Ind. Eng. Chem. Res. 2000, 39, 473-477
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Extraction and Modeling of Lavender Flower Essential Oil Using Supercritical Carbon Dioxide Mesut Akgu 1 n, Nalan A. Akgu 1 n, and Salih Dinc¸ er* Chemical Engineering Department, Yildiz Technical University, Sisli, 80270 Istanbul, Turkey
Essential oil was extracted from lavender flowers (Lavandula Stoechas subspecies Cariensis Boiss) in a semicontinuous system by supercritical CO2. Experiments were carried out in the ranges of 8-14 MPa pressures, 308-323 K temperatures, and 1.092-2.184 × 10-3 kg/min carbon dioxide flow rates. The extraction process was modeled by a quasi-steady-state model as a function of extraction time, flow rate, pressure, and temperature. The model contains only one adjustable parameter, the intraparticle diffusion coefficient (effective diffusivity) De. The model using the best fit of De correlates the data satisfactorily. Introduction Supercritical fluid extraction (SCE) has been demonstrated as an alternative to conventional separation processes in the production of essential oils for a long time. It has been widely accepted by many investigators that SCE provides a rapid and quantitative method for extracting essential oils from aromatic plants that compares favorably with steam distillation.1-5 Analysis of the SCE products and assessment of their composition showed that the extracts had higher yield, finer odor, and longer shelf-life.1-3 Moreover, because supercritical fluids have liquidlike density and gaslike viscosity and diffusivity, they have high mass-transfer characteristics and their effectiveness can be controlled by small changes in temperature and pressure.6 The majority of experiments in the literature on supercritical extraction of natural products are complicated by several parameters such as the properties of plants (e.g., particle size, moisture content, polarity of the essential oil components, and molecular weight) and operating conditions (e.g., pressure, temperature, extraction time, raw material/solvent ratio, entrainer, etc.). However, some or all of these parameters can affect the extraction rate and the composition of the extracts. This influence can be explained by different diffusion times due to structural differences of various kinds of aromatic plants. Goto et al.7 extracted essential oils from mint, ginger, and tomato seeds, and they noted that the extraction behavior of natural materials cannot be represented by a single mechanism and illustrated that the extraction behavior for seeds, flowers, and leaves must be explained by different mechanisms. Reverchon and Poletto,8 Catchpole et al.,9 and Roy et al.10 pointed out that the intraparticle diffusion is dominant in the extraction process when extracting sage, celery, and ginger, which have woody structure. Lavender flowers are aromatic plants with three main species (true lavender, spike lavender, and lavandin) and numerous subspecies. Although the essential oils of the true lavender, lavandin, and their subspecies contain up to 65% linalool and linalyl acetate, which are used widely in perfumery,11 the essential oil of the * To whom correspondence should be addressed. Telephone: +90(212) 224 49 68. Fax: +90(212) 224 49 68. E-mail: dincer@ yildiz.edu.tr.
spike species contains over 70% camphor and fenchone, which are used as pharmaceutical products.12,13 Adasoglu et al.14 optimized the SCE of spike lavender (Lavandula Stoechas subspecies Cariensis Boiss) by response surface methodology. True lavender essential oil was extracted using supercritical CO2 as the solvent, and the chromatographic analysis of the oil extracted was studied by Reverchon et al.11 In this work, the extraction of essential oil from lavender flowers by supercritical CO2 in a semicontinuous system was studied. The extraction rate of essential oil was investigated as a function of temperature, pressure, and CO2 flow rate. The extraction process was modeled for various experimental conditions by applying mass balance in the packed-bed extractor.15 Mathematical Model Extraction of a solute from the solid matrix occurs in three stages of diffusion of fluid to particle pores, dissolution of extractable matter in the fluid, and transfer to the bulk fluid. Therefore, several models have been developed based on empirical kinetic models or differential mass balances for the fixed bed. Poletto and Reverchon16 simulated the extraction of seeds and flowers by differential mass balance, equilibrium, and lumped parameter models. Goto et al.17 adapted the shrinking-core model, which has been used in solidfluid reactions, adsorption, and ion exchange, to SCE of natural materials. Roy et al.10 modeled the SCE of ginger oil successfully by the shrinking-core model. In this work, the shrinking-core model was applied to the extraction of lavender flowers. Actually, this model is called as quasi-steady-state model because of the assumption of no axial dispersion in the fixed bed.17 Moreover, the following assumptions are also made during model solution: (1) the extraction is an irreversible desorption process, (2) the matrix is a porous material where lavender oil is uniformly distributed throughout the particle, (3) the system is isothermal, (4) the physical properties of the fluid are constant during the extraction. Based on these assumptions, the material balance in the extractor is described as17
∂C ∂C 1 - 3kf +ν )[C - Ci(R)] ∂t ∂z R
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(1)
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Average solid-phase oil concentration (q j ) variation with time is equated to the rate of mass transfer of the solute within the external film surrounding the particle:
∂q j 3kf ) [C - Ci(R)] ∂t R
(2)
The diffusion to the outer region in the particle is expressed by
( )
De ∂ 2 ∂Ci r )0 ∂r r2 ∂r
(3)
The average solid-phase oil concentration is described as a function of the particle diameter:
()
rc q j ) q0 R
3
(4)
The initial and boundary conditions are given as follows:
(
rc ) R
at t ) 0
(5)
C)0
at t ) 0
(6)
C)0
at z ) 0
(7)
∂C/∂z ) 0
at z ) L
(8)
Ci ) Csat
at r ) rc
(9)
) kf[C - Ci(R)]
(10)
De
)
∂Ci ∂r
r)R
The equations and the initial and boundary conditions can be expressed in terms of the following dimensionless variables:
X ) C/Csat; Xi ) Ci/Csat; ξ ) r/R; Z ) z/L; j /q0; a ) νR2/DeL; θ ) (De/R2)t; yj ) q b ) Csat/q0; Bi ) kfR/De The numerical integration of eqs 1-4, written in terms of dimensionless variables, was accomplished by using MATHCAD (MathSoft, Inc., MathcadPlus6.0). The yield (cumulative amount of extract up to time θ) is given as
yield )
ab 1-
∫0θX dθ
(11)
Experimental Section Material. The spike lavender flower (L. Stoechas subspecies C. Boiss), growing wild in nature and wellknown as Gargan in the Aydin-C¸ ine region in Turkey, was used in this work. These flowers were harvested in May, dried in shade, frozen in liquid nitrogen, and then crushed manually to the desired size. The particle size was about 1.2 mm. The initial oil content was 1.51 wt %. A total of 10 g of the sample was used in each experimental run. The essential oil components were identified by GC as β-pinene, R-pinene, camphene, DL-limonene, fenchone, cis- and trans-linalool oxide, camphor, linalool, linalyl acetate, R-fenchyl acetate, caryophyllene oxide, R-cadinol, tert-sobrelol, etc. The major components were camphor (43.74%) and fenchone (33.14%).
Figure 1. Experimental setup: (1) CO2 cylinder; (2) syringe pump; (3) extractor; (4) pressure gauge; (5) expansion valve; (6) collection bottle; (7) wet-test meter; (8) heating bath.
Experimental Setup and Procedure. Experiments were performed in a semicontinuous extraction system schematically shown in Figure 1. The experimental apparatus consists of a tubular extractor (50 cm length × 1 cm i.d.; internal volume 38 mL), which is placed into a constant-temperature bath, and a syringe pump (ISCO, model 260 D). Carbon dioxide was compressed into the extractor by the syringe pump, after the extractor temperature reached the desired value. The flow rates of CO2 were maintained at 1.2, 1.7, and 2.5 mL/min at pump conditions. These flow rates were converted to the interstitial velocities at the column conditions by using the appropriate densities and the bed dimensions. The temperature was controlled by an immersion circulator (Lauda GMBH & Co., model Lauda M/2; controlled to (0.1 °C accuracy). CO2containing essential oil was passed through the collection bottle containing methanol placed into the ice bath. The amount of essential oil trapped in the methanol was determined by GC for every 30 min. Analytical Procedure. The Unicam model 610 GC was used for the analysis of essential oil extracted. The separation was achieved by using a capillary column (EC-WAX Carbowax, 30 m × 0.32 mm i.d., film thickness 0.25 µm). Argon was used as the carrier gas. GC was temperature-programmed as follows: from 60 °C, 1 °C/min to 63 °C, and then 20 °C/min to 210 °C. The concentration of essential oil components was computed from GC peak areas using calibration curves. Results and Discussion Experimental Results. Figure 2 shows the effect of CO2 flow rate, where yield is plotted versus extraction time and the amount of CO2 consumed in parts a and b, respectively. It is seen that the extraction rate is not affected significantly by the CO2 flow rate. The extraction rate increases linearly with time in the early stages of extraction. This kind of linearity indicates that the solute concentration in the fluid at the outlet of the extractor is constant, and hence in the first part of the extraction, the fluid leaving the system appears to be at equilibrium conditions. A similar explanation was given by Reverchon and Poletto,8 Goto et al.,7 and Roy et al.,18 when extracting various flowers, peppermint, and tomato seeds, respectively. However, the saturation solubility values measured by assuming the essential oil, containing over 75 wt % fenchone and camphor, consists of fenchone and camphor only were much higher than the solubilities calculated from the slopes of curves given in Figures 2b, 3, and 4, implying that the extraction process is not controlled by solubility but by intraparticle diffusivity. The effect of temperature on the extraction yield at 10 MPa is shown in Figure 3. The extraction rate increases because of the increase in the solute solubility where the effect of the vapor pressure increase overcame
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Figure 4. Effect of pressure on the extraction yield of lavender essential oil at 1.456 × 10-3 kg/min of CO2 flow rate and 323 K versus amount of CO2 consumed.
Figure 2. Effect of CO2 flow rate on the extraction yield of lavender essential oil (a) versus extraction time at 323 K and 10 MPa and (b) versus amount of CO2 consumed at 323 K and 10 MPa.
Figure 3. Effect of temperature on the extraction yield of lavender essential oil at 1.456 × 10-3 kg/min of CO2 flow rate and 10 MPa versus amount of CO2 consumed.
the effect of the solvent density decrease. Roy et al.10 also pointed out that the extraction rate increased with increasing temperature because of increasing solute vapor pressure in their studies on ginger oil. Figure 4 shows the effect of pressure on the extraction yield at 323 K. Because the essential oils have lower solubility in CO2 below the solute-solvent mixture critical pressure which is about 9.6 MPa at 323 K,19 the extraction rate at 8 MPa was lower than both of those at 12 and 14 MPa. At 12 and 14 MPa, however, the extraction rates were close to each other because of the miscibility of essential oil with the supercritical CO2. A similar explanation was given by Roy et al.,10 and they emphasized that solubility is controlled by a balance between solvent density and solute vapor pressure changes. Modeling Results. The physical properties of both the solvent and solute at the experimental conditions
are shown in Table 1. The density of CO2 was extrapolated from IUPAC20 tables. The viscosity of CO2 was calculated by the residual viscosity correlation of Jossi, Stiel, and Thodos.21 The binary diffusivity D12 of the lavender essential oil-CO2 system at the experimental conditions was estimated by the method of Akgerman et al.22 They measured the diffusion coefficients of naphthalene, phenanthrene, and hexachlorobenzene in supercritical CO2 using the Taylor dispersion technique and correlated their data. The same correlation was used here for the lavender flower essential oil components in supercritical CO2 which are organic and nonpolar in nature. The external mass-transfer coefficient, kf, was estimated from the Lee-Holder correlation.23 Lee and Holder23 measured the mass-transfer coefficient of naphthalene and toluene in a packed-bed column under supercritical conditions of CO2 by a chromatographic method and developed a generalized masstransfer correlation, which considers both the effects of natural and forced convection. The correlation is valid for 0.3 < Re < 135, which is in the range of our application. The bed void fraction, , was calculated as about 0.9 by compressing 10 g of lavender flower into a vise. The density of the solid matrix was 688 kg/m3. The initial average solid matrix essential oil concentration, q j 0, was calculated as about 10.42 kg/m3. Csat is determined as a total concentration of fenchone and camphor because the lavender essential oil in this study contains over 75 wt % fenchone and camphor. It is 0.243 kg/m3 at 12-14 MPa and 323 K and 0.0514 kg/m3 at 8 MPa and 323 K.19 The intraparticle diffusivity (effective diffusivity), De, is found by fitting model results to experimental results, and it is different for different kinds of plant material. For example, De is 2.5 × 10-10 m2/s for ginger oil10 and 5.1 × 10-12, 3.1 × 10-12, and 1.4 × 10-12 m2/s for coriander, sage, and celery, respectively.9 Reverchon et al.24 explained that these differences could be related to the different mass-transfer resistances because of different types of cell structure and mechanisms of solute extraction. Roy et al.10 pointed out that this could also be related to the different diffusion resistances due to the different solute nature such as molecular size, hydrophilic property, etc. In this work, the best fit was obtained as De ) 1.2 × 10-11 m2/s for the lavender flower used. The solid lines in Figures 2-4 show the quasi-steadystate model results. The model using the best fit of De correlates the experimental data satisfactorily. Although the initial stages of extraction show a linear relation-
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Table 1. Physical Parameters of SC-CO2 at Various Experimental Conditions pressure (MPa)
temperature (K)
Q × 103 (kg/min)
u × 104 (m/s)
ν × 104 (m/s)
F × 10-2 (kg/m3)
Re
D12 × 108 (m2/s)
µ × 104 (kg‚m/s)
kf × 104 (m/s)
80 100
323 308 313 323 323 323 323 323
1.456 1.456 1.456 1.456 1.092 2.184 1.456 1.456
12.6 4.16 4.94 7.4 5.24 10.9 5.8 4.7
14 4.62 5.50 8.22 5.82 12.1 6.44 5.22
2.22 7.10 6.15 4.08 4.08 4.08 5.25 6.42
1.52 0.61 0.75 1.13 0.80 1.66 0.89 0.70
3.084 1.407 1.594 2.167 2.167 2.167 1.824 1.565
2.210 5.792 4.848 3.208 3.210 3.210 4.095 5.160
20 6.81 8.44 10 13.1 13.5 10 8.03
120 140
ship, the slopes of the curves indicate that the extraction is not controlled by solubility but by intraparticle diffusivity. Roy et al.10 state that solutes in the outer parts of flower particles are extracted much faster than the solutes in the inner parts of flower particles at the beginning of extraction. This is attributed to masstransfer/diffusion limitations. As the extraction time increases, the diffusion of solutes from inner parts to the bulk phase are more difficult because the solid-phase oil concentration decreases; therefore, the extraction rate also decreases.
Xi: dimensionless concentration in pores yj: dimensionless average solid-phase concentration Z: dimensionless bed height variable z: bed height variable Greek Letters : bed void fraction θ: dimensionless time µ: viscosity, kg‚m/s ν: interstitial fluid velocity, m/s ξ: dimensionless radius F: density, kg/m3
Conclusions The essential oil of the lavender flower used was extracted using supercritical carbon dioxide to determine the effects of solvent flow rate, temperature, and pressure on the extraction rate. The experimental results show that the extraction rate is not affected significantly by the CO2 flow rate. Although the extraction rate increases linearly with time in the early stages of extraction, the extraction process is not limited by the solubility of solute in the supercritical CO2. Thus, intraparticle mass-transfer/diffusion limitations are dominant. The extraction rate increases with increasing temperature because of increasing vapor pressure of the components. The extraction rate increases with increasing pressure because of the solubility increase of essential oil components. Moreover, the extraction process was modeled using the quasi-steady-state model with one adjustable parameter, the intraparticle diffusion coefficient. The model using the best fit of De correlates the data satisfactorily. Nomenclature a: dimensionless interstitial velocity b: dimensionless parameter defined as Csat/qo Bi: biot number C: bulk fluid phase concentration, kg/m3 Ci: concentration in the particle pores, kg/m3 Csat: saturation concentration of solute in the fluid phase, kg/m3 D12: binary diffusion coefficient, m2/s De: intraparticle diffusion coefficient (effective diffusivity), m2/s kf: mass-transfer coefficient, m/s L: extractor length, m Q: flow rate, kg/min q j : average solid-phase oil concentration, kg/m3 qo: initial solid-phase oil concentration, kg/m3 R: particle radius, m Re: Reynolds number r: particle radius variable rc: critical particle diameter variable t: time u: superficial velocity, m/s X: dimensionless concentration in the bulk fluid phase
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Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 477 (15) Akgu¨n, M. Investigation of Thermodynamic and Transport Properties in Supercritical Fluid Extraction. Ph.D. Dissertation, Yildiz Technical University, Istanbul, Turkey, 1999. (16) Poletto, M.; Reverchon, E. Comparison of Models for Supercritical Fluid Extraction of Seed and Essential Oils in Relation to the Mass-Transfer Rate. Ind. Eng. Chem. Res. 1996, 35, 3680. (17) Goto, M.; Roy, B. C.; Hirose, T. Shrinking-Core Leaching Model for Supercritical Fluid Extraction. J. Supercrit. Fluids 1996, 9, 128. (18) Roy, B. C.; Goto, M.; Hirose, T.; Navaro, O.; Hortacsu, O. Extraction Rates of Oil from Tomato Seeds with Supercritical Carbon Dioxide. J. Chem. Eng. Jpn. 1994, 27 (6), 768. (19) Akgu¨n, M.; Akgu¨n, N. A.; Dinc¸ er, S. Phase Behavior of Essential Oil Components in Supercritical Carbon Dioxide. J. Supercrit. Fluids 1999, 15, 117. (20) IUPAC, International Thermodynamic Tables of the Fluid State Carbon Dioxide; Angus, S., Amstrong, B., de Reuck, K. M., Eds.; Pergamon Press: New York, 1976.
(21) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th ed.; McGraw-Hill: New York, 1987. (22) Akgerman, A.; Erkey, C.; Orejuela, M. Limiting Diffusion Coefficients of Heavy Molecular Weight Organic Contaminants in Supercritical Carbon Dioxide. Ind. Eng. Chem. Res. 1996, 35, 911. (23) Lee, C. H.; Holder, G. D. Use of Supercritical Fluid Chromatography for Obtaining Mass Transfer Coefficients in Fluid-Solid Systems at Supercritical Conditions. Ind. Eng. Chem. Res. 1995, 34, 906. (24) Reverchon, E.; Donsi, G.; Osseo, L. S. Modelling of Supercritical Fluid Extraction from Herbaceous Matrices. Ind. Eng. Chem. Res. 1993, 32, 2721.
Received for review July 1, 1999 Revised manuscript received November 9, 1999 Accepted November 11, 1999 IE9904798
