Fractionation of Used Frying Oil by Supercritical CO2 and Cosolvents

2410

Ind. Eng. Chem. Res. 2010, 49, 2410–2418

Fractionation of Used Frying Oil by Supercritical CO2 and Cosolvents Jesusa Rincoń,* Rafael Camarillo, Luis Rodrı´guez, and Virginia Ancillo Departamento de Ingenierıá Quı´mica, Facultad de Ciencias del Medio Ambiente, UniVersidad de Castilla-La Mancha, AVenida Carlos III, s/n, 45071 Toledo, Spain

Supercritical extraction with pure and modified CO2 has been used for the fractionation of waste frying oil at different temperature and pressure conditions (25-80 °C and 300-400 kg/cm2). The cosolvents used to modify the CO2 behavior were ethanol, methanol, acetone, and hexane. They were selected because of their capacity to form hydrogen bonds. Both extraction rate and oil yield were larger under high density conditions (high pressure and low temperature). Further, when cosolvents were used, higher values of both variables were attained at softer operating pressures. Regarding the effect of cosolvents on these variables, it was found to follow this order: ethanol > methanol > acetone > hexane. The relative separation efficiency (RSE) of triglycerides (TG) from polar compounds with molecular weight higher than triglycerides (HMWC) was not greatly dependent on the operating conditions (pressure, temperature, cosolvent type, and cosolvent concentration). However, better values of the separation efficiency of TG from polar compounds with molecular weight lower than triglycerides (LMWC) were attained under low density conditions and using hexane as cosolvent. On the basis of the results relative to extraction rate, oil yield, and separation efficiency, the best extraction conditions were selected to separate a purified TG fraction from used frying oil. They were P ) 300 kg/cm2, T ) 40 °C, cosolvent type ) hexane, and cosolvent concentration ) 0.1 g of hexane/g of solvent. However, when processing the oil at these conditions, the polar content of the lipid fraction recovered still was about twice the fresh frying oil polar content. For this reason, a two-stage supercritical fluid extraction was performed. The composition of the fraction recovered in the two-stage process was very close to that of the fresh oil, a fact that clearly shows that the supercritical extraction with CO2 + hexane is an efficient method for the purification of the TG fraction of waste frying oil. 1. Introduction Frying oils are purified fats from plant origin used in deepfat frying, a popular process throughout the world for cooking many foods. Their compositions vary depending on the vegetable source they originate in (olive oil, sunflower oil, soybean oil, corn oil, etc.) but, in general, they consist of a major portion of triglycerides (about 95%) and small amounts of unsaponificables and phospholipids.1 Used frying oil is the waste that remains after fresh frying oil degradation occurs due to its repeated use in several frying processes. During deep-fat frying, elevated temperature (about 180 °C), air oxygen, and humidity provoke a complex series of reactions, such as hydrolysis, thermal oxidation, and polymerization, that alters the initial oil composition. Thus, used frying oil mainly consists of triglycerides (about 70%) and the products of oil degradation: free fatty acids, mono- and diglycerides, oxidized triglycerides, and oligomeric triglycerides or polymers.2,3 Waste frying oils are generally discarded because oxidized lipids degrade the quality of fried foods. However, since the discarded used-frying oil still has a large portion of nondegraded triglycerides, economic considerations have stimulated the interest in its purification. Considering the characteristics of used oil components, i. e., that triglycerides are nonpolar materials while oil degradation products are polar compounds,2,4 the purification of used frying oil may be achieved by separating the nonpolar and nondegraded triglycerides from the undesirable polar degradation products. The triglycerides fraction could be used as a raw material in different added value manufacturing industries as surfactants, cosmetics, paintings, biodegradable lubricants, biodiesel, fodder, * To whom correspondence should be addressed. E-mail: [email protected].

etc., whereas the rest of compounds could be re-evaluated by means of incineration, with recovery of calorific power.5 One of the technologies that can be applied for the purification of used frying oil is supercritical fluid extraction. Their main advantages are high speed, high selectivity tunable with single changes in operation pressure and/or temperature, energy savings regarding solvent removal, cleanness, and environmental friendliness.6-8 However, we should not forget some drawbacks: costs in gas compression and confinement and smaller solute solubilities in high pressure solvents in comparison to conventional solvents. Nevertheless, the low solubility problem may be solved using solvent modifiers or cosolvents.6,7,9 Some authors have proposed the recovery of nondegraded triglycerides from waste frying oil with supercritical CO2. For example, Yoon and co-workers analyzed the triglycerides (TG) separation in a batch extractor.10 Later on, the process was performed continuously in a mixer-settled unit5 and in a continuous tower.11 Promising results were reported in these earlier works. However, considering that the solvent power of supercritical CO2 is relatively poor and that it can be increased by adding small amounts of cosolvents,7,9 in this work, four cosolvents have been used (ethanol, methanol, acetone, and hexane) to improve the efficiency of the pure solvent (CO2) to separate the triglyceride fraction of the frying oil. Accordingly, the specific objectives of this paper will be to establish (1) the general trends in the extraction rates and yields of the supercritical extraction of waste frying oil with pure and modified CO2, (2) the effect of operating variables on the composition of the oil recovered with pure and modified CO2, and (3) the optimum process conditions for the efficient separation of triglycerides from the products of oil degradation.

10.1021/ie901871w  2010 American Chemical Society Published on Web 02/02/2010

Ind. Eng. Chem. Res., Vol. 49, No. 5, 2010

2411

Figure 1. Flow diagram of the semicontinuos extraction setup.

2. Materials and Experimental Procedures 2.1. Materials. The fresh frying oil was refined sunflower oil provided by Diasol (Borges, Spain). Used frying oil was obtained from fresh oil by heating it for 14 h at 195 °C in a frying machine. According to the literature, the composition of the waste oil obtained by heating is similar to that attained in a true frying process and has the advantage that it does not contain food scraps.3,12 After the heating process, the used frying oil was stored in hermetic bottles, in a N2 atmosphere, in the dark, and at a temperature of 0 °C to avoid further oil degradation because of oil contact with oxygen, light, and humidity. Liquid CO2 (purity 99.99%) was supplied by Praxair S.A. (Spain). Cosolvents methanol, ethanol, acetone, and hexane, all of them HPLC grade, were supplied by Sigma Aldrich (U.S.A.). Petroleum ether (Panreac, Spain), diethyl ether (Panreac, Spain), silica gel (Merck, U.S.A.), sea sand (Panreac, Spain), diisopropyl ether (Fisher Scientific, U.S.A.), oleyl-rac-glycerol (Sigma Aldrich, U.S.A.), tetrahydrofuran (Panreac, Spain), and cyclohexane (Panreac, Spain) were used for oil characterization. 2.2. Apparatus and Extraction Procedure. The experiments were developed in the semicontinuous apparatus schematically shown in Figure 1. Liquid CO2 from a stainless-steel cylinder (SC) was cooled (CS), filtered (F), and compressed by a membrane pump (P) (HG-140, LEWA, Germany). The pressure was regulated by a backpressure regulator (BPR) (BP-66, GO Regulator, U.S.A.) 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 g). To keep the extractor temperature at the desired value, this equipment was submerged in a thermostatic bath with mineral oil. The oil-laden gas from the extractor was passed through a heated metering valve (MV) where the compressed solvent was expanded. The extracted oil was collected in a receiver at ambient temperature (RE1), and the cosolvent was collected in a second receiver at 0 °C (RE2). The gas flow through the extractor (4 L/min) was measured with a rotameter (R). Extracts recovered in receiver RE1 were heated at 60 °C for 60 min in order to eliminate any cosolvent that could contaminate the oil. The oil collected was gravimetrically quantified and properly stored for further analysis. In experiments with cosolvents, in

order to ensure that the cosolvent was always present in the supercritical phase, all extraction tests were performed at pressure and temperature conditions above the critical point of the modified solvent.13 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: pressure ) 350 kg/cm2, temperature ) 40 °C, CO2 flow ) 4 L/min, and oil load ) 25 g. In these experiments, the extraction yields were similar (37.5, 37.0, 37.1, and 37.3%) indicating that reproducibility of the data was good. Nevertheless, to minimize experimental errors, each run was replicated twice. 2.3. Analytical Procedure. The analysis of the extract composition was performed according to the analytical method proposed by Dobarganes and co-workers.14 First of all, the precision of the method was evaluated by performing 10 replicated measurements of a fresh oil sample. Average and standard deviation values were 7.3% and 0.4% for polar compounds, 3.4% and 0.3% for polar compounds with molecular weight higher than triglycerides (HMWC), and 3.9% and 0.3% for polar compounds with molecular weight lower than triglycerides (LMWC). Briefly, the method may be described as follows. First, the separation of polar and nonpolar fractions of the extracted samples was accomplished by silica column chromatography. Then, for the analysis of the polar fraction, a high performance size exclusion liquid chromatograph (Waters 1515 isocratic HPLC pump, U.S.A.) equipped with a refractive index detector (Waters 2410,U.S.A.) was used. The sample was dissolved in tetrahydrofuran (THF) (final concentration 15 mg/ mL). Two different aliquots of each sample were injected into the HPSEC. The flow rate of mobile phase (THF) was 1 mL/ min. HP Plgel 5 µm 100 Å and HP Plgel 5 µm 500 Å columns were connected in series and used for the analysis. The detection of polymers and LMWC was made by a refractive index detector at 35 °C. The composition of the HMWC and LMWC in the samples was determined by calculating the relative peak areas of each peak. Reported experimental values of HMWC and LMWC concentrations for each sample are the average between the two aliquots measured by HPSEC.

2412


Figure 3. Percentages of polar compounds in the oil extracted with pure CO2.

Figure 2. Extraction yields attained with pure CO2.

3. Results and Discussion The used frying oil composition is very complex.2,3 It contains undegraded triglycerides and a great variety of polar compounds (polymers, dimers, diglycerides, oxidized glycerides, fatty acids, aldehydes, ketones, etc.) resulting from deteriorative reactions (including thermal oxidation, hydrolysis, and polymerization) that occur during the frying process. However, some authors have considered it to be a mixture of three pseudocomponents:10,11 Triglycerides and polar compounds with higher and lower molecular weight than triglycerides. On this basis, used frying oil may be treated to obtain a rich triglyceride fraction, with similar composition to fresh frying oil, by separating the polar components from triglycerides. Therefore, the aim of the fractionation will be to obtain a highly pure triglyceride fraction with high yield. In the following, the extraction yields and the characteristics of the fractions recovered with pure and modified CO2 under various process conditions are discussed. Later on, the best extraction conditions to perform the extraction are selected and the composition of the lipid fraction recovered is compared to that of fresh frying oil. 3.1. Extraction Yields with Pure CO2. The cumulative yields of collected oil during the supercritical fluid extraction of used frying oil with CO2 under various operating conditions are shown in Figure 2. At all pressures and temperatures investigated, it can be observed that the extraction yields increase with time until reaching an asymptotic value that depends on the operating conditions. With liquid CO2, the asymptotic yield is around the 70-75%, achieved approximately after 8 h of extraction. With supercritical CO2, the asymptotic yield is lower, reached later, and depends more on the operating conditions. The former two findings should be attributed to the higher densities of the liquid solvent, as compared to those of the supercritical solvent, while the last one may be imputed to that, in the experimental range analyzed, the liquid fluid density range (967-1005 kg/m3) is smaller than that of the supercritical solvent (746-957 kg/m3), as calculated from ref 15.

Results in Figure 2 also show that the extraction rate is constant at the early stages of extraction and, then, decreases gradually as the extraction proceeds. Most probably, it is due to the solubility difference between low and high molecular weight compounds (the former extracted with preference at the beginning of the extraction, as it will be shown below) and the lack of solute at later stages of extraction.6,7 The effect of operating conditions on extraction yields and extraction rates is also presented in Figure 2. It can be observed that, at a given temperature, the higher the pressure, the higher is the extraction yield, and at a given pressure, the lower the temperature, the higher is the yield. Also, it can be appreciated that the extraction rate under low density conditions (low pressure and high temperature) was lower than under high density conditions (high pressure and low temperature). These results are in good agreement with those previously reported for similar systems5,10,11,16,17 and, as usual, they may be imputed to the effect of pressure and temperature on fluid density (and, therefore, on fluid solvent power, oil solubility, and extraction yield). Thus, at constant pressure, solvent density, oil solubility, and extraction yield decrease as temperature rises. On the other hand, isothermal increases of pressure lead to higher solvent densities, solubilities, and yields. Nevertheless, it should be recalled that oil solubilities (and yields) in compressed gases depend not only on solvent density but also on solute vapor pressure,6,7,16,17 although this effect does not seem to significantly affect the oil solubility and extraction yield in the experimental range analyzed in this work. Finally, it should be pointed out that the solubilities of the recovered lipids (calculated from the straight slope of Figure 2 extraction curve at the beginning of the process) varied from 4.0 to 14.1 g of lipid/kg of CO2, depending on the extraction conditions used. These values fairly agree with those reported in the literature for TG solubility in CO2.10,16,18,19 3.2. Characteristics of the Fractions Recovered with Pure CO2. Similarly to the fresh frying oil, the lipid fraction recovered from the used frying oil consists of apolar tryglicerides and a polar fraction. In Figure 3, the mass percentages of polar compounds in the extracts obtained with pure CO2 are shown for the different experimental conditions analyzed. As it can be observed, although very smoothly, this percentage tends to decrease with time. In principle, the selective extraction of polar compounds with an apolar solvent like CO2 can be explained if the molecular


Figure 4. Composition of the oil extracted with pure CO2.

weight of the extracted polar compounds is smaller than that of apolar triglycerides. In order to prove this hypothesis, the evolution with time of the extract composition was analyzed. Results obtained are shown in Figure 4 for some of the experimental conditions investigated, although similar results were found at the other conditions tested. It can be seen that the mass percentage of LMWC (mainly oxidized tryglicerides, diglycerides, and fatty acids) in the lipid fractions recovered at the beginning of the extraction is slightly larger than in those obtained at the end of the process. On the contrary, the percentages of HMWC (polymers and dimers) follow the opposite trend. Therefore, it may be concluded that these results confirm the suggested hypothesis. Other authors have reported similar results when studying the solubility behavior of ternary systems of lipids in supercritical CO2.17 Figure 3 also shows the effect of pressure and temperature on the percentage of polar compounds in the extracts recovered. It can be appreciated that, regardless the fluid state (liquid or supercritical), the variable augments with isothermal pressure increases and isobaric temperature decreases, i.e., the polar compounds percentage increases with growing solvent densities because the fluid solvent power is larger at higher densities and, therefore, it may solubilize the higher molecular weight polar compounds that lower fluid densities can not solubilize.6-8,20,21 Regarding the effect of pressure and temperature on the composition of the polar fraction, Figure 4 shows that the mass percentage of HMWC slightly rises with isothermal pressure increases, while the LMWC percentage decreases. The opposite trend was found at isobaric increases of temperature. Once more, these results should be imputed to the fluid density and solvent

2413

power and to its variation with pressure and temperature conditions.6-8,20,21 3.3. Extraction Yields with Modified CO2. Earlier studies on the extraction of used frying oil with CO25,10,11 and results presented above demonstrate that polar compounds are inevitably coextracted with TG when the extraction is performed using pure CO2, since the separation efficiency is greatly dependent on extraction conditions. In general, polar compounds and TG are more effectively separated in the low density region (see Figure 3) but, at such low density conditions, the extraction yields and rates are rather low (see Figure 2). Then, in order to improve the extraction efficiency, alternative separation schemes should be investigated. For this reason, and considering that the use of cosolvents to modify the CO2 behavior can be appropriate to produce rich TG fractions from oily residues,5,22-25 we have investigated the effect of adding cosolvents to CO2 on the recovery of the TG fraction from used frying oil. This analysis has not been performed at all the experimental conditions studied with pure CO2 but in a subset of them. Specifically, it has been carried out at those pressure and temperature combinations leading to the smallest fluid densities (i.e., to extracts with the lowest polar content) that, simultaneously, permitted evaluating the influence of operating variables (pressure and temperature) on the cosolvent effects. On the basis of these criteria, the selected conditions were 300 kg/cm2 and 80 °C, 350 kg/cm2 and 80 °C, and 300 kg/cm2 and 40 °C. As regards to the cosolvents analyzed (ethanol, methanol, acetone, and hexane), substances with both different capacity to form hydrogen bonds and proved experience in the supercritical fluid extraction of triglycerides have been selected. Two levels of concentration (0.05 and 0.1 g of cosolvent/g of solvent) have been used according to values found in the bibliography. Figure 5 shows the evolution with time of extraction yields in the extractions performed with the four cosolvents (ethanol, methanol, acetone, and hexane) at different concentrations (0.05 and 0.1 g of cosolvent/g of solvent). The effects of pressure and temperature on extraction yields for the different cosolvents and concentrations analyzed are also shown in the figure. It can be observed that, just as in the pure solvent case, the yields increase with time up to reaching an asymptotic value that is larger or smaller and reached sooner or later, depending on the fluid solvent power (i.e., depending on the operating conditions). Figure 5 also shows that the extraction yields increase with cosolvent concentration, a fact that may be explained by considering the system modifications that cosolvent addition implies. In effect, on the one side, the density of the modified solvent (and, therefore, its solvent power) increases with the amount of cosolvent added7,9,26 and, on the other side, cosolvent addition usually enlarges the interactions between solute and modified solvent because new specific interactions may appear.7,9,27-29 Related to the effect of the different cosolvents, it is shown in Figure 5 that, at both concentrations analyzed, the extractions yields attained with modified CO2 are larger than those corresponding to pure CO2. Concretely, it has been found that the effects of the cosolvents on yields follow this order: ethanol > methanol > acetone > hexane. Obviously, this cosolvent effect should be attributed to specific interactions between their functional groups and those of used frying oil components. Furthermore, by considering the type of solvents used and the composition of used frying oil,2,3 results attained may be imputed to the cosolvent ability to form hydrogen bonds with used oil components, since all cosolvents (but hexane) and used

2414


Figure 5. Extraction yields attained with modified CO2. Table 1. Polarizability and r and β Kamlet-Taft Solvatochromic Parameters of the Cosolvents31 cosolvent

R

β

R*, Angstrong3

methanol ethanol acetone hexane

0.93 0.83 0.08 0

0.62 0.77 0.48 0

5.1 3.3 6.4 11.9

oil compounds (polymers, dimers, diglycerides, oxidized triglycerides, fatty acids, etc.), although to different extents, can participate in hydrogen bonding, either as hydrogen bond donors or acceptors.30 In effect, according to the R and β Kamlet-Taft solvatochromic parameters of the cosolvents31 (Table 1), that respectively represent the hydrogen-bond-donor and acceptor capabilities of the cosolvent, it may be stated that methanol (R ) 0.93 and β ) 0.62) is a better hydrogen bond donor and worse

hydrogen bond acceptor than ethanol (R ) 0.83 and β ) 0.77). Therefore, the larger cosolvent effect observed with ethanol could be attributed to its larger capability to participate in hydrogen bonding as a hydrogen bond acceptor. Related to acetone, since its solvatochromic parameters are R ) 0.08 and β ) 0.48, its ability to form hydrogen bonds is less than that of the two alcohols and, as a sequel, the smaller the yield is attained when it is used as cosolvent instead of alcohols. In regards to hexane, its solvatochromic parameters are R ) 0.0 and β ) 0.0, i.e., hexane has no capacity to form hydrogen bonds. This fact might explain that its effect on extraction yields was smaller than with the other cosolvents since only the density effect, and no specific interactions, seems to contribute to the cosolvent effect. In relation to the influence of temperature on yields when cosolvents are used (Figure 5), in the experimental range analyzed, it is found that, although the effect is quite small, isobaric increases of temperature lead to decreasing yields. Just as in the case of the use of pure CO2, this finding should be imputed to the smaller fluid density at higher temperatures29 but also to the weakening of solute-cosolvent interactions due to that the molecular energies of both, solute and cosolvent, increase with temperature.32 Finally, contrary to what occurred with pure CO2, the pressure effect on extractions yields when cosolvents were used was found not significant (Figure 5). It is probably due to that, in the pressure range analyzed (300-350 kg/cm2) and at the temperature level investigated (80 °C), the factors causing the extraction yields to change (density variation of the modified solvent and solute-cosolvent interactions) were not high enough to produce observable effects.7,26 Extraction yields presented above for the CO2 + ethanol modified solvent are similar to those reported by other authors for similar systems.5 However, it should be noted that, in this earlier work, neither detailed information on the influence of pressure and temperature on the cosolvent effect was given nor the composition of the recovered fractions reported. On the other hand, results with the other cosolvents cannot be compared to other investigations because, to the authors knowledge, ethanol has been the only cosolvent used to improve the efficiency of CO2 to selectively extract the TG fraction from used frying oil. Regarding the solubilities of the recovered fractions (determined from the straight slope of Figure 5 extraction curves at the beginning of the process), they were found to be in the ranges 8.3-27.5, 6.3-19.9, 7.5-18.8, and 5.8-15.2 g of lipid/ kg of solvent when the cosolvents used were ethanol, methanol, acetone, and hexane, respectively. Comparing these figures with those corresponding to the pure solvent, it is found that higher lipids solubilities are attained at softer operation pressures when cosolvents are used. 3.4. Characteristics of the Fractions Recovered with Modified CO2. On broad outline, results related to the evolution with time of the composition of the extract recovered with modified CO2 and to the effect of pressure and temperature on such composition were similar to those obtained with pure CO2 and, therefore, they will not be commented on here. However, the effect of the different cosolvents on the characteristics of the lipid fraction extracted with modified CO2 will be discussed in detail. It should be noticed that the analysis of the used frying oil extraction with a solvent + cosolvent mixture is extremely complex because there are many substances involved and a considerable number of potential interactions among them.


Figure 6. Percentages of polar compounds in the oil extracted with modified CO2.

Nevertheless, by considering the cosolvents nature and the used frying oil composition,2,3 it may be expected that, when adding cosolvents to CO2, those polar components with molecular size similar or smaller than triglycerides will be extracted easier than the nonpolar fraction components (triglycerides). It is so because all cosolvents, but hexane, and all used frying oil polar components (polymers, dimers, triglycerides, oxidized triglycerides, diglycerides and fatty acids, among others) have the ability to form hydrogen bonds,30 a capacity whose magnitude varies between large (in the case of acids, alcohols, and aldehydes) and moderate (ketones, esters, and ethers). In effect, in Figure 6, it is shown that at 300 kg/cm2 and 40 °C the larger polar compounds percentages correspond to the extracts obtained with ethanol, followed by methanol, acetone, and hexane, with the hexane percentage comparable to that obtained with pure CO2. Similar results were found at the other conditions tested (300 kg/cm2 at 80 °C and 350 kg/cm2 at 80 °C). According to the above comments, it probably must be attributed to the ability of ethanol, methanol, and acetone to form hydrogen bonds with the polar compounds that conform the oil more easily than with triglycerides. Thus, the larger selectivity for the polar fraction by alcohols must be attributed to their ability to participate in hydrogen bonding, either as donor or acceptor molecules (see their R and β solvatochromic parameters in Table 1), a fact that potentially increases the number of ways in which they can interact with the polar components of the oil. However, solvatochromic parameters of acetone indicate that it can only participate as a hydrogen bond acceptor in this type of bonding; which could explain its smaller affinity by polar compounds of the oil. On the other hand, the

2415

larger selectivity of ethanol (compared to methanol) could be imputed to its greater capacity to accept hydrogen bonds since the β parameter of the alcohols increases with alcohol chain length.28 As far as the smaller selectivity for the polar components of hexane, it must be imputed to its apolar character and incapacity to form hydrogen bonds with these polar components. Finally, the fact that the selectivity of the CO2 + hexane modified solvent was similar to that of pure CO2, it should be further commented. According to the high affinity of alkanes (like hexane) and triglycerides,9,33,34 there should be an expectation of a reduction in the polar compound percentage of the extract obtained with the modified solvent. Nevertheless, the addition of hexane to CO2 also entails both a density increase of the solvent (and, therefore, of its capacity to solubilize substances) and a rise in the number of dipole-dipole induced interactions between hexane, highly polarizable (R*hexane ) 11.9 Å3), and the polar components of the used oil. Obviously, these last two effects favor the polar components extraction and would compensate the effect derived from the high affinity of alkanes and triglycerides. Consequently, the selectivity of the CO2 + hexane modified solvent results is similar to that of pure CO2. Other authors29,35,36 have investigated the effect of these cosolvents in simpler systems (ternary) than those analyzed here (multicomponent). Their results roughly agree with those commented above, regardless of the lipid or nonlipid nature of the solute used in the investigations. In relation to the effect of cosolvent concentration, when methanol, ethanol, and acetone are used as cosolvents, it may be observed in Figure 6 that the percentage of polar compounds in the extract is larger than with pure CO2 and increases with cosolvent concentration, probably because polar solute-cosolvent interactions also increase that way.23,37 However, although it is not shown in any figure, it should be noted that the magnitude of the observed variation depends on the operating conditions. Specifically, it was more significant at those pressure and temperature conditions (300 kg/cm2 and 80 °C) leading to lower solvent density and less significant in the more dense systems (300 kg/cm2 at 40 °C and 350 kg/cm2 at 80 °C). It can be due to the differences between the local cosolvent concentration around polar solutes and global cosolvent concentration in the system decrease with increasing solvent density.13,38 Furthermore, in the case of ethanol, it may also have contributed to this effect the fact that their molecules, even at supercritical conditions, show a great tendency to selfcombine, a trend that increases with the concentration of alcohol as diverse spectroscopic studies have demonstrated.38 When hexane is used, as opposed to what happens with the other cosolvents or modifiers, the percentage of polar compounds in the extract is similar to that obtained with pure CO2, regardless of the cosolvent concentration used. Taking into account this result and the fact that extraction yield increases with hexane concentration (Figure 5), it may be stated that the solubility of both polar and nonpolar components increases at the same extent when adding hexane to the system, regardless of the hexane concentration used. It must probably be imputed, on the one hand, to the well-known affinity between alkanes (hexane) and triglycerides9,33,34 and, on the other hand, to the high polarizability of hexane (R*hexane ) 11.9 Å3), the most polarizable of the cosolvents analyzed (R*acetone ) 6.4 Å3, R*ethanol ) 5.1 Å3, R*methanol ) 3.3 Å3). The first fact would increase the triglycerides solubility, whereas the second one would raise that of polar components due to dipole-dipole induced interactions between these components and hexane. It

2416


hydrogen bonding between these polar components and alcohols. On the other hand, the fact that the LMWC and HMWC fractions show opposite behaviors should be imputed to the polarizability effect (dipole-induced dipole interaction) that may prevail over hydrogen bonding when LMWC interacts with hexane due to their larger capability (as compared to HMWC) to induce dipoles on the hexane molecules. Finally, related to the effect of cosolvent concentration on the composition of the polar fraction (not shown), for all cosolvents and concentration levels analyzed, it has been found that the variable does not affect significantly the selectivity of the modified solvent for a particular pseudocomponent of the polar fraction. It could be attributed to the complicated interactions between components of complex systems so that single effects may cancel and no net effect is observed in the experimental range analyzed.29 3.5. Relative Separation Efficiency. For the optimization of the separation process, it is very important to consider the efficient separation of each solute. It has been shown above that yields and extraction rates increase when the extraction is performed at the high density region (high pressure and low temperature) and when cosolvents were used, although these operational conditions usually lead to poor separation selectivity for TG due to multifunctional interactions between the numerous system components. In this section, in order to quantitatively select the most favorable conditions for the efficient separation of the TG fraction, the relative separation efficiency concept (RSE) is used.39 The RSE excludes the effect of feed composition that affects the separation efficiency and indicates the degree of separation difficulty in the extraction. It is calculated from the following equation: Figure 7. Composition of the oil extracted with modified CO2.

should also be pointed out that, since experiments have been performed in the dense supercritical region (Fr > 1.3) where the fluid is almost uncompressible,36 the modification of the solvent density due to cosolvent addition should not have contributed to a great extent to the solubility variation of both types of substances (triglycerides and polar compounds). Regarding the cosolvent effect on the composition of the polar fraction recovered, for the smallest cosolvent concentration analyzed (0.05 g of cosolvent/g of solvent) Figure 7 shows that when acetone or hexane were used the percentage of HMWC is smaller than in extractions performed with ethanol or methanol, whereas it is larger for the percentage of LMWC, i.e., the percentage of those compounds that can be dissolved more easily. Similar results were attained at the other concentration tested (0.1 g of cosolvent/g of solvent). These results can be explained by considering that different preferential intermolecular forces among the different polar components and cosolvents arise. Thus, the greater affinity of alcohols for the HMWC fraction is probably due to hydrogen bonding between alcohols and these compounds. The fact that a lesser affinity had been observed with acetone (a cosolvent that can also participate in hydrogen bonding, although only as an acceptor compound) reflects the larger capacity of donor molecules, like alcohols (with donor and acceptor capacities), to form these bonds with HMWC. Finally, hexane does not have the ability to interact with HMWC by hydrogen bonding (according to their solvatochromic parameters: R ) 0.0 and β ) 0.0), but due to its high polarizability (R* ) 11.9 Å3), some type of dipoleinduced dipole interaction between their molecules and those of HMWC could be expected. Nevertheless, results attained indicate that the magnitude of this interaction is smaller than

RSE ) (Xi/XTG)extract/(Xi/XTG)feed where Xi ) mass of TG, HMWC, or LMWC in the feed or extract and XTG ) mass of TG in the feed or extract. From this equation, the RSE value for TG is always 1. As the RSE value of other compounds is far from 1, the separation of other compounds from TG is easier. The RSE values obtained in this work at the different extraction conditions analyzed are shown in Figure 8. These results clearly show that the RSE values of LMWC are greatly dependent on pressure, temperature, cosolvent type, and cosolvent concentration, while it is not the case for the HMWC fraction. This means that the separation efficiency between LMWC and TG with pure and modified CO2 is greatly dependent on extraction conditions. Specifically, it can be seen that LMWC and TG are more effectively separated at the low density region (low pressure and high temperature) and using hexane as cosolvent. On the other hand, the extraction rate is acceptable at these conditions, as shown in Figure 5. Regarding the RSE of the HMWC fraction, Figure 8 shows that it does not depend significantly on extraction conditions. Thus, it has been demonstrated that the extraction under the low density region and using hexane as cosolvent leads to the best results. Specifically, it has been found that the most favorable conditions for the separation of TG from the polar fraction, at reasonable values of both extraction rate and separation efficiency, are P ) 300 kg/cm2, T ) 40 °C, cosolvent type ) hexane, and cosolvent concentration ) 10%. However, when processing the oil at these conditions, the polar content of the lipid fraction recovered still was about twice that of the fresh frying oil (see Figure 6 and Table 2) and, for this reason, a two-stage supercritical fluid extraction was


2417

Table 2. Comparison of the Composition of the Fresh Frying Oil, Used Oil, and Treated Oilsa lipid fraction

fresh frying oil

used frying oil

one-stage treated oilb

two-stage treated oilb

TG fraction (%) polar fraction (%) HMWC fraction LMWC fraction

92.7 7.3 3.4 3.9

70.1 29.9 13.0 16.9

84.9 15.1 2.5 12.6

90.9 9.1 1.6 7.5

a P ) 300 kg/cm2, T ) 40°C, cosolvent type ) hexane, cosolvent concentration ) 10%. b Composition of the fraction obtained after 4 h of extraction.

Figure 9. Triglyceride yield and composition of the oil recovered in the two-stage supercritical extraction.

extraction under the optimized single extraction conditions (first stage) was reprocessed later at the same conditions (second stage). Operating in this manner, we simulated a continuous fractionation with a second extractor. The reason to reprocess the fraction obtained after 4 h of extraction was that it contained about 95% of the TG treated and, as shown in Figure 5, from that time (4 h), the extraction rate was rather low. The contents of TG, HMWC, and LMWC in the lipid fractions recovered during the two stage process are shown in Figure 9. Also, in this figure, it is shown the overall TG yield of the two-stage process (i.e., that obtained considering the triglyceride percentage in the extract recovered in the second extractor and that only 95% of the triglycerides fed to the first extractor were treated in the second extractor). In summary, the results presented indicate that the supercritical extraction with CO2 + hexane is a very efficient method for the removal of HMWC, especially when it is compared to adsorbents treatments,2,40 and that the technology is also efficient to remove LMWC since their level in the fraction recovered in the two-stage process is very close to that of the fresh frying oil (Table 2). 4. Conclusions

Figure 8. Relative separation efficiencies from TG of HMWC and LMWC under different extraction conditions.

performed in order to get closer values of both compositions. In the two-stage process, the lipid fraction obtained after 4 h of

Results presented here clearly show, for the first time, that the separation of unoxidized triglycerides from used frying oil by supercritical CO2 may successfully be improved using cosolvents, specifically hexane. Both extraction rate and separation efficiency increase when this cosolvent is used. The separation of HMWC and TG by this technology is very effective and almost independent of the operating conditions. The separation of LMWC is more difficult since coextraction of LMWC and TG is almost unavoidable. However, it has been shown here that by proper selection of extraction schemes and operational variables lipid fractions with similar composition

2418


to that of fresh frying oil can be obtained. The extracts obtained by this method could be used in such diverse industries as biodiesel production or cosmetics. Acknowledgment The authors thank financial support of this work by the Regional Government of Castilla-La Mancha for the Project with reference PBI-05-048. Literature Cited (1) Weiss, T. J. Food oils and their uses; AVI Publishing Company: Westport, CT, 1983. (2) Yates, R. A.; Caldwell, J. D. Regeneration of oils used for deep frying: a comparison of active filter aids. J. Am. Oil Chem. Soc. 1993, 70 (5), 507. (3) 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. (4) Paquot, C.; Hautfenne, A. IUPAC Standard Methods for the Analysis of Oils, Fats and DeriVatiVes; Blackwell Scientific Publications: Oxford, 1987. (5) Devittori, C. M.; Perrut, M.; Maiewski, W. Method for fractionating cooking oils. European Patent 6,204,401, 2001. (6) Stahl, E.; Quirin, K. W.; Gerard, D. Dense Gases for Extraction and Refining; Springer-Verlag: Berlin, 1988. (7) Luque de Castro, M. D.; Valcarcel, M.; Tena, M. T. Extraccioń con Fluidos Supercrı´ticos; Reverte´: Barcelona, 1993. (8) Rincoń, J.; Can˜izares, P.; Garcıá, M. T.; Gracia, I. Regeneration of Used Lubricant Oil by Propane Extraction. Ind. Eng. Chem. Res. 2003, 42, 4867. ¨ stu¨ndag˘, O.; Temelli, F. Solubility behaviour of ternary (9) Gu¨çlu¨-U systems of lipids, cosolvents and supercritical carbon dioxide and processing aspects. J. Supercrit. Fluids 2005, 36, 1. (10) Yoon, J.; Han, B.-S.; Kang, Y.-C.; Kim, K. H.; Jung, M. Y.; Kwon, Y. A. Purification of used frying oil by supercritical carbon dioxide extraction. Food Chem. 2000, 71, 275. (11) Sesti Osseó, L.; Caputo, G.; Gracia, I.; Reverchon, E. Continuous Fractionation of Used Frying Oil by Supercritical CO2. J. Am. Oil. Chem. Soc. 2004, 81, 879. (12) Fritsch, C. W. Measurements of frying fat deterioration: a brief review. J. Am. Oil Chem. Soc. 1981, 5, 272. (13) Gurdial, G. S.; Foster, N. R. Solubility of o-hydroxybenzoic acid in supercritical carbon dioxide. Ind. Eng. Chem. Res. 1991, 30, 575. (14) Dobarganes, M. C.; Pe´rez-Camino, M. C.; Ma´rquez-Ruı´z, G. High performance size exclusion chromatography of polar compounds in heated and non-heated fats. Fat Sci. Technol. 1988, 90, 308. (15) NIST Thermophysical Properties of Fluid System, 2007. National Institute of Standards and Technology. http://webbook.nist.gov/chemistry/ fluid (accessed November, 2009). ¨ stu¨ndag˘, O.; Temelli, F. Correlating the solubility behavior (16) Gu¨çlu¨-U of fatty acids, mono-, di-, and triglycerides, and fatty acid esters in supercritical carbon dioxide. Ind. Eng. Chem. Res. 2000, 39, 4756. ¨ stu¨ndag˘, O.; Temelli, F. Solubility behavior of ternary (17) Gu¨çlu¨-U systems of lipids in supercritical carbon dioxide. J. Supercrit. Fluids 2006, 38, 275. (18) King, M. B.; Bott, T. R.; Barr, M. J.; Mahmud, R. S.; Sanders, N. Equilibrium and rate data for the extraction of lipids using compressed carbon dioxide. Sep. Sci. Technol. 1987, 22 (2&3), 1103.

(19) Kwon, Y. A. Solubility interactions between saturated triglycerides and cholesterol in supercritical carbon dioxide. Food Sci. Biotechnol. 1998, 7, 286. (20) Rincoń, J.; Can˜izares, P.; Garcıá, M. T. Regeneration of Used Lubricant Oil by Ethane Extraction. J. Supercrit. Fluids. 2007, 39, 315. (21) Rincoń, J.; Can˜izares, P.; Garcıá, M. T. Improvement of the wasteoil vacuum-distillation recycling by continuous extraction with dense propane. Ind. Eng. Chem. Res. 2007, 46, 266–272. (22) Cocero, M. J.; Calvo, L. Supercritical fluid extraction of sunflower seed oil with CO2-ethanol mixtures. J. Am. Oil Chem. Soc. 1996, 73, 1573. (23) Catchpole, O. J.; Grey, J. B.; Noermark, K. A. Solubility of fish oil components in supercritical CO2 and CO2 + ethanol mixtures. J. Chem. Eng. Data 1998, 43, 1091. (24) Lucas, A.; Rincoń, J.; Gracia, I. Influence of Operation Variables on Quality Parameters of Olive Husk Oil extracted with Supercritical CO2. II. Three-Step Sequential Extraction. J. Am. Oil Chem. Soc. 2003, 80 (2), 181. (25) Lucas, A.; Gracia, I.; Garcıá, M. T.; Rincoń, J. Solubility Determination and Model Prediction of Olive Husk Oil in Supercritical Carbon Dioxide and Cosolvents. Ind. Eng. Chem. Res. 2007, 46 (14), 5061. (26) Perry, R. H.; Green D. W.; Maloney, J. O. Manual del Ingeniero Quı´mico; McGraw-Hill: Mexico, 1992. (27) Kim, S.; Johnston, K. P. Effects of supercritical solvents on the rates of homogeneous chemical reactions. ACS Symp. Ser. 1986, 42, 329. (28) Ting, S. S. T.; Macnaughton, S. J.; Tomasko, D. L.; Foster, N. R. Solubility of naproxen in supercritical carbon dioxide with and without cosolvents. Ind. Eng. Chem. Res. 1993, 32, 1471. (29) Foster, N. R.; Singh, S. L.; Yun, J.; Tomasko, D. L.; Macnaughton, S. J. Polar and nonpolar cosolvent effects on the solubility of cholesterol in supercritical fluids. Ind. Eng. Chem. Res. 1993, 32, 2849. (30) Burrel, H. The challenge of the solubility parameter concept. J. Paint Technol. 1955, 40, 1968. (31) Barton, A. F. M. CRC Handbook of solubility parameters and other cohesion parameters; CRC Press, Inc.: Boca Raton, FL, 1985. (32) Huang, Z.; Kawi, S.; Chiew, Y. C. Solubility of cholesterol and its esters in supercritical carbon dioxide with and without cosolvents. J. Supercrit. Fluids 2004, 30 (1), 25. (33) Brunner, G.; Peter, S. On the solubility of glycerides and fatty acids in compressed gases in the presence of an entrainer. Sep. Sci. Technol. 1982, 17, 199. (34) Straver, E. J. M.; de Roo, J. L.; Peters, C. J.; de Swaan Arons, J. Phase behaviour of the binary system propane and tristearin. J. Supercrit. Fluids 1998, 11, 139. (35) Vedaraman, N.; Brunner, G.; Srinivasa Kannan, C.; Muralidharan, C.; Rao, P. G.; Raghavan, K. V. Extraction of cholesterol from cattle brain using supercritical carbon dioxide. J. Supercrit. Fluids 2004, 32, 231. (36) Dobbs, J. M.; Wong, J. M.; Lahiere, R. J.; Johnston, K. P. Modification of supercritical fluid phase behavior using polar cosolvents. Ind. Eng. Chem. Res. 1987, 25, 56. (37) Zhong, M.; Han, B.; Yan, H. Solubility of stearic acid in supercritical CO2 with cosolvents. J. Supercrit. Fluids 1997, 10, 113. (38) Fulton, J. L.; Yee, G. G.; Smith, R. D. Hydrogen bonding of methyl alcohol-d in supercritical carbon dioxide and supercritical ethane solutions. J. Am. Chem. Soc. 1991, 113 (22), 8327. (39) Liang, J. H.; Yeh, A. I. Process conditions for separating fatty acid esters by supercritical CO2. J. Am. Oil Chem. Soc. 1991, 68, 687. (40) Yates, R. A.; Caldwell, J. D. Adsortive capacity of active filter aids for used cooking oil. J. Am. Oil Chem. Soc. 1992, 69, 894.

ReceiVed for reView November 27, 2009 ReVised manuscript receiVed January 21, 2010 Accepted January 22, 2010 IE901871W

Fractionation of Used Frying Oil by Supercritical CO2 and Cosolvents

Recommend Documents