American Journal of Analytical Chemistry, 2013, 4, 763-770
Published Online December 2013 (
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Supercritical Equilibrium Data of the Systems
Carbon Dioxide—Linalool and Carbon
Dioxide—Orange Essential Oil
Claudio Capparucci, Sara Frattari, Fausto Gironi
Dipartimento di Ingegneria Chimica Materiali Ambiente,
“Sapienza” Università di Roma, Rome, Italy
Received October 5, 2013; revised November 12, 2013; accepted November 28, 2013
Copyright © 2013 Claudio Capparucci et al. This is an open access article distributed under the Creative Commons Attribution Li-
cense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
In this paper experimental equilibrium data on the system supercritical CO2-orange essential oil and the system super-
critical CO2-linalool are reported at 323.15 K and 343.15 K, for pressures in the ranges of 7.6 - 13.5 MPa. The behavior
of the system supercritical CO2-orange essential oil was represented by means of thermodynamic model, based on
Peng-Robinson equation of state. To this aim the orange essential oil was represented by a mixture of limonene, linalool
and β-caryophyllene, selected to represent the classes of monoterpenes, oxygenated terpenes and sesquiterpenes respec-
tively. The model uses only regression parameters calculated from binary sub-systems, CO2-limonene and CO2-
β-caryophyllene (taken from literature) and CO2-linalool (calculated from the fitting of original data reported in the
present work) thus being predictive with respect to the multicomponent mixture.
Keywords: Supercritical; Extraction; Carbon Dioxide; Orange Essential Oil; Linalool; Phase Equilibrium
1. Introduction
The so called citrus essential oils are by means of a me-
chanical extraction process: they are extensively used in
the food (beverage, sweets etc.), cosmetics and pharma-
ceutical industry, mainly because of its fragrance and
These natural matrices are very complex and contain
an extremely large number of components (more than
one hundred), and, in the classical approach, components
are lumped in classes of homogeneous physical and
chemical characteristics. Each class is then represented
by a single component (the component showing the higher
concentration is the one usually chosen). In the field of
citrus essential oils three classes of more or less volatile
components are usually considered: the monoterpenes,
unsaturated hydrocarbon with 10 carbon atoms, the oxy-
genated terpenes which show the same number of carbon
atoms but contain an oxygenated group (alcoholic, ald-
heidic, esteric or ketonic) and the sesquiterpenes, mole-
cules with a higher number (15) of carbon atoms [1].
Citrus essential oils are characterized by different
concentrations of the three classes of compounds; in par-
ticular lemon essential oil has a content of terpenes lower
(approximately 94%) than the ones contained in orange
essential oil (98% - 99%). On the contrary the monoter-
pene concentration in bergamot essential oil is very dif-
ferent (equal to about 50%) [2,3]. The fragrance of the
oil mainly depends on the type and concentration of
oxygenated components present in it, while the terpenes
provide a very poor contribution. The deterpenation
process consists in the selective extraction of terpenes
from natural matrix, thus producing a more valuable raf-
finate enriched in oxygenated compounds [4-6]. This
process can be carried out through a thermal process
(mainly vacuum distillation) or by supercritical extrac-
tion with CO2 as solvent. In fact supercritical CO2 has a
solvent power on monoterpene compounds higher than
the power it has on the oxygenated ones, so an extract
enriched in monoterpenes and a raffinate enriched in
oxygenates can be obtained.
The extraction process may be continuous or discon-
tinuous and can be carried out in different apparatus (a
simple extraction vessel or a countercurrent column with
enrichment and exhaustion sections, with internal or ex-
ternal reflux). The modelization and optimization of the
extraction equipment require the knowledge of the equi-
librium data of the system CO2-natural complex mixture
(for example orange essential oil), as a function of opera-
tional temperature and pressure. Such experimental data
are generally poor, except in the case of lemon essential
oil, which has been studied most extensively [7-10].
Anyway the available experimental data generally refer
to solubility data e.g. data on solubility of CO2 in the oil
and solubility of oil in supercritical CO2. In some works
the CO2-citrus essential oil system is assimilated to a
pseudo binary system and no information is given about
the equilibrium concentrations of main classes of com-
ponents (monoterpenes, oxygenated terpenes and ses-
quiterpenes) in supercritical and liquid phases.
Just a few orange essential oil experimental data are
actually available, furthermore, the oil is assimilated to
a two pseudocomponents [11,12], monoterpenes and
“aroma” components. The aim of this paper is to present
experimental equilibrium data of the system CO2-orange
essential oil. In particular, in the range of temperature
and pressure widely used in deterpenation process (i.e.
temperatures ranging from 323.15 K to 343.15 K and
pressures from 7 to 13 MPa), solubility data and equilib-
rium concentration data of CO2 and different classes
(monoterpenes, oxygenated terpenes and sesquiterpenes),
in supercritical and liquid phases will be presented. This
approach is similar to the one followed recently to study
the system CO2-Lemon essential oil [10].
Furthermore, the thermodynamic modelization of the
system CO2-orange essential oil will be presented by
means of Peng Robinson equation of state (PREOS). As
pointed out above, the complex mixture is assimilated to
a simple mixture of the components that are assumed to
represent each class. For example in the previous papers
dealing with lemon essential oil [5,6,10], limonene, citral
e β-caryophyllene have been assumed to represent mo-
noterpene, oxygenated terpenes and sesquiterpene classes.
In the modelization only binary interaction parameters
between CO2 and each component in the mixture repre-
senting the oil are used to increase the reliability of the
method. These parameters are obtained from regression
of binary equilibrium data of the systems CO2-limonene,
CO2-Citral, CO2-β-caryophyllene in the temperature
range of interest, and its values are reported in literature.
Dealing with orange essential oil, the same compounds
can be selected to represent monoterpene and sequiter-
pene classes, but for oxygenate class another compound,
linalool, is more suitable for the modelization of the
whole class, instead of cytral.
Unfortunately scarce experimental data about the sys-
tem CO2-linalool are available, so equilibrium data about
this system at temperature of 323.15 K and 343.15 K and
pressure ranging from 7.6 to 13.2 MPa have been meas-
ured in this work. Optimal values of binary interaction
parameters for the system CO2-linalool have been deter-
mined by the fitting of experimental data by means of PR
Once all the binary interaction parameters are avail-
able and after having defined the concentration of vari-
ous components in the oil, the model can be applied to
the modelization of the CO2-orange essential oil system.
The method is purely predictive because no other regres-
sion parameter has been used to represent equilibrium
data of the multicomponent system CO2-orange essential
A comparison between predicted and experimental
values of equilibrium data of the system CO2-orange
essential oil shows that the model is able to represent
both solubility data and equilibrium compositions of liq-
uid and supercritical phases at 323.15 K and 343.15 K. In
particular this model correctly evaluates equilibrium ra-
tios of the four components in the mixture (CO2, limo-
nene, linalool and β- caryophyllene) at different opera-
tional pressure and temperature: therefore, the model can
be used in a process simulator to design and to optimize
the deterpenation equipment.
2. Materials and Methods
2.1. Materials
The essential oil used in this work was purchased from
Simone Gatto (Sicily, Italy), and was obtained by cold-
pressing orange peels. The oil was stored at 248 K and
filtered before use in order to remove waxes. The compo-
sition of the oil was determined by means of Gas Chro-
matographic (GC) analysis, according to the procedure
reported in detail elsewhere [5]. All components reported
in the GC analysis of orange oil were grouped in terms of
the three main classes of compounds: monoterpenes (mt),
oxygenated terpenes (ox), and sesquiterpenes (st).
The mean composition of the feed oil, obtained from
five GC analyses, resulted to be (mass percent, average
value and standard deviation): mt 99.05% ± 0.25%, ox
0.82% ± 0.21% and st 0.13% ± 0.05%. The same proce-
dure, repeated five times, was also applied in the meas-
urements of the composition of supercritical and liquid
phases in the equilibrium experiments.
CO2 used in this work is 99.9% pure (Siad, Italy). Li-
nalool (dimethyl-1,6-octadien-3-ol) 97% pure was pur-
chased from Sigma-Aldrich (FW 154.25) and was used
without further purification
Ethanol p.a. 99.8% (Fluka Chemical), was utilized to
dilute (1:10) the samples of oil before the GC analysis.
2.2. Experimental Methods
The apparatus used in the present work was developed
and utilized in previous works [13-15] dealing with
high-pressure phase equilibrium measurements of binary
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systems composed of CO2 and relevant compounds of
lemon essential oil and measurements of the pseudo bi-
nary system CO2-lemon essential oil [10]. In particular
work [13] reports a detailed description of both the ap-
paratus and the experimental methodology.
The experimental plant is composed of two cylindrical
chambers of 170 cm3 each (i.d. 16 mm), equipped with
heating jackets for temperature control. One of the two
chambers (the equilibrium chamber) is initially loaded
with about 30 grams of liquid (linalool or orange essen-
tial oil) whereas the other (gas chamber) is initially
empty. At the beginning of the run CO2 is fed to both
chambers up to the desired pressure. The apparatus is
equipped with a recirculation pump that extracts the
gaseous phase from the bottom of the gas chamber and
re-injected it at the bottom of the equilibrium chamber to
increase mass transfer between supercritical and liquid
According to our experience on this apparatus, a re-
circulation time equal to almost 6 hours and a flow rate
of recirculating supercritical phase equal to about 500
Nliters/min have to be utilized to be sure to attain equi-
librium conditions.
After the recirculation period, the two chambers are
separated by closing valves in order to allow the whole
content of the gas chamber to be sampled without dis-
turbing the equilibrium conditions in the equilibrium
chamber. The sampling of gas and liquid phases is car-
ried out according to the procedure described in detail
elsewhere [14]. The amount of solute collected during
gas and liquid sampling was determined gravimetrically,
whereas the amount of CO2 is determined by means of
the gas meter connected to the experimental apparatus
(accuracy equal to 0.001 Nliters).
The amount of liquid collected from the gas phase
sampling (pure linalool or orange essential oil extract)
resulted to be in the range of 0.13 - 12.3 g, with a sam-
pled volume of CO2 in the range of 19.5 - 56 l (measured
at ambient conditions). As far as the liquid phase sam-
pling is concerned, the amount of liquid withdrawn from
the bottom of the equilibrium chamber resulted to be in
the range of 0.23 - 1.46 g.
A small amount of liquid oil collected from gas and
liquid phase sampling were analyzed chromatographi-
cally in order to determine their composition, on CO2
free basis.
At the end of each experimental run the equilibrium
chamber is depressurized and mass of oil was determined
gravimetrically in order to check the oil mass balances
referred to the whole experimental run. On the contrary
no check can be done by means of CO2 mass balance
because it is not known the mass of CO2 initially fed to
the apparatus. Anyway the volume of CO2 exiting the
chamber in the depressurization step is measured and
both added to the volume of CO2 measured during the
extraction of the gaseous phase from the gas chamber
and to the volume developed during the sampling of the
liquid phase. This way, in the hypothesis that no loss of
CO2 occurred during the experiment, the total amount of
CO2 fed to the apparatus is evaluated. So it is possible to
evaluate the initial overall composition of the system.
3. Experimental Results
3.1. Carbon Dioxide—Linalool
The phase equilibrium data for the binary system
CO2-linalool are measured at 323.15 K and 343.15 K, in
the pressure ranges 8.3 - 10.5 MPa and 10.1 - 13.5 MPa,
respectively, with the experimental set-up and with the
procedure described in the previous Section 2.
Table 1 reports the solubility of linalool in supercriti-
cal CO2 phase (linalool solubility, sg) and the solubility
of CO2 in liquid linalool (CO2 solubility, sl), at the indi-
cated temperatures.
Figure 1 shows the experimental points and the model
curves for the linalool solubility, as a function of the
pressure, at the temperatures of 323.15 K and 343.15 K.
These values are compared with the data from literature
at 313.15, 318.15, 323.15, 328.15 and 333.15 K [16,17].
Linalool solubility is expressed as grams of linalool per
kilogram of CO2. The solubility of linalool measured in
this work, increases with the pressure, at fixed tempera-
Table 1. Experimental solubility of linalool in supercritical
CO2 (sg) and solubility of CO2 in the liquid linalool (sl) at
323.15 K and 343.15 K as function of pressure.
linalool - carbon dioxide
T (K)
P (MPa) sg (g/1000 g) sl (g/g)
8.250 9.815 0.590
8.600 11.856 0.745
8.650 16.589 0.856
9.200 16.927 1.296
9.840 28.429 2.055
10.083 47.985 3.775
10.464 123.540 5.200
10.100 16.610 0.571
10.900 20.007 0.749
11.500 26.380 0.797
11.900 36.215 0.877
12.300 43.015 1.107
12.700 50.171 1.274
13.000 56.699 1.546
13.100 68.163 1.570
13.500 96.704 2.139
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ture, in the range of 9.8 - 123.5 g/kg at 323.15 K, and in
range of 16.6 - 96.7 g/kg at 343.15 K. The trends of lit-
erature data, at the five different temperatures, are con-
sisted with the behavior of the experimental data and
with the model curves presented in this work.
Figure 2 compares the experimental data reported in
this work and the model curves of the CO2 solubility, as
a function of the pressure, at the temperatures of 323.15
and 343.15 K with data from literature at 313.15, 323.15
and 333.15 K [17]. The solubility in liquid phase is ex-
pressed as grams of CO2 per gram of linalool. The solu-
bility of CO2 measured in this work, increases with the
pressure in the ranges of 0.59 - 5.2 g/g at 323.15 K, and
in ranges of 0.57 - 2.14 g/g at 343.15 K and decreases
with the temperature in the considered pressure ranges.
These trends are in agreement with the literature data
Figure 1. Experimental data presented in this work, litera-
ture data and model curve for solubility of linalool in su-
percritical CO2 at 323.15 K and 343.15 K as function of
Figure 2. Experimental data presented in this work,
literature data and model curve for solubility of CO2 in the
liquid linalool at 323.15 K and 343.15 K as function of
even if the values reported in reference [17] are higher
than the values of solubility measured in this work at
323.15 K, after 8.9 MPa. However, it has to be under-
lined that in this region solubility increases greatly.
3.2. Carbon Dioxide—Orange Oil
The gas-liquid phase equilibrium of the system CO2-
orange essential oil was investigated at 323.15 K and
343.15 K, for pressures in the ranges of 7.6 - 9.6 MPa
and 10.1 - 13.2 MPa, respectively. The maximum value
of pressure used is the value in correspondence of which,
at each temperature, there are two phases in equilibrium
conditions. The chosen range of pressure and tempera-
ture values are particularly relevant for this system, since
they represent potential operating parameters for the effi-
cient application of the supercritical deterpenation pro-
cess. The experimental data on the system CO2-orange
essential oil are collected in Table 2 and their trends are
shown in Figures 3 and 4.
Figure 3 shows the solubility of the oil in supercritical
CO2 (sg) as a function of pressure, at the two tempera-
tures under investigation. The data measured in this work
show that at 323.15 K the solubility of the oil increases
from approximately 11 to 21 g/kg, for pressures ranging
from 7.6 MPa to 9.6 MPa; at 343.15 K the solubility in-
creases from approximately 16 to 75 g/kg, for pressures
ranging from 10 MPa to 13 MPa. In the same Figure 3 a
comparison with literature experimental data [12] is also
reported. Actually in Figure 3 it was not possible to rep-
resent the experimental data of solubility of orange es-
sential oil in CO2 [12] as in the original work these data
are reported only in a figure. On the other hand in the
same work is given an empirical correlation of experi-
mental solubility data that perfectly represents the ex-
perimental data themselves. Therefore, to test the relia-
Table 2. Experimental solubility of orange oil in supercriti-
cal CO2 (sg) and the solubility of CO2 in orange essential oil
(sl) at 323.15 K and 343.15 K as function of pressure.
Orange essential oil—carbon dioxide
T (K)
P (MPa) sg (g/1000 g) sl (g/g)
323.15 7.56 11.604 0.549
8.11 11.766 0.978
8.60 10.558 0.902
8.70 11.326 0.828
9.10 15.448 1.113
9.60 20.728 2.218
343.15 10.14 16.079 0.614
11.05 25.567 0.655
12.14 45.086 0.861
13.15 74.859 1.241
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bility of the experimental data reported in this paper,
values obtained from the empirical correlation can be
used. In the comparison, however, it must be taken in to
account that the composition of orange essential oil used
in the work [12] is different from the one of the present
work. In particular, in reference [12] the oil has a weight
fraction of monoterpenes equal to 98.25% whereas a
value of approximately 99% characterizes the oil in the
present work. From the analysis of the figure we see that
the data reported in this work are in agreement with data
reported in reference [12], with relative mean deviations
equal to 21% at 323.15 K and equal to 24% at 343.15 K
Figure 4 shows the solubility of CO2 in the liquid
phase, as a function of pressure. The solubility of the gas
in the liquid phase increases with pressure and decreases
with temperature, for the values of pressure and tem-
perature are under investigation. At 323.15 K, an increase
from approximately 0.5 to 2.2 g/g is observed, whereas the
Figure 3. Experimental data presented in this work, litera-
ture data and model curve for solubility of orange essential
oil in supercritical CO2 at 323.15 K and 343.15 K as func-
tion of pressure.
Figure 4. Experimental data presented in this work, litera-
ture data and model curve for the solubility of CO2 in the
oil phase at 323.15 K and 343.15 K as function of pressure.
solubility increases from 0.6 to 1.2 g/g at 343.15 K. In
Figure 4 the comparison, of obtained experimental data
with the empirical correlation of data [12] is reported:
taking into account the considerations above discussed,
the two data sets are in good agreement with relative
mean deviations equal to 13% at 323.15 K and equal to
16% at 343.15 K.
The experimental data reported in Table 2 can also be
observed in the isothermal P-x-y diagrams reported in
Figure 5, which represents the system as pseudo-binary
(i.e., considering orange essential oil as a pure compo-
nent). The diagram provides an overview of the behavior
of the system at the two temperatures. Each condition of
equilibrium pressure and temperatures corresponds to a
given value of the ratio oil/CO2 used in the test. In fact,
for a constant mass of oil, the added mass of CO2 should
be such as to reach the desired pressure in the apparatus
(of fixed volume). Being the system not actually binary
(i.e., orange essential oil is not a pure compound), it fol-
lows that if another overall composition locus (for exam-
ple by charging in the apparatus a mass of oil equal to 20
g instead of 30 g) is chosen, equilibrium P-x-y lines
would change accordingly.
Table 3 shows the experimental compositions, on
CO2-free basis of phases in equilibrium at different val-
ues of temperature and pressure. The gaseous phase
shows the enrichment in monoterpenes, on the contrary
the liquid phase has only a modest enrichment in oxy-
genated terpenes and sesquiterpenes components. This
behavior depends on the low value of the ratio sol-
vent/feed utilized in the experimental runs: a minimal
amount of oil is solubilized in the supercritical phase,
and then the composition of the liquid phase is almost
coincident with the one of the feed.
No literature experimental data are available for com-
parison because in the works [11,12] besides the mono-
terpenes, only a single “aroma” component is considered.
Figure 5. Pseudo-binary isothermal P-x-y diagrams at
323.15 K and 343.15 K.
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From the analysis of experimental data it is apparent
that the feed mass fraction values of the three classes of
compounds are between the corresponding values in the
gas and the liquid, and therefore mass balance of differ-
ent components are respected. Some experimental runs
(at 323.15 K and 9.1 MPa and 9.6 MPa; at 343.15 K and
11.05 MPa) show that both the supercritical and liquid
phases are richer in monoterpenes than the feed. This
problem is probably caused by some inaccuracies in the
GC-analyses of this complex mixture.
In order to quantify this aspect, a selectivity of the
solvent with respect to the separation of monoterpenes
and oxygenated terpenes + sesquiterpenes compounds
can be defined as follows:
mtox st
mt ox
mtox st
In Equation (1) Ymt, Yox and Yst indicate the weight
fraction of monoterpenes , oxygenated terpenes and ses-
quiterpenes respectively in supercritical phase, whereas
Xmt, Xox and Xst indicate the mass fraction of the same
components in the liquid phase.
From the data reported in Tables 3, not considering
the above-mentioned experimental points showing in-
consistencies in the mass balances, it can be seen that at
323.15 K the selectivity varies from 2 to 5.These values
are higher than literature data [12] which vary from 1.1
to 3 in the same temperature and pressure ranges.
4. Thermodynamic Modeling
System Carbon Dioxide—Linalool
Equilibrium data of the system CO2 linalool reported
in Table 1, were represented by means of PREOS
with the van der Waals mixing rules and two tempera-
ture-independent interaction parameters, as already re-
Table 3. Experimental composition of supercritical and
liquid phases (on solvent free basis).
Orange essential oil compositions
T (K)
P (MPa) Xmt XoxXst Ymt YoxYst
323.15 7.56 0.986 0.0110.003 0.997 0.0020.001
8.11 0.990 0.0090.001 0.997 0.0020.001
8.60 0.986 0.0120.002 0.997 0.0020.001
8.70 0.987 0.0110.002 0.997 0.0020.001
9.10 0.993 0.0060.001 0.995 0.0040.001
9.60 0.993 0.0060.001 0.994 0.0050.001
343.15 10.14 0.990 0.0090.001 0.997 0.0020.001
11.05 0.991 0.0080.001 0.994 0.0050.001
12.14 0.986 0.0120.002 0.994 0.0050.001
13.15 0,992 na na 0.993 na na
ported in our previous works dealing with the thermody-
namic characterization of systems composed by CO2 and
single component of lemon oil (limonene, citral and
β-caryophyllene) [13-15].
Attractive parameters (ai) and covolumes (bi) of CO2
and linalool were calculated from critical properties and
acentric factors through the relationships reported by
Peng and Robinson [18]. To carry out these calculations,
linalool critical parameters and acentric factor were de-
termined by means of group contribution method [19]:
the obtained values are reported in Table 4 together with
the one of CO2. The relationships for the mixture attrac-
tive parameter and covolume are reported below:
 ij
 (3)
In Equations (2) and (3) zi and zj indicate the mole
fractions of the components in the generic phase (i.e., gas
or liquid) and kij and ηij are the binary interaction pa-
rameters (both equal to zero if i = j and kij = kji; ηij = ηji if
i j).
The interaction parameters were calculated according
to a well-defined procedure, by regression of experimen-
tal data, by minimizing of the follow objective function:
iCiEiC iE
ij ij
iiE iE
NjCjEjC jE
ijE jE
yy xx
kNy x
yy xx
Ny x
 
In Equstion (4) y and x indicate the molar fraction in
the gas and liquid phase, respectively, the subscript C
stands for a quantity calculated by the thermodynamic
model, the subscript E refers to experimental data, and
N50 and N70 are the numbers of experimental data at re-
spectively 323.15 K and 343.15 K. The average percent-
age deviation for the binary systems CO2-linalool is
Besides representing the experimental data, Figures 1
and 2 also show the equilibrium solubility curves calcu-
Table 4. Physical properties of CO2 and components of or-
ange essential oil
Physical property
MW Pc (MPa) Tc (K) ω
carbon dioxide44.01 7.376 304.2 0.225
limonene 136.24 2.788 651.17 0.389
linalool 154.25 3.155 646.03 0.6903
β-caryophyllene204.36 2.076 736.15 0.518
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lated with the optimal values of the temperature-inde-
pendent interaction parameters that were determined in
this work and the curves calculated by assuming k = 0
and e η = 0 (completely provisional values). In addi-
tion, Figure 6 provides an overall view of the behavior
of this binary system, at the two temperatures under in-
vestigation. Optimal values of interaction parameters
were used also to correlate experimental data reported in
reference [17] at 333.15 K (see Figure 6). The agreement
seems to be satisfactory also with this set of experimental
The system CO2-orange essential oil was modelized as
composed of 4 components, which are CO2 (the sol-
vent) plus three selected main components representing
the oil: the major monoterpene compound (i.e., limo-
nene), the major oxygenated compound (i.e., linalool),
and a sesquiterpene compound (i.e., β-caryophyllene).
Extensive discussion on this modeling approach is re-
ported in other papers dealing with the modelization of
the system CO2-lemon essential oil [5]. The critical
properties and acentric factor of limonene and β-caryo-
phyllene are reported in Table 4 whereas the values of
interaction parameters for system CO2-limonene and
CO2-β-caryophyllene taken from literature [10,15] are
As for the binary-interaction parameters between oil
compounds, meanings the sub-systems not involving
CO2, they were assumed equal to zero. On the whole, the
complete set of values of the temperature-independent
interaction parameters used for modeling the system
CO2-orange essential oil is reported in Table 5. It is here
underlined that the only empirical parameters utilized by
the thermodynamic model are the interaction parameters
referred to binary sub-systems CO2-orange essential oil
components. Therefore, the model is predictive with re-
spect to the multicomponent system.
Figure 6. Experimental data presented in this work, data
from reference [17] and equilibrium values obtained by
means of PREOS at 323.15 K, 333.15 K and 343.15 K with
kij and ηij reported in Table 5.
The capability of the model of predicting the behavior
of the system CO2-orange essential oil was tested
through a series of phase equilibrium calculations at con-
stant pressure, temperature, and volume. In particular the
volume of the apparatus (340 cm3), the amount of loaded
oil (30 g) and the oil composition (obviously in terms of
limonene, linalool and β-caryophyllene representing
monoterpenes, oxygenated compounds, and sesquiter-
penes respectively) were fixed besides the values of
pressure and temperature. By means of a specific soft-
ware which takes into account the phase equilibrium
equations (i.e., isofugacity conditions) together with the
mass balances and the volume constraint on the system,
the amount and composition of the gas and liquid phases
at equilibrium is provided as output. In particular, it is
here noted that the amount of CO2 in the system is an
output parameter of the calculation.
Figures 3 and 4 show that the model correctly pro-
vides the solubilities in the gas and the liquid phases for
the system CO2-orange oil. Actually the model overesti-
mates the solubility of CO2 in the liquid phase at 343.15
K. The amount of CO2 which is necessary to guarantee
the specified pressure to be established is also very well
predicted (see Figure 5). The capability of the model of
providing a good representation of the pseudo-binary
system is summarized in Figure 6, which shows calcu-
lated and predicted values of the mass fractions of the
two phases in equilibrium.
The comparison between calculated and experimental
values of equilibrium composition can also be carried out.
In particular negligible values of relative deviation be-
tween calculated and experimental weight fraction of
monoterpenes in liquid and supercritical phases are
evaluated. On the contrary high values of the same de-
viations for oxygenated and sesquiterpenes weight frac-
tions are obtained. These high deviations are due also to
the inaccuracies of the CG analysis.
5. Conclusion
The gas-liquid phase equilibrium of the system
CO2-orange essential oil was experimentally determined
in a constant-volume apparatus working at 323.15 K and
343.15 K, in the pressure ranges of interest for the deter-
penation process. The experiments were carried out with
an orange oil rich in monoterpenes (mt 99.05%, ox
Table 5. Binary interactions parameters for the systems
CO2-components of orange essential oil.
parameters CO2-limoneneCO2-linalool CO2-β-caryophyllene
kij 0.089 0.016 0.089
ηij 0.013 0.069 0.005
reference [18] this work [X]
Open Access AJAC
Open Access AJAC
0,82%, st 0,13%). In the investigated pressure range,
solubility of CO2 in liquid phase and solubility of oil in
the supercritical phase were determined. Furthermore, the
composition of the gas and the liquid phase were deter-
mined, in terms of three main classes of components. A
thermodynamic model based on PREOS, including van
der Waals mixing rules corrected by two temperature-
independent binary interaction parameters, was used in
order to predict equilibrium data for the system CO2-
orange oil. The comparison is satisfactory because the
model is capable of providing a good representation both
of the experimental solubility data and the phases’ com-
positions. The selected model is predictive with respect
to the multicomponent system, being the regression para-
meters calculated only on data of selected binary sub-
6. Acknowledgements
The authors thank the student Gabriele Cintia Lattanzi
who performed some experimental runs during his bachelor
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