The goal for this section is to present a phenomenological description of the extrac- tion process of recovered components in SD. A description of the VO distillation process is used to present the steps that occur in the model. Although this general scheme is suitable for all single-stage processes, differential remarks are presented when applied to solid or liquid raw materials.
Oil recovery from the aromatic plant takes place in four sequential stages: (1) Promoted by temperature increase, oil is released from inside the plant to its outer surface; (2) Oil vaporizes, taking vaporization heat from the steam; (3) Vapor oil molecules at the raw material surface must diffuse into the steam stream in a mass transfer process; and (4) Vapor oil molecules carried along by the steam are con- densed and decanted. A simplifi ed scheme of this sequential staged process is shown in Figure 2.1.2. A description of these four stages is detailed next.
2.1.4.1 Oil Release
When a liquid product is steam distilled, the whole load is directly accessible by the steam, and volatile compounds are ready to be vaporized as soon as they reach their boiling temperature. This is the case with oil refi ning and deodorizing, and under these circumstances, the oil release stage must be omitted, and vaporization should be taken as the starting point.
TAF-62379-08-0606-C002.indd 14
In the case of solid materials, as it is in VO distillation, at least a portion of the recoverable components is not in contact with steam, and it must fl ow out of the solid before it can be vaporized. The mechanism by which this oil is released out of the plant depends on where it is located. Two main oil locations and release mechanisms are described in the literature.
2.1.4.1.1 Seeds, Fruits, or Roots
The solid shows an isotropic material behavior, with a uniform distribution of oil. Coriander seeds [8, 9] or aniseed grains [10] have been successfully described using this model, where diffusion inside the solid matrix is assumed.
2.1.4.1.2 Leaves or Flowers
Oil is deposited on the surface of the plant inside fragile glandular trichomes. In other oil extraction processes, such as supercritical CO2 extraction [11, 12] or micro- wave extraction [13], the disruption of all or a signifi cant part of the trichomes has been demonstrated. However, in SD, the integrity of the wall containing the oil inside the trichome has been verifi ed by SEM (scanning electron microscopy) [13–15], and an exudation model has been proposed in this case where the oil slowly permeates through membranes and cuticle [8, 14, 15].
Because the oil release stage is a slow transfer mechanism, it is usually the controlling stage in the fi nal part of the distillation, mainly in ground particles where diffusion inside the particle is the main resistance to oil recovery (see 2.1.4.1.1). This is the main reason why seeds and roots are usually crushed before distillation.
2.1.4.2 Vaporization
Vaporization occurs at the liquid–vapor interface. In this process molecules of com- ponents in the liquid phase move to the vapor phase, according to their volatilities.
Raw material 1) Oil release
Steam
2) Vaporization 3) Mass transfer
Distiller
Condenser
Oil
Water
Decanter
FIGURE 2.1.2 Schematic representation of extraction steps in steam distillation of essential oils.
TAF-62379-08-0606-C002.indd 15
The relation between compositions in both phases is regulated by the usual vapor– liquid equilibrium expression:
y Pi x f i i i o i = γ ˆφ , (2.1.1)
where P is the total or operation pressure, xi and yi are the molar fractions of each
component in the liquid and vapor phases, respectively, γi is the activity coeffi cient of component i in the liquid phase, fi
o
the standard state fugacity of pure component i, and ˆφi
V
the fugacity coeffi cient of component i in the vapor phase. These terms may be simplifi ed, assuming ideal gas behavior, calculated from experimental mea- surements or estimated from group contribution methods.
In the case of oil refi ning and deodorization, the process is carried out under a vacuum (a few millibars) and high temperatures (381–543 K) in a single liquid phase. However, in VO steam distillation, the presence of condensed water wet- ting the plant surface, together with the fl ow of VO released by the plant, lead to the formation of two immiscible liquid phases, in direct contact with steam. If water and volatile (or oil) phases are considered totally immiscible, by Dalton’s law, then
P Pwvap PC vap
= + , (2.1.2)
where P is the total pressure, and Pw vap
and PC
vap are the water and volatile sub-
stances vapor pressures, respectively. The presence of liquid water in a separated phase reduces the boiling temperature of the mixture because its contribution to the vapor pressure allows the liquid to boil at a lower temperature.
2.1.4.3 Mass Transfer
Molecules of vaporized components at the liquid–vapor interface must go into the steam stream by a mass transfer process. Mechanisms involve diffusion and convec- tive mass transfer.
In VO distillation, steam fl ows through a porous bed of solid material, wetted by the liquid oil–water phases, and conventional mass transfer correlation coeffi cients [16, 17], Kg, may be used to calculate the molar fl ow of volatilized components, mi,
incorporating into the global steam stream: mi K Sg yi yi
G
=
(
–)
, (2.1.3)where S is the transfer surface of contact between the porous bed and the steam, and yi and yi
G
are the vapor phase mole fractions of component i in the liquid–vapor interface and in the global steam stream, respectively.
In oil refi ning and deodorization, mass transfer is usually considered as a limita- tion to vapor–liquid equilibrium and, instead of mass transfer coeffi cients, a stage
TAF-62379-08-0606-C002.indd 16
effi ciency parameter is used. This is the conventional practice in distillation, where Murphree effi ciency is used to correct equilibrium deviations caused by mass trans- fer limitations and other effi ciency-reducing phenomena, such as liquid droplets car- ried out by the steam fl ow. Distillation is discussed in depth in Chapter 3.
2.1.4.4 Distillate Condensation
Vapor leaving the distiller is condensed in the water cooled external condenser. In a total condenser no change in fl ow or composition takes place, because all vapors are condensed into a liquid phase.
2.1.5 NOMENCLATURE
Symbol Defi nition
Units in SI system
Dimensions in
M, N, L, T, and
fi
o Standard state fugacity of pure
component i
Pa ML−1 T−2
Kg Mass transfer correlation coeffi cient kmol s−1 m−2 NT−1 L−2
L Total moles of liquid in the still kmol N
P Pressure Pa ML−1 T−2
Pi
vap Vapor pressure of component i Pa ML−1 T−2
Pw
vap Vapor pressure of water Pa ML−1 T−2
S Transfer surface of contact between the porous bed and the steam
m2 m2
xi Component i liquid molar fraction at
the vapor–liquid interphase
— —
yi Component i vapor molar fraction at
the vapor–liquid interphase
— —
yi G
Component i vapor molar fraction at the global steam stream
— —
Greek letter ˆ
φiV Fugacity coeffi cient of component i
in the vapor phase
— —
γi Activity coeffi cient of component i
in the liquid phase
— —
2.1.6 REFERENCES
1. Günther, E. 1948. The essential oils. Vol. 1 of History and origin in plants production
analysis. New York: Krieger Publishing.
2. Ullmann. 2007. Flavors and fragrances: Essential oils. In Ullmann’s encyclopedia of
industrial chemistry. Hoboken, NJ: John Wiley & Sons.
3. Di Cara, A., Jr. 1983. Essential oils. In Encyclopedia of chemical processing and
design, Vol. 19, edited by J. J. McKetta, 352–381. New York: Marcel Dekker–Taylor &
Francis–CRC.
TAF-62379-08-0606-C002.indd 17
4. Mookherjee, B. O., and R. Wilson. 2001. Oils essential. In Kirk-Othmer encyclopedia
of chemical technology, ECT (CD) Vol. 17. New York: John Wiley & Sons.
5. Masango, P. 2005. Cleaner production of essential oils by steam distillation. Journal of
Cleaner Production 13:833–839.
6. Muñoz, F. 2002. Plantas medicinales y aromáticas: Estudio, cultivo y procesado. Madrid: Ediciones Mundi-Prensa.
7. Peter, K. V. 2004. Handbook of herbs and spices. London: Woodhead Publishing. 8. Benyoussef, E. H., S. Hasni, R. Belabbes, and J. M. Bessiere. 2002. Modélisation du
transfert de matiére lors de l`extraction de l´huile essentielle des fruits de coriandre.
Chemical Engineering Journal 85:1–5.
9. Sovová, H., and S. A. Aleksovski. 2006. Mathematical model for hydrodistillation of essential oils. Flavour Fragrance Journal 21:881–889.
10. Romdhane, M., and C. Tizaoui. 2005. The kinetic modelling of a steam distillation unit for the extraction of aniseed (Pimpinella anisum) essential oil. Journal of Chemical
Technology and Biotechnology 80:759–766.
11. Zizovic, I., M. Stamenic´ , A. Orlovic´, and D. Skala. 2007. Mathematical modelling of essential oil SFE on the micro-scale—Classifi cation of plant material. 5th International Symposium on High Pressure Process Technology and Chemical Engineering, Segovia (Spain), June 24–27.
12. Mukhopadhyay, M. 2000. Natural extracts using supercritical carbon dioxide. New York: CRC Press.
13. Iriti, M., G. Colnaghi, F. Chemat, J. Smadja, F. Faoro, and F. A. Visinoni. 2006. Histo- cytochemistry and scanning electron microscopy of lavender glandular trichomes following conventional and microwave-assisted hydrodistillation of essential oils: A comparative study. Flavour Fragrance Journal 21:704–712.
14. Cerpa, M. G. 2007. Hidrodestilación de aceites esenciales. Doctoral diss., Department of Chemical Engineering and Environmental Technology, University of Valladolid, Spain.
15. Cerpa, M. G., R. B. Mato, and M. J. Cocero. 2008. Modeling steam distillation of essential oils: Application to lavandin super oil. AIChE Journal 54 (4): 909–917. 16. Knudsen, J. G., H. C. Hottel, A. F. Sarofi m, et al. 1999. Heat and mass transfer. In
Perry´s chemical engineers handbook, 7th ed., edited by R. H. Perry and D. W. Green.
New York: McGraw-Hill.
17. Rexwinkel, G., A. B. M. Heesink, and W. P. M. Van Swaaij. 1997. Mass transfer in packed beds at low Peclet numbers—Wrong experiments or wrong interpretations?
Chemical Engineering Science 52 (21–22): 3995–4003.
2.2 DEACIDIFICATION OF VEGETABLE OILS BY STRIPPING