Supercritical Equilibrium Data of the Systems Carbon Dioxide—Linalool and Carbon Dioxide—Orange Essential Oil

doi:10.4236/ajac.2013.412093

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American Journal of Analytical Chemistry, 2013, 4, 763-770

Published Online December 2013 (http://www.scirp.org/journal/ajac)

http://dx.doi.org/10.4236/ajac.2013.412093

Open Access AJAC

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

Email: [email protected]

Received October 5, 2013; revised November 12, 2013; accepted November 28, 2013

cense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

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

flavor.

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

C. CAPPARUCCI ET AL.

764

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

EOS.

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

oil.

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

Open Access AJAC

C. CAPPARUCCI ET AL. 765

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

phases.

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

323.15

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

343.15

13.500 96.704 2.139

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C. CAPPARUCCI ET AL.

766

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

pressure.

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

pressure.

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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C. CAPPARUCCI ET AL. 767

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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768

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

YYY

SXXX













(1)

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:



mijij

azzaak









 ij



(2)



mij

bzz





















 (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









 



















(4)

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

4.2%.

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

Compound

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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C. CAPPARUCCI ET AL. 769

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

data.

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

reported.

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.

Interaction

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

C. CAPPARUCCI ET AL.

Open Access AJAC

770

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-

systems.

6. Acknowledgements

The authors thank the student Gabriele Cintia Lattanzi

who performed some experimental runs during his bachelor

thesis.

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