High-pressure apparatus for phase equilibria studies: solubility of fatty acid esters in supercritical CO2

Journal of Supercritical Fluids 16 (1999) 11–20www.elsevier.com/locate/supflu

High-pressure apparatus for phase equilibria studies:solubility of fatty acid esters in supercritical CO2

C. Crampon a, G. Charbit a,*, E. Neau ba Laboratoire d’Etudes et d’Applications de Procedes Separatifs, Universite d’Aix-Marseille, Case 512,

Av. Escadrille Normandie Niemen, 13397 Marseille Cedex 20, Franceb Laboratoire de Chimie Physique de Marseille, Universite de la Mediterranee, Case 901, 163, Av. de Luminy,

13288 Marseille cedex 9, France

Received 14 May 1998; received in revised form 7 April 1999; accepted 22 April 1999

Abstract

Starting from a screw-type manual pump, a high-pressure variable-volume view cell was designed to perform staticmeasurements of equilibria involving CO2 and fatty acid esters. Though the design of the cell allows both the upperand the lower phases to be sampled, the synthetic method was chosen for this study. The efficiencies of both theapparatus and the method were first checked through preliminary experiments related to a compound already studied,namely methyl oleate. The good agreement between our measurements and those published in the literature allowedus to study other compounds, more precisely the ethyl esters of myristic, palmitic and stearic acids. These measurementswere obtained at three different temperatures, 313.15, 323.15 and 333.15 K, and at pressures ranging from 1 to18 MPa. The data were correlated using the modified Peng–Robinson equation of state with the classical quadraticmixing rules. An interesting feature of the apparatus is its low consumption of substance for a rather large amountof acquired data. © 1999 Elsevier Science B.V. All rights reserved.

Keywords: Fatty acid esters; Phase equilibria; Solubility; Supercritical CO2; Variable-volume view cell

1. Introduction pounds, but they have the disadvantages in thatthey use toxic and inflammable solvents and exces-

Because of their attractive contents in healthy sively high temperatures causing their degradation.components, long-chained unsaturated fatty acids Extraction by supercritical fluids may constitutehave been used in a wide range of applications in an alternative to such problems and is, for thisfood-stuffs and pharmaceutical industries [1]. It reason, the subject of growing interest.has been shown that they may have a beneficial Supercritical carbon dioxide (SC-CO2) is com-impact on several diseases [2–5]. monly used, but in this particular case, a problem

Liquid–liquid extraction and distillation are the arises from the low solubility of fatty acids inconventional methods used to recover such com- SC-CO2, and so their direct extraction is not

realistic from an industrial point of view. Trans-esterification of these acids yielding esters is suit-* Corresponding author. Tel.: +33-04-91-28-86-43;able since the latter ones are fairly soluble infax: +33-04-91-02-35-72.

E-mail address: [email protected] (G. Charbit) SC-CO2. Initially, methanol was used, but because

0896-8446/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved.PII: S0896-8446 ( 99 ) 00021-2

12 C. Crampon et al. / Journal of Supercritical Fluids 16 (1999) 11–20

of its toxicity, it has been superseded by ethanol, 2. Phase equilibrium measurementsand so ethyl esters are recovered.

The design of an extraction operation needs key Three different methods are usually used toperform experimental studies of phase equilibriaparameters such as equilibrium data in the operat-

ing conditions. Though systems involving CO2 and under supercritical conditions, and the procedureto be used depends on the information requiredmethyl esters have been extensively studied, there

is a lack of experimental results devoted to [1,6,12,13]. The synthetic and analytical methodsare static procedures, whereas in the dynamicCO2/ethyl ester systems [6 ]

This study has a dual aim: method, the supercritical fluid is continually sweptthrough the cell. The main features of these meth-$ set-up of a simple apparatus allowing precise

measurements combined with a low consump- ods were summarized by Staby [1], Brunner [12]and McHugh and Krukonis [13]. The dynamiction of substance,

$ extension of a predictive thermodynamic model method, which only provides information on thecomposition of the light phase, is thus not conve-for phase equilibria.

The present work essentially deals with the first nient in this case. Analysis systems, such as chro-matography or UV spectroscopy, are required forgoal. It describes both the apparatus and the

measurement method, and reports data related to the determination of the phase composition in theanalytical method, and in most cases, sampling offour esters at three temperatures in a wide range

of pressures. In order to check the efficiency of both phases is performed. In the synthetic method,no sampling is necessary since a known composi-both the set-up and the method, it was necessary

to perform the measurements in a system that has tion mixture is introduced in an optical cell, andthe phase transitions resulting from pressure varia-already been studied in the literature. Since the

binary mixture CO2/methyl oleate has been the tions are studied by direct visualization. Theabsence of sampling, and the fact that the precisionobject of several studies [7–11], it was taken as

reference. of the measurements depends essentially on the

Fig. 1. Experimental set-up.

13C. Crampon et al. / Journal of Supercritical Fluids 16 (1999) 11–20

precision of the weighing, led us to set-up an Each revolution of the manual pump is registeredby a counter and induces a 0.3 cm3 variation ofapparatus using this procedure.the volume.

The pressure is measured by an electronic2.1. Apparatustransmitter (HAENNI ED 510); overpressures areprevented by a rupture disc. The front zone of theThe experimental set-up is shown in Fig. 1. Itapparatus is heated by the circulation of waterconsists of a high-pressure view cell with variable-coming from a thermostated bath. The screw fillervolume, stainless-steel construction, a magneticcap at the top of the cell includes a port for astirrer, a thermostatic water bath, a camera (Modelplatinum resistance. A purge drain is fitted at the#1352-5000, from COHU Inc.), a TV monitor,base of the cell for easy cleaning.temperature and pressure gauges. The main part

The cell may work in both the synthetic andof the set-up is the cell, the details from which arethe analytical modes. For the latter, it is equippedshown in Fig. 2. The cell was designed by Topwith two sampling ports at the upper and lowerIndustries S.A. A three-hand screw-type manualparts of the chamber, each sampling line includingpump was set to a thermostated chamber, equippeda three-way micro-valve.with a sapphire window to enable direct visualiza-

tion of the medium.Preliminary calibrations showed that the global 2.2. Materials

volume of the cell may vary from 4.05 to14.35 cm3; these values take into account the dead CO2 (99.9% purity) was supplied by ‘I’Air

Liquide’. The methyl and ethyl esters, from Sigma,volumes generated by the different equipments ofthe apparatus (sampling lines, valves, gauges). had a stated purity higher than 99%.

Fig. 2. High-pressure view cell.


2.3. Procedure performed as described above. After three addi-tions of CO2, it is considered that the cumulativeerrors due to the successive weighing operationsThe results of this study were obtained accord-

ing to the synthetic method, and the experimental induce an inadequate uncertainty of the composi-tion. As a consequence, the cell is drained off, andprocedure is as follows.

First, a known amount of ester is loaded in the a new amount of ester is introduced for furthermeasurements.cell. With this loading, the ester is placed in a

cupel and a first weighing of the whole is per-formed. The solute is then introduced in the cell,

2.4. Accuracy of measurementsand the empty cupel is weighed.Before starting the experiment, the confined air

The uncertainty on the gravimetric measure-of the cell is evacuated as follows. The cell isments is of the order of ±5 mg for the carbonclosed, and carbon dioxide is introduced until adioxide and ±0.05 mg for the ester. The uncer-pressure of 0.4 MPa is reached. After a few min-tainty in binary compositions is essentially due toutes, the gas phase in the cell is evacuated. ThisCO2; indeed, the CO2 weight is known with lessprocedure is repeated five times. Since the chemi-precision, and the errors with CO2 are cumulative.cals studied here have a very low volatility at

Taking account of the respective uncertainties0.4 MPa, it is assumed that the amount of solutewith the CO2 and ester weights, and the cumulativeeliminated during the degassing is negligible.errors due to the method, we calculated that theAfter eliminating the air, a known amount oferrors on CO2 mole fractions would never exceedCO2 is brought into the cell by a high-pressure0.003.calibrated bomb, which is weighed before and after

The cell pressure measurement is accurate tothe introduction of CO2. The mixture thus±0.02 MPa, and the temperature controlled toobtained has a known composition, and thewithin ±0.2 K.medium is biphasic at this pressure level. When

the system is equilibrated at the chosen temper-ature, the piston is progressively moved in orderto increase the pressure and thus to bring thebiphasic system to a monophasic system. Once the 3. Experimental results and correlationpressure has stabilized, it is slowly decreased untila second phase appears. The medium is thus 3.1. Experimental datasuccessively compressed and decompressed inorder to define the narrowest range of pressures Methyl oleate, frequently encountered in the

literature [7–11], was chosen to check the experi-for the phase transition. The stabilization has tobe reached after each modification of pressure, but mental set-up, and ethyl myristate, palmitate, and

stearate led to new data in CO2. For each system,due to the small cell volume and the efficiency ofthe stirring, the experiment requires no more than the equilibrium transition pressures were isother-

mally measured at three temperatures: 313.15,3 h, which is a relatively short period of timecompared to that reported in the literature. 323.15 and 333.15 K. Tables 1–4 report the corre-

sponding data that are shown in Figs. 3–8.For a given global composition, three increasingtemperatures, 313.15, 323.15, and 333.15 K, are The transition points were easily obtained and

were reproduced to within 5×10−2 MPa. Ourstudied.The second point of an isotherm, which corres- experiments were performed in a large range of

pressures and compositions; however, the presentponds to a higher CO2 mole fraction, is obtainedby the following method. An additional weighed experimental set-up did not enable us to perform

measurements at low pressures ( less than 1 MPa).amount of CO2 is introduced in the cell, thusmodifying the composition of the previous mix- As a consequence, the lower part of the equilibrium

curves was not obtained.ture. Measurements of the transition pressures are


Table 1 Table 2Equilibrium solubility data of ethyl myristate in supercriticalEquilibrium solubility data of methyl oleate in supercritical

carbon dioxide at 313.15, 323.15, and 333.15 K carbon dioxide at 313.15, 323.15, and 333.15 K

CO2 mole fraction Pressures (bar) CO2 mole fraction Pressures (bar)

313.15 ( K) 323.15 (K ) 333.15 ( K)313.15 ( K) 323.15 ( K) 333.15 (K )

0.194 10.0 10.7 11.5 0.305 16.5 18.7 21.90.356 22.5 25.7 29.00.211 11.5 13.0 14.0

0.275 14.5 16.0 17.0 0.399 25.0 28.5 34.30.492 41.4 48.0 54.30.287 16.5 17.5 18.5

0.359 19.5 22.8 23.9 0.568 47.5 54.5 61.80.618 57.7 67.0 76.90.458 28.5 31.5 34.4

0.548 39.5 44.0 51.0 0.618 57.5 66.9 76.80.664 60.8 70.6 80.80.570 42.4 48.4 53.6

0.612 50.0 56.5 64.0 0.667 64.8 75.4 87.00.723 69.4 81.4 94.50.651 55.7 65.1 70.9

0.658 58.5 68.5 78.0 0.731 73.5 86.8 101.40.758 74.5 87.7 102.40.692 58.6 70.7 83.0

0.748 71.8 87.9 102.7 0.773 76.3 90.3 106.00.788 79.4 96.2 113.40.774 77.0 92.0 105.5

0.827 85.0 104.0 122.0 0.804 80.1 95.4 113.60.829 83.6 102.7 123.40.875 92.1 119.4 145.4

0.886 97.4 121.7 148.4 0.839 85.1 104.5 125.10.847 85.9 106.6 128.60.916 106.2 134.2 159.2

0.945 123.8 151.5 177.7 0.872 88.8 113.0 136.40.909 92.8 118.9 142.60.960 125.2 154.3 180.3

0.967 126.1 154.8 181.2 0.911 93.3 119.6 143.40.925 – 120.4 143.80.970 123.4 151.0 177.0

0.972 127.0 155.4 180.7 0.938 – 113.7 –0.963 93.4 120.3 144.40.973 122.0 151.0 177.0

0.976 121.5 149.6 – 0.965 93.4 119.6 143.70.982 92.5 115.5 136.20.993 107.5 135.2 160.4

0.995 102.9 129.9 153.6 0.990 91.6 114.9 137.20.991 89.1 111.4 131.50.997 95.5 121.5 146.7

It should be noted that our apparatus and from the critical properties:procedure allowed measurements around the criti-cal point. b=0.07780

RTc

Pc

(2)

3.2. Data correlation

a(T )=0.45724R2T2

cPc

Y(T, v) (3)The experimental P–x data were correlated

using the modified Peng–Robinson equation ofstate proposed by Peneloux and Rauzy [14], which Y(T, v)=G1+mC1−AT

TcB0.44507DH2 , (4)

improves the vapor pressure correlations of mix-tures: where m is calculated from:

m=P=RT

v−b−

a

v2+2bv−b2, (1)

[6.812553(E1.12754+0.517252v−0.003737v2−1)]2,(5)where equation parameters a and b are defined


Table 3 Table 4Equilibrium solubility data of ethyl stearate, in supercriticalEquilibrium solubility data of ethyl palmitate in supercritical

carbon dioxide at 313.15, 323.15, and 333.15 K carbon dioxide at 313.15, 323.15, and 333.15 K

CO2 mole fraction Pressures (bar) CO2 mole fraction Pressures (bar)

313.15 ( K) 323.15 (K ) 333.15 ( K)313.15 ( K) 323.15 ( K) 333.15 (K )

0.325 18.9 20.6 24.0 0.281 14.5 17.5 19.50.487 34.9 41.6 44.30.512 35.1 38.8 42.6

0.565 43.4 50.1 57.8 0.607 48.0 50.0 58.50.700 60.0 71.0 77.00.614 48.8 56.2 64.1

0.642 55.8 65.5 75.2 0.739 79.0 79.0 88.00.759 76.5 89.6 100.40.694 63.0 73.3 82.4

0.720 65.5 73.8 85.6 0.820 83.8 101.2 118.50.905 99.1 129.4 151.30.776 72.5 83.3 97.9

0.839 82.3 104.3 126.8 0.937 133.0 159.1 182.40.952 141.2 165.3 189.20.839 82.8 104.9 128.0

0.884 93.4 118.5 143.3 0.955 141.8 166.4 189.20.961 143.9 167.7 190.80.915 100.4 125.5 149.9

0.915 – 125.0 149.9 0.967 143.7 167.6 190.60.995 109.0 136.2 159.60.939 106.4 133.5 158.8

0.939 106.2 133.4 158.9 0.995 109.4 136.5 159.30.997 101.8 129.2 149.80.945 106.0 133.2 158.0

0.959 108.8 136.9 161.4 0.997 102.1 129.0 149.70.9977 101.2 125.4 146.90.968 109.3 136.3 161.3

0.974 109.1 136.2 160.8 0.9977 101.2 125.7 144.20.9978 99.4 124.0 145.50.985 106.4 131.6 156.2

0.987 100.8 125.2 148.1 0.998 96.1 119.4 139.90.9984 95.1 117.4 137.30.990 94.1 118.1 139.0

0.991 92.0 113.1 131.4 0.9986 94.7 115.8 133.40.991 92.9 115.4 135.20.993 92.1 111.4 127.80.994 91.0 109.7 127.5

lume and temperature function of component i.kij

and lij

are the binary interaction parameters,which must be tuned on experimental data.where v is the acentric factor.

For all systems considered, the pure equation-As the esters used in this study decomposeof-state parameters are given in Table 5. Table 6before reaching the critical temperatures, the pseu-lists the k

ijand l

ijvalues, and the mean deviationsdocritical properties were estimated through the

DP% obtained from the correlation of experimen-Ambrose group contribution method [15,16 ],tal data.using the normal boiling point calculated from the

AMP correlation [17,18]. The acentric factor, v,was estimated from the Edminster equation.

4. DiscussionFor a mixture of p components, the quadraticclassical mixing rules, with respect to mole frac-

4.1. CO2/methyl oleate mixturetions x

i, were used:

As mentioned above, the CO2/methyl oleate hasa=∑i

p∑j

pxixjEa

iaj(1−k

ij) (6)

been the subject of several other papers [7–11],using an analytical technique, involving long

b=∑i

p∑j

pxixj

bi+b

j2

(1−lij

). (7) periods and consumption of a large amount ofproduct, whereas our method should provide fastmeasurements and a low consumption of solute.x

i, b

i, a

iare, respectively, the mole fraction, covo-


(a) (b)

Fig. 3. Comparison of the experimental data obtained with previously published data and with the correlation for the system methyloleate/SC-CO2 at 313.15 K.

Figs. 3–5 compare the results obtained with our studies of Zou et al. [7], Inomata et al. [8], Yuet al. [9], and Cheng et al. [10]. The whole set ofapparatus at different temperatures with literature

data. Figures labelled ‘a’ describe the whole range data, including P–x and P–y measurements, isreported in each figure.of composition, whereas those labelled ‘b’ focus

on compositions ranging from 0.95 to 1. The It can be seen that at 323.15 and 333.15 K, ourdata are in good agreement with those alreadyfigures are plotted using the modified Peng–

Robinson equation of state using parameters k12 published. It is difficult to make measurementsvery near to the critical point, and a certainand l12 obtained from the correlation of the whole

set of literature data. It should be noted that amount of scattering is observed. This is probablywhy, except for Zou et al., few data have beenparameters k12 and l12 were tuned only on P–x

data and considering measurements from the provided by the other authors. Due to this diffi-

(a) (b)



(a) (b)


culty, slight differences appear between our data the fact that the mean deviation DP% is almostthe same for the global correlation and the correla-and those of Zou. Nevertheless, these differences

in pressure never exceed 1 MPa. tion of our data shows a good agreement betweenthe whole data. In particular, the model agreesAt 313.15 K, the published data are more

numerous, and once again, our data fit correctly very well with the data of Inomata et al. and thosefrom this work, even for P–y measurements.with those of the literature. The only discrepancy

existing concerns the data of Cheng et al. However,it must be pointed out that, according to these 4.2. Other estersauthors, their data yield equilibrium curves having‘a characteristic waisted shape’ which is not found For systems involving ethyl myristate, palmitate

and stearate, the experimental data are reportedin any of the other papers cited.Concerning the adjustment of the parameters, in Tables 2–4, and shown in Figs. 6–8. For the

Fig. 6. Equilibrium curve of the system ethyl myristate/ Fig. 7. Equilibrium curve of the system ethyl palmitate/SC-CO2 at 313.15, 323.15, and 333.15 K.SC-CO2 at 313.15, 323.15, and 333.15 K.


Table 5Equation-of-state parameters for the pure components

Components Tb ( K) Tc ( K) Pc (bar) v

CO2 304.21 73.8 0.225Methyl oleate 619.46 774.09 12.783 0.8982Ethyl myristate 583.86 743.41 14.548 0.8062Ethyl palmitate 609.59 765.18 13.152 0.8695Ethyl stearate 631.07 781.54 11.981 0.9281

system CO2/ethyl stearate, our results are com-pared with those already published by Bharathet al. [19]. We have the same observation as thatfor the system CO2/methyl oleate: our data are ingood agreement with those of Bharath et al. except

Fig. 8. Equilibrium curve of the system ethyl stearate/SC-CO2very near to the critical point.at 313.15, 323.15, and 333.15 K.For all the esters considered here, the data were

correlated using the model previously described[Eqs. (1)–(7)]. The following points must be For the ethyl stearate/CO2 system, we also

needed two parameters to fit the data. We cannoted.For the system ethyl myristate/CO2, it was note the good agreement between the data and

the model.clearly possible to use only one parameter kij

because the slope of the curve was weak; therefore, From these results, several conclusions may bedrawn, which are in agreement with conclusionsthe data were correlated using a linear expression

for the covolume [Eq. (7)]. The model fits very from other similar studies:$ Irrespective of the temperature and the esterwell with these data.

For the ethyl palmitate/CO2 system, it was studied, the solubility of the heavy compoundin SC-CO2 increases with pressure. In so far asnecessary to tune two parameters; indeed, the slope

of the curve is stronger. However, the agreement large modifications of the pressure induce slightvariations of the ester solubility in the vaporis better for low pressures.

Table 6Results of the correlation

313.15 K 323.15 K 333.15 K References

k12 l12 DP% k12 l12 DP% k12 l12 DP%

Methyl oleateGlobal correlation 0.0521 0.0226 0.0529 0.0253 0.0573 0.0352

5.01 4.27 4.74 This work3.26 3.23 4.46 [11]6.60 8.07 [9]4.87 3.91 [12]5.20 [13]

Correlation of our data 0.0502 0.0208 4.73 0.0525 0.0252 4.27 0.0549 0.0341 3.86 This workEthyl myristate 0.0502 0.0000 6.89 0.0495 0.0000 6.52 0.0502 0.0000 6.03 This workEthyl palmitate 0.0525 0.0200 4.47 0.0524 0.0200 4.84 0.0524 0.0200 5.59 This workEthyl stearateGlobal correlation 0.0537 0.0283 6.22 0.0550 0.0283 5,95 0.0537 0.0283 5.88 This work

6.91 6.21 7.37 [19]


phase, some scattering of the experimental Referencespoints may be observed for this part of theisotherm. However, despite the difficulty of [1] A. Staby, Application of Supercritical Fluid Techniques on

Fish Oil and Alcohols, Thesis of the Technical Universityexperimentation in this range of composition,of Denmark, Lyngby, 1993.the trend of the curves obtained fits correctly

[2] A. Hirai, T. Terano, H. Saito, Y. Tamura, S. Yoshida,with the data previously published. Clinical and epidemiological studies of eicosapentaenoic

$ For a family of homologous compounds (here, acid in Japan, in: W.E.M. Lands (Ed.), Proc. AOAC(1987) 9.linear long-chained saturated esters), and at a

[3] W.E. Connor, S.L. Connor, Adv. Intern. Med. 35 (1990)fixed temperature and pressure, the larger the139.chain length, the lower the solubility of the ester

[4] J.M. Kremer, D.A. Lawrence, W. Jubiz, R. Di Giacomo,in the vapor phase. R. Rynes, L. Bartholomew, M. Sherman, Arthrit. Rheu-

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[8] H. Inomata, T. Kondo, S. Hiroharna, K. Arai, Y. Suzuki,M. Konno, Fluid Phase Equilibria 46 (1989) 41.

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Phase Equilibrium: Science and Technology, AmericanSociety, Washington, DC, 1989, p. 86.An experimental apparatus has been designed

[11] W.B. Nilsson, E.J. Gauglich, L.K. Hudson, JAOCS 68to perform measurements of equilibria involving(1991) 87.fatty acids esters and CO2. The synthetic method

[12] G. Brunner, Gas Extraction: An Introduction to Funda-was chosen for this study, though the set-up could mentals of Supercritical Fluids and the Application to Sep-operate in the analytical mode. aration Processes, Springer, New York, 1994.

[13] M. McHugh, V. Krukonis, Supercritical Fluid Extraction:Though uncertainties are not negligible, severalPrinciples and Practice, 2nd edition, Butterworths, Boston,advantages were gained with this method. AsMA, 1994.sampling is avoided, it offers an easy use. The

[14] A. Pdneloux, E. Rauzy, Fluid Phase Equilibria 8 (1982) 7.characteristics of the optical cell, combined with [15] D. Ambrose, Correlation and estimation of vapor–liquidthe specificity of the synthetic method, enable the critical properties. I. Critical temperatures of organic com-

pounds. NPL Rep. Chem. 92, National Physical Labora-collection of numerous reproducible data, fittingtory, Teddington, UK, 1978.correctly with previously published data, and in a

[16 ] D. Ambrose, Correlation and estimation of vapor–liquidrelatively short time.critical properties. I. Critical pressures and volumes of

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cal Laboratory, Teddington, UK, 1979.the whole range of composition. Because of both[17] A.B. Macknick, J.M. Prausnitz, Ind. Eng. Chem. Fundam.the small volume of the cell and the useless of

18 (1979) 348.sampling, the experiments we carried out, did not[18] A.B. Macknick, J. Winnick, J.M. Prausnitz, Ind. Eng.

need more than 30 g of each ester. These character- Chem. Fundam. 16 (1977) 392.istics should be attractive in the case of studies [19] R. Bharath, H. Inomata, K. Arai, Fluid Phase Equilibria

50 (1989) 315.devoted to expensive compounds or urgent studies.

Documents

High-pressure apparatus for phase equilibria studies: solubility of fatty acid esters in supercritical CO2