Correlating the Solubilities of Used Frying Oil in High-Pressure

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Correlating the Solubilities of Used Frying Oil in High-Pressure Propane, Carbon Dioxide, and Ethane Jesusa Rinco´n,* Rafael Camarillo, Fabiola Martı´nez, Luis Rodrı´guez, and Virginia Ancillo Departamento de Ingenierı´a Quı´mica, Facultad de Ciencias del Medio Ambiente, UniVersidad de Castilla-La Mancha, AVenida Carlos III, s/n 45071 Toledo, Spain

Solubility of used frying oil in three compressed gases (propane, carbon dioxide, and ethane) has been determined using a dynamic flow method. The aim of the work has been to analyze and compare the solubilities in such gases in order to determine the best processing conditions leading to selective separation of nondegradated triglycerides from polar components. A second objective has been to obtain empirical correlations to predict these solubility data for further scale-up applications. The effect of pressure and temperature on the solubility has shown that the variable increases as solvent density and/or solute vapor pressure increase. Best results have been attained using ethane at 29.4 MPa and 25 °C. At such conditions the solubility of the oil was high (17 g/(kg of ethane)), being triglycerides its major constituents (83.2% by weight), and only 0.4 and 16.4%, respectively, the weight percentages of HMWC and LMWC. HMWC and LMWC refer to polar compounds with molecular weight higher and lower than that of triglycerides. Finally, it has been found that the Chrastil’s model can successfully estimate the solubilities of used frying oil lipids in nonpolar solvents such us propane, carbon dioxide, and ethane. 1. Introduction Frying oils are purified fats from plant origin, such as olive oil, sunflower oil, soybean oil, corn oil, cotton oil, canola oil, peanut oil, cocoa oil, and palm oil, etc. Their chemical composition varies with the type of oil, although, in general, they are made of 98% triacylglycerol or triglycerides, the remaining 2% being unsaponificable matter and phospholipids.1,2 Frying processes operate at elevated temperature (180 °C) in the presence of air oxygen and humidity. It provokes a complex series of changes and reactions that alter the oil both physically and chemically. The principal reactions occurring during frying include formation of conjugated dienes, formation and decomposition of hydroperoxides, formation of low molecular carbonyl compounds, hydrolysis of triglycerides, and polymerization via complex free radical processes.3-5 As a consequence of these transformations, after a time of use frying oils should be discarded because altered lipids degrade the quality of fried foods, becoming a waste. But it should be noted that this waste still has a large portion of nondegradated triglycerides that can be used as a raw material in different added value manufacturing industries such as surfactants, cosmetics, paintings, biodegradable lubricants, biodiesel, and fodder, etc.6 Moreover, the rest of the compounds could be reevaluated by means of incineration, with recovery of calorific power.6 The separation of nondegraded triglycerides from undesirable oxidized polar compounds may be accomplished by using supercritical fluid technology. In fact, several authors have addressed the study of the triglycerides recovery from used frying oil with supercritical CO2. Thus, Yoon and co-workers analyzed the triglycerides separation in a batch extractor.5 Later on the process was performed continuously in a mixer-settled unit6 and in a continuous tower.7 More recently, the use of cosolvents to increase the solvent power of CO2 has also been investigated.8 Very promising results have been reported in these earlier works. * To whom correspondence should be addressed. E-mail: [email protected].

However, considering that hydrocarbon solvents such as ethane9-12 and propane9,13,14 have been reported to be better solvents for lipids than CO2, in this work the solubilities of used frying oil in CO2, propane, and ethane, as well as the compositions of the oil recovered with each solvent, are analyzed and compared in order to determine the best processing conditions leading to the selective separation of the nondegradated triglycerides and polar components that make up the used frying oil. Moreover, with a view to the industrial application of the process, several empirical models15 have been proposed to correlate the solubility data experimentally attained. These models accurately describe the dependence on pressure and temperature of the used frying oil solubility in the different solvents analyzed and can be used with confidence in the mathematical optimization of the industrial processes. 2. Solubility Data Analysis Thermodynamic modeling of solubility in dense fluids is difficult for vegetable oils due to the fact they are multicomponent mixtures. In addition, for the majority of oil components there is a lack of information about physical properties and/or critical and intermolecular energy parameters data requested for equations of state.16 For this reason, empirical models describing solubility as a function of solvent density (pressure and temperature, implicitly) are successfully used. Chrastil’s model15 is based on the hypothesis that each molecule of a solute associates with k molecules of supercritical solvent to form a solvato complex which is in equilibrium with the system. Equation 1 establishes a linear relationship between natural logarithms of solubility of a solute in the high-pressure solvent (S, g/L) and density of pure solvent (F, g/L) as follows: ln S ) k ln F + a +

b T

(1)

In this equation, “k” (association number) is the number of molecules in the solvato complex and represents the density dependence of solubility, “a” is a constant dependent on the

10.1021/ie1015766  2011 American Chemical Society Published on Web 12/03/2010

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Scheme 1. Flow Diagram for the Semicontinuous Extraction Setup

molecular weights of the solute and solvent and the association number, and “b” is a constant dependent on the total heat of reaction and it is related to the temperature (T, K) dependence of solubility at constant density. The concentration and temperature range of the model is determined by the validity range of its approximations. Deviation from the linear relation is expected at high solute concentrations in fluid phase as the solvent density starts to deviate from the density of pure solvent. The solute polarity has also been reported to affect the accuracy of the model. The model was not accurate for highly polar solid solutes up to a reduced density of 1.5, indicating density dependence of the association number.9 In the literature, the units used for solute concentration and density vary, so the regressed constants for eq 1 are dependent on the units used to obtain them.15 It should also be noted that Chrastil-type equations are dimensionally inconsistent. This inconsistency does not represent an impediment to use of the equation as long as the units of each variable are clearly specified. Avoiding this inconsistency, however, has advantages. One way to do this is by using dimensionless variables, such as those shown in eq 2 (which is eq 1 rewritten in dimensionless form): ln S' ) k' ln Fr + R +

β Tr

(2)

where S′ ) S/Fc; Fr ) F/Fc; Tr ) T/Tc; and Tc and Fc are the critical temperature and density of solvent. Parameters in eq 2 are related to those in eq 1 by the following expressions: k' ) k

(3)

R ) a + (k' - 1) ln Fc

(4)

β ) b/Tc

(5)

In this work model parameters k′, R, and β have been estimated from experimental data of the used frying oil solubility using the multivariate regression analysis of the Solver tool from Microsoft Office Excel. Standard deviation (s) and relative

quadratic error (e) of the model predictions have been calculated using the following expressions:

s)




∑ (y

e)




∑ (y - y ) ∑y

e

- yc)2

N-1

(6)

2

e

c

(7)

e

where ye, yc, and N are experimental and calculated values of the dimensionless solubility, and the number of experiments used in the correlation, respectively. 3. Experimental Procedures 3.1. Materials. Refined sunflower oil provided by Diasol (Borges) was heated for 14 h at 195 °C. Afterward, degradated oil was stored in hermetic bottles, in a N2 atmosphere, in the dark and at a temperature of -1.0 °C to avoid oxygen, light, and humidity. Mass percentages of fresh frying oil constituents were 92.7% triglycerides, 3.9% LMWC, and 3.4% HMWC, while those of used frying oil were 70.1% triglycerides, 16.9% LMWC, and 13.0% HMWC. Liquid CO2 (purity, 99.99%), propane (purity. 99.5%), and ethane (purity, 99.4%) were supplied by Praxair S.A. Petroleum ether (Panreac), diethyl ether (Panreac), silica gel (Merck), sea sand (Panreac), diisopropyl ether (Fisher Scientific), oleyl-rac-glycerol (Sigma Aldrich), tetrahydrofuran (Panreac), and cyclohexane (Panreac) were used for the oil characterization. 3.2. Apparatus and Extraction Procedure. The experiments were developed in a semicontinuous apparatus, as shown in Scheme 1.17 Liquid solvent from a stainless steel cylinder (SC) was cooled (CS), filtered (F), and compressed by a membrane pump (P) (HG-140, LEWA). The pressure was regulated by a back-pressure regulator (BPR) (BP-66, GO Regulator) and checked by a manometer (M1). The compressed fluid was passed through the extractor (EX), a 940 cm3 stainless steel cylinder loaded with used frying oil (25-50 g). To keep the extractor temperature at the desired value, it was submerged

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Table 1. Physicochemical Properties of Studied Solvents (from References 19 and 20) and Triolein (from References 21 and 22)

formula MW (g/mol) Pc (MPa) Tc (°C) Fc (g/L) melting point (°C) boiling point (°C)

propane

CO2

ethane

triolein

C3H8 44 4.26 96.9 217 -152.3 -42.1

CO2 44 7.38 31.1 469 -78.5 -56.6

C2H6 30 4.94 32.1 203 -183.3 -88.6

C57H104O6 884 0.46 770.2 -4 to -5 235 to 240

into a thermostatic bath with mineral oil. The oil-laden gas from the extractor was passed through a heated metering valve (MV) where the pressured solvent was expanded, and the extracted oil and solvent were separated. The extract was collected in a receiver (RE) at ambient temperature. The gas flow through the extractor (4 L/min) was measured with a rotameter (R). Extracted oil recovered was gravimetrically quantified and properly stored for further analysis. For each solvent, the accuracy of the experimentally determined extraction yields has been determined by comparing the results attained, after a given extraction time (4 h), from four independent runs carried out under identical conditions (CO2, pressure of 34.3 MPa, temperature of 40 °C, gas flow of 4 L/min, and oil load of 25 g; ethane, 19.6 MPa, 40 °C, 4 L/min, and 50 g; and propane, 4.9 MPa, 40 °C, 4 L/min, and 50 g). The extraction yields obtained were almost similar (differences observed were about 1% in all cases), indicating that reproducibility of the data was good. Nevertheless, to minimize experimental errors all runs were replicated twice. 3.3. Analytical Procedure. The extract composition was determined using the method proposed by Dobarganes et al.,18 whose precision was previously evaluated and found acceptable.8 For the separation of polar and nonpolar fractions of the extracted samples a thin layer chromatography (TLC) and further gravimetric determination were used. For the analysis of the polar fraction, a high-performance size exclusion liquid cromatograph (HPSEC; Waters 1515 isocratic HPLC pump) equipped with refractive index detector (Waters 2410) was used. The sample was dissolved in tetrahydrofuran (THF; final concentration, 15 mg/L). Two different aliquots of each sample were injected into the HPSEC. The flow rate of the mobile phase (THF) was 1 mL/min. HP PIgel, 5 µm, 100 Å and HP PIgel, 5 µm, 500 Å columns were connected in series and used for the analysis. The detection of polymers and LMWC was made by refractive index detector at 35 °C. The compositions of HMWC and LMWC in the samples were determined by calculating the relative peak areas of each peak. 4. Results and Discussion Before analyzing the effect of pressure and temperature on used frying oil solubility, preliminary experiments were performed to determine the maximum values of the operational variables (solvent flow rate and initial oil load in the extractor), ensuring that no oil dragging occurred during the extraction runs. Results obtained are reported in the following section. On the other hand, Table 1 summarizes some useful physicochemical properties of studied solvents.19,20 Regarding the used frying oil, considering its variable and complex composition,3,4,18 only the properties of triolein (a model compound selected as representative of triglycerides, the major fraction of used frying oil) are supplied.21 4.1. Previous Tests. Two series of experiments with each solvent were completed. In the first series, some operation conditions were constant: propane (pressure, 4.9 MPa,; tem-

Figure 1. Experimental (symbols) and estimated (Chrastil’s model, lines) solubilities for used oil in propane, carbon dioxide, and ethane.

perature, 40 °C), ethane (19.6 MPa; 40 °C), or carbon dioxide (34.3 MPa; 40 °C), and solvent flow of 4 L/min. The length of the experiments was proportional to the initial oil load in the extractor (40, 100, 200, 300, and 400 min for experiments with 10, 25, 50, 75, and 100 g, respectively). For all solvents, a similar trend in yield (mass of extracted oil/mass of used oil) versus initial oil load in the extractor was observed (not shown). This yield was constant until a certain oil load (50 g for propane and ethane and 25 g for carbon dioxide), and increased at oil loads surpassing such values, a fact that may be imputed to oil dragging. This hypothesis was confirmed by measuring the polar contents of the extracts, since this parameter was constant and far from the value of untreated used frying oil (29.9% polar compounds) when the initial oil loads were below the limits indicated above and, however, increased approaching the used oil polar content value (29.9%) when the loads surpassed such limits. Regarding the solvent flow rate, a second series of experiments was completed. In this case, pressure and temperature had the same values as those in the first series of previous experiments for each solvent. The values of the initial oil load in the extractor were those optimized in the previous step for each solvent (50 g for propane and ethane, and 25 g for CO2). The solvent flow rates tested were (6, 4, 2, and 1 L/min) and the extraction times of these experiments (100, 150, 300, and 600 min, respectively). Experimental data collected showed trends similar to those for oil loading. Extraction yields and polar content of the extracted oil were constant for the smaller solvent flow rates, up to a limit flow from which both dependent variables (yield and polar content) increased with increasing solvent flow rates. In other words, for all three solvents it was found that the maximum solvent flow rate to be used without oil dragging was 4 L/min. From these results, the initial oil loads of 25 g (for CO2) and 50 g (ethane and propane) and the flow rate of 4 L/min were established for performing subsequent extraction experiments. 4.2. Solubility of Used Frying Oil in Propane. The solubility of used oil was investigated in the temperature and pressure ranges from 25 to 110 °C and from 3.9 to 5.9 MPa, respectively, using the dynamic flow-type technique.5,16 Figure 1 shows the natural logarithms of the experimental solubility of used frying oil in propane in dimensionless form (divided by the critical density of propane, 217 g/L)23 versus natural logarithms of the solvent reduced densities for the nine experiments developed with propane.

Ind. Eng. Chem. Res., Vol. 50, No. 2, 2011 Table 2. Values of Constants Calculated for Solubility Modeling of Frying Oil in Propane, Carbon Dioxide, and Ethane, with Standard Deviations and Relative Quadratic Error solvent

k′

R

β

s

e

propane CO2 ethane

8.42 4.31 6.45

-5.28 -6.64 -6.63

-3.14 -0.42 -1.68

0.0036 0.0035 0.0027

0.01 0.03 0.02

It can be inferred from this figure that the solvation power of liquid propane (0.8 < ln Fr < 0.9, T e Tc ) 96.9 °C, and P e Pc ) 4.26 MPa) is clearly higher than that of supercritical propane (0.4 < ln Fr < 0.6, T g Tc, and P g Pc), since the values of natural logarithms of dimensionless solubility for the first one are less negative. Regarding the effects of pressure and temperature, it has been observed that solubilities decrease isobarically with temperature but increase isothermally with pressure. Because the variation of the solvent density also follows this trend, it is obvious that results presented are related to the value of this parameter. Moreover, the fact that the liquid propane density (479-507 g/L) was noticeably higher than that of supercritical propane (329-376 g/L)23 within the range essayed would explain the higher solubilities attained in liquid propane. The values for the fitting constants used in the Chrastil’s solubility modeling for propane are shown in the first row of Table 2. All constants have been calculated by minimizing s and e between experimental and calculated dimensionless S′. In Figure 1, the solubility predictions obtained from the empirical correlation has been compared to the experimental results. It can be observed that Chrastil’s empirical model (lines) fits the experimental results with exactitude. 4.3. Solubility of Used Frying Oil in CO2. In the case of CO2, the operation temperature and pressure ranges were 25-80 °C and 29.4-39.2 MPa, respectively. Figure 1 shows the changes in dimensionless used frying oil solubility in CO2 as a function of the solvent reduced density. These experimental values have also been fitted to the Chrastil’s model (lines). It can be seen that the tendencies are similar to those for propane. Thus, solvation power of liquid CO2 (0.72 < ln Fr < 0.77) is higher than supercritical CO2 (0.45 < ln Fr < 0.72), which may be explained in the light of the effect of pressure and temperature on carbon dioxide density,24 even though we should mention some particularities. First, solubilities are smaller than those for propane, which is coherent with a lower capacity of CO2 to dissolve triglycerides.9,13,14 Moreover, unlike propane, solubilities do not exhibit discontinuity between liquid and supercritical regions, because, also unlike propane, densities for liquid (967-1005 g/L) and supercritical states (746-957 g/L)23 are close. Regarding the Chrastil’s model fitting, a good agreement between experimental and estimated values is observed again (Figure 1). The values obtained for the equation parameters, as well as the standard deviation and relative quadratic error, are shown in row 2 of Table 2. 4.4. Solubility of Used Frying Oil in Ethane. Finally, for ethane, operation temperature and pressure ranges were 25-80 °C and 14.7-29.4 MPa, respectively. Figure 1 shows experimental data together with lines obtained from Chrastil’s model. It can be observed that ethane has an intermediate capacity to dissolve used frying oil, being much lower than propane but higher than CO2. As expected, this result agrees with data reported in the bibliography9-14,16 for oil, triglycerides, the major components of used frying oil (70.1% by weight), and even mineral lubricant oil.25,26 In this case, the

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solvation power of liquid ethane (0.69 < ln Fr < 0.77) is not always higher than supercritical ethane (0.49 < ln Fr < 0.73), a tendency previously observed with both propane and carbon dioxide. This result is partly because the densities of liquid ethane in some experiments (408 and 424 g/L, at 14.7 MPa, 25 °C and 19.6 MPa, 25 °C, respectively) are very close to those of supercritical ethane23 (402 and 419 g/L, at 19.6 MPa, 40 °C and 29.4 MPa, 40 °C, respectively). Since the solubility of the substances does not depend only on the solvent density (decreasing with temperature at constant pressure) but also on the vapor pressure of the solutes (increasing with temperature at constant pressure), it seems to be that in these experiments with ethane the second effect prevails over the first one.21,24 Finally, it can be observed that the solubility values estimated with Chrastil’s model are in good agreement with experimental results (Figure 1), he model parameters being tthose appearing in row three of Table 2. 4.5. Further Comments on the Used Frying Oil Solubility in the Three Solvents. To determine the effect of working in the vicinity of the critical point27 on the used frying oil solubilities, a further analysis of the solubility in terms of the reduced density of the solvents is also carried out. Thus the solubilities of the oil considering as reference the solubilities in carbon dioxide have been calculated within a solvent reduced density range between 1.25 and 2. It is observed that oil solubilities at the lowest reduced density (Fr ) 1.25) are similar in the three solvents, whereas at the highest reduced density (Fr ) 2) the solubility of used frying oil in propane and ethane is, respectively, 3 and 1.3 times higher than that in CO2; i.e., the relative effect of the hydrocarbon solvents in the vicinity of the critical point is smaller than at higher distances. Taking into consideration these results, as well as the critical constants for the three solvents (Table 1), it can be stated that hydrocarbon fluids (especially propane) dissolve better than CO2 used frying oil at all conditions (severe and milder). This should be imputed to the nonpolar nature of both the major portion of the oil (about 70% are nonpolar triglycerides) and hydrocarbon solvents. With regard to the values of the Chrastil’s model constants found for propane, compared to the other solvents, it can be seen (Table 2) that the magnitude order of the association number (k′) for propane is higher than those for ethane and CO2 (in this order), in accordance with the oil solubility trend previously described. With reference to the β parameter, it has a negative value for all solvents, indicating an inverse relationship between the natural logarithm of S′ and 1/T or an increase in the solubility with temperature at constant density, which only can be due to an increase in the vapor pressure of solutes.9 Finally, parameter R does not follow any regular tendency. Some papers have examined the values for constants in Chrastil-type equations describing the solubility of lipids in supercritical solvents. Most of them report results for multicomponent systems comprising different kinds of oils and highpressure CO2, which is the most studied solvent.9,16 Gu¨c¸lu¨¨ stu¨ndag and Temelli9 studied and correlated the solubility U behavior of fatty acids, mono-, di-, and triglycerides, and fatty acid esters in supercritical carbon dioxide with Chrastil-type equations. For triglycerides, as stated above the major used oil component, they obtained values in the range of 4.13-13.14 for association number k′, which are coherent with those obtained in this work. Regarding the values for β, which have been recalculated from parameters for inconsistent Chrastil-type equations appearing in that paper, they were in the range of -6.76 to -1.17 for triglycerides. Thus, the parameter indicative

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Figure 2. Mass percentages of the HMWC fraction in extracts from experiments with different solvents and experimental conditions.

Figure 3. Mass percentages of the LMWC fraction in extracts from experiments with different solvents and experimental conditions.

of the relative effect of temperature on the vapor pressure of solutes is always negative, as it can be observed in this work for all solvents (Table 2). These authors did not find any trend for the third parameter (R). 4.6. Composition of the Oil Recovered with the Different Solvents. To selectively separate the nondegradated triglycerides and polar components that make up the used frying oil, it is important to know not only the used frying oil solubility in the solvents but also the composition of the extracts recovered. For this reason, the mass percentages of polar compounds (HMWC and LMWC) in the extracts attained with each solvent have been determined. Results found are depicted in Figures 2 and 3 for the HMWC and LMWC fractions, respectively. It can be observed that, in general terms, the HMWC fraction is poorly solved in the three solvents (Figure 2), with average percentages in the extracts of about 1% by weight for ethane, 4% for CO2, and 8% for propane. However, the LMWC fraction is more easily extracted from used frying oil than the HMWC fraction, since average compositions in the range of 10-20% by weight are found in all cases (Figure 3). The relative affinity of each solvent for this fraction follows the same order described for HMWC. It would not have been unwise to assume a priori that CO2 should extract both HMWC and LMWC fractions better than hydrocarbons, since this nonpolar solvent has a quadrupolar

moment of -8.32 × 10-26 esu,28 which may facilitate certain interactions with polar substances, such as weak acid-base or electrical forces. Nevertheless, this hypothesis is in conflict with the results for propane, which could be attributed to a possible behavior of HMWC and LMWC fractions more as nonpolar hydrocarbonated chains than as molecules with one or more polar functional groups. In this sense, it should be noted that similar results have been found for other binary systems, such as the study reporting higher solubility of cholesterol in hydrocarbons than in carbon dioxide, in spite of the OH group in its molecule.27 The higher affinity of CO2 for polar compounds, compared to ethane, an effect particularly well seen in the case of the HMWC fraction, could be related to the predominance of attractive dipole-dipole forces over the dispersion forces between nonpolar molecules. Another hypothesis for the highest solubilities of HMWC and LMWC in propane could be that only the major triglyceride fraction was more soluble in this solvent, and this fraction, when dissolved into the supercritical fluid, “pulls” the other ones (above all, solutes with lower molecular weights) due to specific interactions.5,7,21 However, there is some discrepancy about this statement since several authors have also reported that a large presence of triglycerides in the solvent could saturate it and prevent it from dissolving more polar solutes.9,14,24 For one reason or another, this high tendency of propane to dissolve polar fractions becomes a handicap when evaluating its use in the proposed process because the coextraction of triglycerides together with HMWC and LMWC seems unavoidable. To sum up, if we take into consideration both used frying oil solubility (sections 4.2-4.5) and purity of the extracted oil (section 4.6), the optimum solvent would be ethane. At a pressure of 29.4 MPa and a temperature of 25 °C the used frying oil solubility is 17 g of oil/(kg of solvent), its composition being 83.2% triglycerides and 16.8% polar material. Although this composition is halfway between untreated used frying oil (70.1% triglycerides, 29.9% polar material) and fresh oil (92.7% triglycerides, 7.3% polar material), it is closer to fresh oil than that obtained with CO2 (81.1% triglycerides, 18.9% polar material) and propane (76.2% triglycerides, 23.8% polar material) at similar solubility values. It should be noticed that, although the compositions of ethane and CO2 extracts commented above are quite close, the operational conditions leading to the oil solubility indicated (17 g oil/kg solvent) are milder with ethane than with CO2 (29.4 MPa and 25 °C with ethane vs 39.2 MPa and 25 °C with CO2). Finally, it should be mentioned that the composition of ethane recovered oil is good enough for further processing in several industries such as biodiesel production29 or biodegradable lubricant manufacture.30 5. Conclusions Development of high-pressure technology in used frying oil processing requires a good understanding and modeling of the solubilities of the compounds of interest in solvents. In this sense, it has been shown that dimensionless solubilities of frying oil can be related to experimental conditions by means of Chrastil-type empirical equations. Thus, within pressure and temperature ranges essayed, these solubilities increase with isobarical decreases of temperature and with isothermal increases in pressure. This result is associated with the increase in density, although there have been some exceptions confirming that solutility is also a function of the vapor pressure of solutes. Hydrocarbon fluids (especially propane) are better than CO2 for dissolving triglycerides (major oil components). The polar

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fractions were found to dissolve in propane better than in CO2 and ethane, in this order (ethane < CO2 < propane). These results may be imputed to the following: in the case of triglycerides, to the fact that its nonpolar nature plays the most important role on their solubility behavior. With regard to the polar fractions, their higher solubility in propane (compared to CO2) should be imputed to the effect of the nonpolar hydrocarbon chains in its structure prevailing over that of their polar functional groups. In relation to the higher solubility in CO2, compared to ethane, it may be related to the higher intensity of attractive dipole-dipole forces compared to dispersion forces between nonpolar molecules. For all of these reasons, ethane would be the best solvent in this case, since it combines high solubilities for triglycerides with moderate and low solubilities for LMWC and HMWC, respectively. Finally, experimental results show that the selective separation of triglycerides and LMWC is more difficult than the selective separation of triglycerides and HMWC, since an important coextraction of LMWC together with triglycerides occurs with the three solvents. One possible measure to increase the selectivity toward nondegradated triglycerides could be the addition of cosolvents to the main solvent. Acknowledgment We thank Regional Government of Castilla-La Mancha (Project with reference PBI-05-048) for financial support of this work. Literature Cited (1) Souci, S. W.; Fachmann, W.; Kraut, H. Food Composition and Nutrition Tables; Medpharm: Stuttgart, Germany, 1994. (2) Pantzaris, T. P. Palm Oil in Frying of Food; Technomic: Lancaster, PA, 1999. (3) White, P. J. Methods for measuring changes in deep-fat frying oils. Food Technol. 1991, 45, 75. (4) Takeoka, G. R.; Full, G. H.; Dao, L. T. Effect of heating on the characteristics and chemical composition of selected frying oils and fats. J. Agric. Food Chem. 1997, 45, 3244. (5) Yoon, J.; Han, B.; Kong Hwan, K.; Yhung Jung, M.; An Kwon, Y. Analytical, nutritional and clinical methods section: Purification of used frying oil by supercritical carbon dioxide extraction. Food Chem. 2000, 71, 275. (6) Perrut, M.; Majewski, W. Procede de fractionnement d’une huile de cuisson. WO Patent 00/78902 A2, 2000. (7) Sesti-Osse´o, L.; Caputo, G.; Gracia, I.; Reverchon, E. Continuous fractionation of used frying oil by supercritical CO2. J. Am. Oil Chem. Soc. 2004, 81, 879. (8) Rinco´n, J.; Camarillo, R.; Rodrı´guez, L.; Ancillo, V. Fractionation of used frying oil by supercritical CO2 and cosolvents. Ind. Eng. Chem. Res. 2010, 49, 2410. ¨ stu¨ndag, O.; Temelli, F. Correlating the solubility behaviour (9) Gu¨c¸lu¨-U of fatty acids, mono-, di-, and triglycerides, and fatty acid esters in supercritical carbon dioxide. Ind. Eng. Chem. Res. 2000, 39, 4756. (10) Borch-Jensen, C.; Mollerup, J. Phase equilibria of long-chain polyunsaturated fish oil fatty acid ethyl esters and carbon dioxide, ethane,

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ReceiVed for reView July 23, 2010 ReVised manuscript receiVed October 26, 2010 Accepted November 16, 2010 IE1015766