , . 2 = (Q/2)(1 + A S M ) .
G Z
G for - ~< te < qJk Z
(A15)
The solid-phase concentration and the a m o u n t of extract are calculated from eqs (A8) and (A9) for each half separately as functions of the quantities ~t/1,2 =
~
(A14)
(1 + ASM)W,
Y1.2 = Y/(1 + ASM),
Nxo
1 - l l n { 1 + [ e x p ( Y ) - 1]exp
Z1,2 = Z/(1 + ASM) °46 .
(AI6)
The mass of extract obtained from the whole bed is for ~ >i qJk(A9)
E = NXo 0.5
..-7"2-{~iZi, Yi, hki, G) . '
i=1Nixo
(A17)