ARTICLE pubs.acs.org/IECR
Solubility of Used Frying Oil in High Pressure CO2Cosolvent Mixtures Jesusa Rincon,* Rafael Camarillo, Luis Rodríguez, and Virginia Ancillo Departamento de Ingeniería Química, Facultad de Ciencias Ambientales y Bioquímica, Universidad de Castilla-La Mancha, Avenida Carlos III, s/n 45071 Toledo, Spain ABSTRACT: The solubility of used frying oil in high pressure CO2 modified with four cosolvents (ethanol, methanol, acetone, and hexane) has been determined using a dynamic flow method. The aim of the work has been to analyze and compare the oil solubilities in CO2cosolvent mixtures to those attained with pure CO2 in order to determine how the use of cosolvents affects the recovery of the undegraded triglyceride fraction of the waste oil. Another objective has been to obtain an empirical correlation to predict these solubility data for further scale-up applications. The pressure, temperature, and concentration ranges used to investigate the cosolvent effect were 3035 MPa, 313353 K, and 0.050.1 g of cosolvent/g of solvent, respectively. In such ranges the classical empirical models cannot be used since they do not account for the presence of a cosolvent. Therefore, a modification of the classical empirical correlations is proposed by introducing a cosolvent-concentration-dependent term. The modified model can be used to estimate the used oil solubilities in solventcosolvent mixtures and provides a better agreement with the experimental data, compared to the earlier models.
1. INTRODUCTION Used frying oil is the waste that remains after fresh frying oil degradation occurs due to its repeated use in several frying processes. Used frying oil mainly consists of triglycerides (about 70% by weight) and the products of oil degradation: free fatty acids, mono- and diglycerides, oxidized triglycerides, and oligomeric triglycerides or polymers.1,2 To recycle the nondegraded triglycerides that are still present in the waste oil, several authors have investigated their possible recovery with supercritical solvents.37 In particular, the more recent studies have focused on the increase of the CO2 solvent efficiency by adding small amounts of solvent modifiers or cosolvents.8,9 Precisely, in one of these works it has been reported that by using proper extraction procedures the composition of the oil recovered could be very close to that of the fresh frying oil.9 On the other hand, with regard to the possible industrial application of the supercritical CO2 technology, the optimization and assessment of the supercritical process by means of empirical modeling is essential. The model should accurately describe the dependence of the solubility on pressure, temperature, cosolvent type, and concentration in the experimental range analyzed. To correlate the solubility data, several empirical models have been proposed.1012 However, in the case of mixed solvents the variation of the solubility cannot be explained exclusively in terms of temperature and solvent density, and the addition of cosolventconcentration-dependent terms to the classical models becomes of prime importance to accurately predict the solubilities.13 Accordingly, a major aim of this paper has been to analyze the solubility of used frying oil in CO2 and CO2cosolvent mixtures to propose a modified correlation incorporating separately the effect of cosolvent concentration and that provoked by the density change that cosolvent addition produces on the main solvent.14 Moreover, with a view to the industrial application of the process, the compositions of the oils recovered with each solvent (pure CO2 and CO2cosolvent mixtures) are analyzed and compared in order to determine the best processing conditions leading to r 2011 American Chemical Society
the selective separation of the nondegraded triglycerides and polar components that make up the used frying oil.
2. SOLUBILITY DATA ANALYSIS Thermodynamic modeling of solubility in compressed fluids is difficult for used vegetable oils because they are complex mixtures constituted of a large number of components, many of which are lacking information about physical properties and/ or critical and intermolecular energy parameter data.14 For this reason, empirical models describing solubility as a function of temperature and solvent density (pressure and temperature, implicitly) are successfully used.7 2.1. Chrastil-Type Models. The most usual empirical models described and tested satisfactorily for determining the solubilities of lipids are the Chrastil correlation10 and its subsequent modifications by Adachi and Lu11 and del Valle and Aguilera.12 In these models the effects of temperature and pressure are both subsumed in the solvent density. Chrastil’s model10 (eq 1) is based on the hypothesis that each solute molecule associates with k molecules of supercritical solvent to form a solvato complex, which is in equilibrium with the system. It establishes a linear relationship between the natural logarithm of the solute solubility in the high pressure solvent (S, g/L) and the density of the pure solvent (F, g/L), as follows: b ð1Þ S ¼ Fk exp a + T In eq 1, 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 molecular weights of Received: January 28, 2011 Accepted: June 14, 2011 Revised: June 14, 2011 Published: June 14, 2011 9314
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solute and solvent and the association number, and b is a constant dependent on the heat of dissolution reaction and is related to the temperature (T, K) dependence of solubility at constant density. As indicated above, several modifications of the Chrastil model have been proposed to improve their solubility predictions in high pressure fluids.11,12 For example, the Adachi and Lu modification,11 represented by eq 2, includes the following density dependence on the association number, k: b S ¼ Fk exp a + ð2Þ k ¼ c + dF + eF2 T On the other hand, the modification by del Valle and Aguilera12 (eq 3) introduces a fourth independent term (c/T2) to express a further temperature dependence. The effect of this addition is to possibly enhance the T effect. b c k S ¼ F exp a + + 2 ð3Þ T T 2.2. Dimensionless Chrastil-Type Models. Chrastil-type equations are dimensionally inconsistent. For example, when in eq 1 both variables solubility and density are expressed in grams per liter, as the association constant k generally differs from 1,10 the density term will be raised to a power other than 1 and the terms on both sides will have different units. Although this inconsistency does not represent an impediment to using the equation, as long as the units of each variable are specified, it is clear that there are some advantages when expressing it in dimensionless or reduced form. The main one is that parameters obtained by authors using different units will be identical and thus easier to compare. On the other hand, it should be noticed that dimensional inconsistencies may be easily avoided in Chrastil-type models by using dimensionless variables15 like those shown in eq 4: B 0 S ¼ Fkr exp A + ð4Þ Tr
where S* = S/Fc, Fr = F/Fc, and Tr = T/Tc; Tc and Fc are the critical temperature and critical density of solvent, respectively. Parameters in eq 4 are related to those in eq 1 by the expressions k0 = k, A = a + [(k0 1) ln Fc], and B = b/Tc. Similarly, the models by Adachi and Lu (eq 2) and del Valle and Aguilera (eq 3) can be also expressed in dimensionless forms: B k0 ð5Þ k0 ¼ C + DFr + EFr 2 S ¼ Fr exp A + Tr B C 0 S ¼ Fkr exp A + + 2 Tr Tr
ð6Þ
Equations 46, as well as their corresponding dimensional forms (eqs 13), have been developed for pure solvents. However, some authors have used them to predict the solubilities of substances in solvents modified with cosolvents by considering the solvent density (F) to be the density of the solventcosolvent mixture (Fm).8,13,16,17 In such cases the mixture density has usually been estimated from the expression Fm ¼
FmEOS EOS FCO2 FCO 2
ð7Þ
EOS represent, respectively, the densities for where FEOS CO2 and Fm pure CO2 and the CO2cosolvent mixture, both calculated using an appropriate equation of state such as the well-known PengRobinson equation of state,18 the one used in this work. Obviously, when using this method one assumes that the errors in calculated compressibility factors are similar for pure CO2 and CO2 mixtures.16,17 Nevertheless, as will be shown below, the cosolvent effect cannot usually be explained only by the change in density that the addition of cosolvent provokes to the main fluid,13 and therefore, the Chrastil-type models should be modified in order to accurately correlate and predict the used frying oil solubility in mixed solvents. Specifically, the modification of eq 4 proposed in this paper consists of introducing a multiplying term to account for the effect of the cosolvent concentration on the solubility of the 00 solute in the supercritical phase CkcoMr : BM 00 0 00 Sco ¼ SCkcoMr ¼ Fkr M CkcoMr exp AM + ð8Þ Tr
where Sco * and S* are the oil solubilities in the mixed and pure CO2 solvent (as estimated from eq 4), respectively, and the parameter kM00 is related to the effect of cosolvent concentration on solubility and Ccor = Cco/Fc; Cco is the cosolvent concentration (g/L) and Fc is the critical density of mixed solvent (g/L). Parameters kM0 , AM, and BM are related to parameters k, a, and b from eq 1 according to the expressions kM0 = k, AM = a + [(kM0 + kM00 1) ln Fc], and BM = b/Tc.
3. EXPERIMENTAL PROCEDURES 3.1. Materials. Refined sunflower oil provided by Diasol (Borges, Spain) was heated for 14 h at 195 °C. Afterward, degraded oil was stored in hermetic bottles, in a N2 atmosphere, in the dark, and at a temperature of 1.0 °C to avoid oil contact with oxygen, light, and humidity. Mass percentages of fresh frying oil constituents were 92.7% triglycerides, 3.9% LMWC, and 3.4% HMWC; those of used frying oil were 70.1% triglycerides, 16.9% LMWC, and 13.0% HMWC. The acronyms “LMWC” and “HMWC” designate, respectively, the polar compounds with molecular weights lower and higher than that of triglycerides. It has been reported in the literature that oil degradation by heating is similar to that obtained in true frying processes regarding parameters such as the percentage of polar compounds and conjugated diolefins.2,19 Liquid CO2 (purity 99.99%) was supplied by Praxair S.A. (Spain). Cosolvents methanol, ethanol, acetone, and hexane, all of HPLC grade, were supplied by Sigma-Aldrich (USA). Petroleum ether (Panreac, Spain), diethyl ether (Panreac, Spain), silica gel (Merck, USA), sea sand (Panreac, Spain), diisopropyl ether (Fisher Scientific, USA), oleyl-rac-glycerol (Sigma-Aldrich, USA), tetrahydrofuran (Panreac, Spain), and cyclohexane (Panreac, Spain) were used for the oil characterization. 3.2. Apparatus and Extraction Procedure. The experiments were developed in a semicontinuous apparatus, as shown in Figure 1.9 Liquid CO2 from a cylinder (SC) was cooled (CS), filtered (F), and compressed by a membrane pump (P) (HG140, LEWA, Germany). The pressure was regulated by a backpressure regulator (BPR) (BP-66, GO Regulator, USA) 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 9315
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Figure 1. Schematic diagram of the experimental extraction system.
temperature at the desired value, it was submerged into 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, and the extracted oil and solvent separated. The extracted oil was collected in a receiver at ambient temperature (RE1). The cosolvent was recovered 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 to eliminate cosolvent traces, since no changes in the oil composition were observed under this treatment in previous tests. The oil collected was gravimetrically quantified and properly stored for further analysis. Finally, note that experiments were performed at pressure and temperature conditions above the pseudocritical point of the modified solvent (calculated from the average values of the critical constants of the mixture components) in order to ensure that the cosolvent was present in the supercritical phase. 3.3. Analytical Methods. The extract composition was determined using the method proposed by Dobarganes et al.,20 the precision of which was previously evaluated and found acceptable.9 For the separation of polar and nonpolar fractions of the extracted samples, thin layer chromatography (TLC) and further gravimetric determination were used. For the analysis of the polar fraction, a high performance size exclusion liquid chromatograph (HPSEC; Waters 1515 isocratic HPLC pump, USA) equipped with a refractive index detector (Waters 2410, USA) 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 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 the 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. 3.4. Calculation of Solubilities. The solubilities have been measured using the dynamic flow-type technique.7 Dynamic methods are characterized by the passage of a solvent through a sample to achieve the saturation of the exit solvent with the sample. Then, the solubility at each condition is determined from the slope of the linear part of the extraction curve (grams of oil/ grams of solvent) at the beginning of each extraction.4,13
The reproducibility of the experimentally determined solubilities has been determined by comparing the results from four independent runs carried out under identical conditions: pressure (P) = 30 MPa, temperature (T) = 313 K, CO2 flow (Q) = 4 L/min (flow rate in standard conditions), and cosolvent concentration (Cco) = 0.1 g of ethanol/g of CO2. In these experiments the solubilities were similar (27.5, 27.0, 26.6, and 27.6 g of oil/g of solvent), indicating that reproducibility of the data was good. Nevertheless, to minimize experimental errors, each run was replicated twice.
4. RESULTS AND DISCUSSION In a previous study we showed that the supercritical CO2 extraction of triglycerides from used frying oil could successfully be improved using cosolvents such as methanol, ethanol, acetone, and hexane.9 In the present work, considering that the oil solubility behavior is essential for the industrial application of the process, the solubilities of the used frying oil in CO2cosolvent mixtures have been determined. Specifically, the experiments have been designed to study the effect of pressure, temperature, and cosolvent on the used frying oil solubility. They have been performed using heated refined oil instead of actual used frying oil. For the purpose of this work its composition simulated well that of actual used frying oil.4,5,9,21 A first group of experiments was performed without cosolvents in order to determine the best processing conditions leading to a rich triglyceride fraction with low polar content. The pressure and temperature ranges used in these experiments were 3040 MPa and 298353 K, respectively. Results obtained have been discussed in a previous paper,7 and only a brief summary of the solubility data reported is presented in Table 1, just to compare the solubility values attained with both pure CO2 and CO2cosolvent mixtures. Then, once the results obtained with pure CO2 were analyzed, the experiments with cosolvents were carried in the subset of experimental conditions that led to extracts with lower concentrations of polar components:7,9 30 MPa and 353 K, 35 MPa and 353 K, and 30 MPa and 313 K. The cosolvents used (methanol, ethanol, acetone, and hexane) were selected attending to both their different capacities to form hydrogen bonds and their proven efficiencies in the supercritical extraction of triglycerides.8 9316
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Table 1. Oil Solubilities, Cosolvent Effects, and Compositions of the Oil Obtained in Experiments with Pure CO2 and CO2Cosolvents operating conditions
extract composition (mass %) a
concn (g of cosolvent/g
solvent density
oil solubility
cosolvent
of solvent)
(g/L)
(mg/g)
effect, Ψ
0
967
10.1
0 0.05
910 825
6.7 11.2
0.1
938
27.5
4.10
24.0
20.7
0.05
789
9.9
1.47
18.2
16.2
0.1
925
25.9
3.86
23.2
20.1
0.05
853
8.9
1.32
16.7
14.4
acetone
0.1
926
22.9
3.41
17.9
15.7
hexane
0.05
849
6.7
1.00
13.4
11.9
313 353
hexane none
0.1 0
898 746
17.5 4.0
2.61
13.1 15.0
11.4 12.5
30
353
ethanol
0.05
788
8.3
2.07
22.9
19.3
30
353
ethanol
0.1
820
26.0
6.50
28.1
23.8
30
353
methanol
0.05
773
7.7
1.92
18.2
15.6
30
353
methanol
0.1
794
23.8
5.95
26.9
3.8
30
353
acetone
0.05
774
7.3
1.82
15.3
13.9
30
353
acetone
0.1
792
19.0
4.75
21.1
18.4
30 30
353 353
hexane hexane
0.05 0.1
742 741
6.4 16.2
1.60 4.05
12.5 13.0
10.9 11.7
35
298
none
0
987
12.0
17.5
11.3
35
313
none
0
935
7.8
15.4
12.8
35
353
none
0
789
4.9
14.3
12.2
35
353
ethanol
0.05
820
9.6
1.95
22.9
18.8
35
353
ethanol
0.1
843
24.5
5.00
25.2
20.3
35
353
methanol
0.05
807
8.6
1.75
18.6
15.1
35 35
353 353
methanol acetone
0.1 0.05
820 807
23.5 7.6
4.79 1.55
25.1 16.0
20.4 13.3
35
353
acetone
0.1
817
20.5
4.18
19.0
16.1
35
353
hexane
0.05
774
6.4
1.30
12.5
10.9
35
353
hexane
0.1
776
16.4
3.34
12.8
11.0
40
298
none
0
1005
14.1
18.6
11.4
40
313
none
0
957
7.2
15.5
11.3
40
353
none
0
823
6.0
16.4
11.5
P (MPa)
T (K)
30
298
none
30 30
313 313
none ethanol
30
313
ethanol
30
313
methanol
30
313
methanol
30
313
acetone
30
313
30
313
30 30
cosolvent
TPCb
LMWCc
16.8
13.9
1.67
14.9 22.3
13.4 19.6
a
As estimated from eq 7. b TPC, total polar compounds percentage in the dissolved oil. TPC + triglycerides percentage = 100. TPC = LMWC + HMWC; HMWC, percentage of compounds with molecular weights higher than that of triglycerides in the polar fraction of the dissolved oil. c LMWC, percentage of compounds with molecular weights lower than that of triglycerides in the polar fraction of the dissolved oil.
Regarding the cosolvent concentration, two levels (0.05 and 0.1 g of cosolvent/g of solvent) were used according to values found in the literature.14 4.1. Solubility of Used Frying Oil: Effect of Pressure and Temperature. Table 1 shows the oil solubilities in pure and modified CO2, together with the corresponding fluid densities calculated using the PengRobinson equation of state.18 The composition of the oil dissolved in each solvent will be discussed in section 4.5. It can be seen that the oil solubility ranges are 8.327.5, 7.725.9, 7.322.9, and 6.417.5 mg/g solvent when the cosolvents used are ethanol, methanol, acetone, and hexane, respectively. When comparing these results to those corresponding to the pure solvent (414.1 mg/g solvent), it is found that higher oil solubilities are attained at lower operation pressures if cosolvents are used. On the other hand, for all cosolvents, the
largest solubility values are obtained at 313 K, 30 MPa, and 0.1 g of cosolvent/g of solvent, i.e., at the highest cosolvent concentration and solvent density tested. Likewise, with pure CO2 the highest oil solubility was attained at the highest CO2 density (i.e., at 298 K and 40 MPa). Table 1 also shows that, in the ranges of pressure and temperatures analyzed, the oil solubility increases isobarically when temperature decreases and isothermally if pressure increases, regardless of the solvent (pure or modified) and cosolvent used. The influence of pressure is due to that increasing values of the variable lead to increases in the fluid density and, therefore, in the solvent power of the fluid.22 The effect of temperature is not so clear, since an isobaric increase of temperature gives rise to two competitive effects: a growing vapor pressure of solutes, which enhances their solubilities, and a 9317
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decrease in density and solvation power of the solvent, which hinders its solubility.14,16,22,23 Considering the results of experiments in Table 1 performed at 313 and 353 K (other conditions: 30 MPa, 0.05 g of cosolvent/g of solvent, and 30 MPa, 0.1 g of cosolvent/g of solvent), we can state that the second effect prevails over the first one. It should not be forgotten either that a rise in temperature also implies the weakening of solutecosolvent interactions caused by increasing molecular energies of species.24 Finally, it should be noticed that other authors have reported similar effects of pressure and temperature when studying the solubilities of lipidic substances within similar operating intervals.25 4.2. Solubility of Used Frying Oil: Effect of Cosolvent. The oil solubility data reported in Table 1 allow quantifying the effect of each cosolvent on the solubility enhancement by determining the “cosolvent effect”, a parameter symbolized by Ψ, that can be defined by the following expression:17 Ψ ¼ yternary =ybinary
ð9Þ
where, for a given pressure and temperature, yternary is the solubility of the oil obtained in a ternary system, i.e., the solubility of the oil in the mixed solvent (CO2 + cosolvent), and ybinary is the solubility of the oil in CO2 (binary system). The parameter Ψ has been evaluated for all mixed solvents studied, and they are presented in column 7 of Table 1. From Ψ values reported three important facts related to the use of cosolvents can be inferred: 1. The solubility of the oil is considerably increased when the primary solvent (CO2) is modified with cosolvents, with the cosolvent effect following the order ethanol > methanol > acetone > hexane. 2. The solubility increases with cosolvent concentration. 3. The effect of the cosolvent concentration prevails over that of pressure and temperature. These findings are similar for all cosolvents studied. To explain the solubility enhancement (1), two factors may be considered: the density variation of the solvent when cosolvent is added and the interactions between solute and cosolvent. The addition of a cosolvent to a compressed pure fluid generally provokes an increase in the bulk density (and solvent power) of the mixed solvent,8,14,26 which contributes to reinforcing the physical interactions (dipoledipole, dipoleinduced dipole, and induced dipoleinduced dipole) between solute molecules and mixed solvent and, therefore, to enhancing the solubility of the solutes.8,14,16,27,28 This effect is expected in the region near the critical point, where isothermal compressibility is larger. For pressures and temperatures far from this region, where the fluid is less compressible, the increase in bulk density is less significant and produces only small increases in solubility.28 Consequently, the effect of density cannot justify completely the important solubility enhancement observed. To check this fact, i.e., if the solubility increase could be explained considering the density effect alone, the contribution of the variable to such an increase has been examined by representing in Figure 2 the oil solubility versus the density of CO2 and CO2cosolvent mixtures at both temperatures investigated (313 and 353 K). The mixture densities have been estimated using eq 7.8,13,16,17 The model calculations are also included in Figure 2. If the solubility increase had been entirely due to the increased density of the cosolvent mixture, the oil solubility lines corresponding
Figure 2. Solubility of used frying oil in SC-CO2 with cosolvents (triangles, 313 K; squares, 353 K; solid symbols, experimental data; open symbols, data calculated by eq 8).
to pure and modified CO2 should have overlapped.16 However, since they do not coincide, the solubility enhancement cannot be imputed exclusively to the density increase produced by cosolvent addition. According to this finding, and considering that all cosolvents (but hexane) and used oil components (polymers, dimers, diglycerides, oxidized triglycerides, fatty acids, etc.)1,2 can participate in hydrogen bonds, either as donors or acceptors,29 the solubility enhancement could be attributed to hydrogen bonding interactions between solute and cosolvent molecules.8 In effect, the larger cosolvent effect of alcohols (Ψethanol > Ψmethanol), compared to those of acetone and hexane, may be explained in the following terms. According to the R and β KamletTaft solvatochromic parameters of the cosolvents,30 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 consequence, a smaller solubility is attained when it is used as cosolvent instead of alcohols. With regard to hexane, its solvatochromic parameters are R = 0.0 and β = 0.0; i.e., hexane has no capacity to form hydrogen bonds. This 9318
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Table 2. Constants of Solubility Models for Pure and Mixed Solvents (eqs 4, 5, and 6), Average Absolute Percentage Deviations, and Standard Deviations system CO27
empirical model
k0
CO2 + methanol
CO2 + acetone
CO2 + hexane
a
B
C
D
E
eq 4
4.17
6.74
0.50
eq 4 eq 4
4.27 4.38
6.65 6.60
0.43 0.34
eq 5
6.75
0.48
4.63
0.03
0.03
eq 5
6.71
0.42
4.75
0.02
0.01
6.58
0.36
4.87
0.09
0.02
eq 5
CO2 + ethanol
A
eq 6
3.99
6.73
0.49
0.01
eq 6
4.10
6.65
0.41
0.07
eq 6
4.22
6.58
0.34
0.14
eq 4 eq 4
8.51 8.81
0.11 0.03
2.76 2.66
eq 4
9.12
0.06
2.57
eq 4
8.12
0.13
2.82
eq 4
8.43
0.05
2.90
eq 4
8.73
0.03
3.01
eq 4
7.80
0.26
2.95
eq 4
8.10
0.17
2.85
eq 4 eq 4
8.41 5.63
0.07 0.95
2.75 3.21
eq 4
5.95
0.85
3.07
eq 4
6.26
0.74
2.98
AAPDa
sb
11.87
0.0039
11.77
0.0033
11.93
0.0042
42.16
0.0070
36.25
0.0078
43.67
0.0060
32.98
0.0052
Average absolute percentage deviation. b Standard deviation.
fact might explain why its effect on oil solubility was smaller than those of the other cosolvents. Nevertheless, data reported in Table 1 show an increase in the oil solubility when hexane is used as cosolvent. It should be imputed to both the well-known affinity between alkanes (hexane) and triglycerides8,31,32 and to the high polarizability of hexane (R*hexane = 11.9 Å3),30 the most polarizable of the cosolvents analyzed according to the polarizability values (R*acetone = 6.4 Å3, R*ethanol = 5.1 Å3, R*methanol = 3.3 Å3).30 The first fact would increase the triglyceride solubility, whereas the second one would raise that of polar components due to dipoledipole induced interactions between these components and hexane. Regarding the second experimental finding observed (2), the fact that the oil solubility was enhanced with increasing cosolvent concentrations, data presented in Table 1 show that the solubility increase is not linear with respect to cosolvent concentration increases in the experimental range investigated; the effect is smaller at the low cosolvent concentration zone (00.05 g of cosolvent/g of solvent) than at the higher concentration range (0.050.1 g of cosolvent/g of solvent). This nonlinear behavior is typical of the systems containing strong specific interactions,29,33 and in this particular case, it is probably due to the fact that, at the lowest cosolvent concentration tested (0.05 g of cosolvent/g of solvent), a significant part of the cosolvent added may be involved in interactions with CO2, ineffective from the point of view of increasing the solubility of the oil. Obviously, this fact reduces the bonding sites available for solvation between solute and cosolvent.34 Regarding the solventcosolvent interactions, they may occur because CO2 has a quadrupolar moment of 8.32 1026 esu,35 which may facilitate certain interactions with cosolvents, such as attractive dipole dipole forces. Although these solventcosolvent interactions also
should occur at the highest concentration analyzed (0.1 g of cosolvent/g of solvent), the experimental solubility data reported in this paper suggest that the increase of ineffective interactions associated with higher amounts of cosolvent in the system becomes smaller as the concentration of cosolvent arises. Finally, in relation to the third experimental evidence observed (3), the fact that the effect of cosolvent concentration on oil solubility is higher than the effect of the pressure and temperature (by means of the modification of the solvent density) demonstrates that, for operating conditions far from the critical point,36,37 the density effect is smaller than that produced by specific interactions between the cosolvent added and the binary system components (oil and CO2). 4.3. Solubility of Used Frying Oil: Correlation of Results. Table 2 shows the values of the fitting parameters obtained in the modeling of the used frying oil solubility in pure CO2 by the empirical equations of Chrastil,10 del Valle and Aguilera,12 and Adachi and Lu.11 The parameters of these equations have been calculated through regression by minimizing the average absolute percentage deviation (AAPD). Moreover, the 95% confidence intervals of the fitted parameters are reported for all models investigated. A good agreement is observed (Figure 3 and Table 2) between experimental and calculated solubilities with all empirical models in the experimental range investigated (298353 K and 3040 MPa). Dashed lines bordering the area within the relative error of (25% are added in Figure 3 in order to give a visual idea of the goodness of fit. Further, a statistical inference technique based on hypothesis contrast was used in order to determine if the differences between the models studied were statistically significant. At a 95% confidence level we found that differences between the results predicted by correlations 46 were not statistically significant. For this reason, correlations 9319
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Industrial & Engineering Chemistry Research 5 and 6 (models by Adachi and Lu11 and del Valle and Aguilera,12 respectively) were not further used to model the oil solubility data obtained with mixed solvents (CO2 + cosolvent), and only the simplest one (Chrastil’s model, eq 4) was employed with such solvents. In other words, all empirical models fit the solubility data with similar exactitude, in spite of the successive modifications to the first proposed correlation (Chrastil).
Figure 3. Comparison between experimental and calculated solubilities of used frying oil in pure CO2.
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Considering the fact that the density effect prevails in the experimental range investigated, it could be expected that the del Valle and Aguilera12 temperature-dependence modification did not improve the Chrastil results. However, this was not the case of the Adachi and Lu11 modification that introduces a density dependency to the association number. Although it is evident that a relation between the density and the association number exists, this k0 /Fr dependence is not significant in the interval studied (as proven via a statistical significance t test at 95% confidence level), producing only a mean variation for k0 values of 10% with respect to the k0 obtained for the Chrastil model. Then, this small variation does not seem to be large enough to improve the data fitting by Chrastil’s equation. As stated above, it was for these reasons that we applied the Chrastil correlation to model the oil solubility in mixed solvents. The corresponding parameters (k0 , A, B), average absolute percentage deviations (AAPD), and standard deviations (s) for mixed solvents are also shown in Table 2. When comparing the fitting results for both systems (pure CO2 and CO2cosolvent mixtures), it can be observed that values of AAPD and s are higher for the mixed solvents. This is not a surprising result since the Chrastil model should have limitations when predicting the solubility in solvent mixtures because, as shown before (Figure 2), the cosolvent effect cannot be explained exclusively by the change in density that the addition of cosolvent provokes to the main compressed fluid. It is for this reason that some
Figure 4. Comparison between experimental and calculated solubilities of used frying oil in CO2 + ethanol, CO2 + methanol, CO2 + acetone, and CO2 + hexane. 9320
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Industrial & Engineering Chemistry Research
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Table 3. Constants of the Solubility Model for Mixed Solvents (eq 11), Average Absolute Percentage Deviations, and Standard Deviations system CO2 + ethanol
CO2 + methanol
CO2 + acetone
CO2 + hexane
a
k0
A
B
D
8.51
0.11
2.76
5.09
8.81
0.03
2.66
6.39
9.12
0.06
2.57
7.99
8.12
0.13
2.82
5.48
8.43 8.73
0.05 0.03
2.90 3.01
6.68 8.18
7.80
0.26
2.95
5.06
8.10
0.17
2.85
6.45
8.41
0.07
2.75
7.87
5.63
0.95
3.21
6.96
5.95
0.85
3.07
7.35
6.26
0.74
2.98
8.84
AAPDa
sb
30.46
0.0045
Table 4. Constants of the Solubility Model for Mixed Solvents (eq 8), Average Absolute Percentage Deviations, and Standard Deviations system CO2 + ethanol
CO2 + methanol 30.50
0.0057
32.44
0.0038
CO2 + acetone
CO2 + hexane 25.72
0.0037
Average absolute percentage deviation. b Standard deviation.
authors13 have introduced an additional term into the Chrastil model (eq 1) to consider the cosolvent concentration effect to correlate and predict the oil solubility in mixed solvents: b k ð10Þ Sco ¼ F exp a + + dCco T where Sco is the solubility in the high pressure mixed solvent, F is the mixed solvent density (estimated from eq 7), and dCco accounts for all other cosolvent effects different from that derived from the solvent density change. It should be noticed that this equation suffers from the same problem of dimensional inconsistency as the original Chrastil model (eq 1). However, similarly to the previous dimensional models,1012 it can be converted to its dimensionless form: B 0 Sco ¼ Fkr exp A + + DCcor ð11Þ Tr where S*co = Sco/Fc is the dimensionless solubility in the mixed solvent (Fc, critical density of mixed solvent), Fr and Tr are the reduced mixed solvent density and temperature, respectively, and Ccor = Cco/Fc; Cco is the cosolvent concentration. Parameters k0 , A, B, and D are related to those of eq 10 according to the expressions k0 = k, A = a + [(k 1) ln Fc], B = b/Tc, and D = dFc. Figure 4 compares the experimental oil solubilities in the CO2ethanol, CO2methanol, CO2acetone, and CO2hexane mixtures to those calculated using the modified (eq 11, squares) and unmodified Chrastil models (eq 4, circles). It can be observed that the modified Chrastil model provides a better agreement between experimental and calculated solubilities, and therefore lower AAPD and s, as shown in Tables 2 and 3. Since similar results were obtained when CO2 was modified with all cosolvents, it may be stated that models developed to correlate solubility data in mixed solvents should consider both cosolvent concentration and the density change that causes cosolvent addition. 4.4. New Solubility Correlation for Pure and Modified Solvents. Although the modified model (eq 11) enhances predictions by the Chrastil model, in this work a new dimensionless Chrastil-type correlation is proposed (eq 8) to further improve the solubility estimations in modified solvents.
a
kM0
kM00
AM
BM
3.95
0.79
0.54
0.63
4.33
0.81
0.44
0.53
4.70
0.83
0.34
0.43
5.25
0.78
0.26
0.62
5.63 6.01
0.82 0.86
0.16 0.06
0.50 0.38
1.48
1.00
0.58
0.68
1.85
1.04
0.47
0.54
2.22
1.08
0.36
0.40
0.20
1.23
0.25
0.94
0.70
1.27
0.14
0.82
1.01
1.31
0.03
0.70
AAPDa
sb
11.95
0.0037
18.70
0.0053
10.71
0.0026
15.73
0.0025
Average absolute percentage deviation. b Standard deviation.
In the following, predictions from this new model (eq 8) are checked and compared to those by eq 11. Table 4 shows the values of the parameters (kM0 , kM00 , AM, BM) together with the absolute average percentage deviations (AAPD) and the standard deviations (s) for eq 8. Moreover, the 95% confidence intervals of the fitted parameters are reported for this model. As in the above models, the parameters were determined through data regression by minimizing the absolute average percentage deviations (AAPD) between experimental and calculated solubilities. Since for each cosolvent the parameters of the modified model were calculated using together all data obtained at 313353 K, 3035 MPa, and 0.050.1 g cosolvent per g of solvent, the proposed dimensionless modified Chrastil correlation can be used to calculate the oil solubility in all CO2cosolvent mixtures in the experimental range studied. Figure 4 compares the solubility predictions in CO2 + ethanol, CO2 + methanol, CO2 + acetone, and CO2 + hexane mixtures when the unmodified Chrastil model (eq 4) and the two Chrastil modified models (eqs 8 and 11) are used. It can be observed that predictions from eq 8 (Table 4) improve those from eq 4 (Table 2) and eq 11 (Table 3). Indeed, it produces quite acceptable estimations considering its wide applicability. Moreover, AAPD and s values are similar to those obtained for pure CO2 with Chrastil model7 (Table 2). In conclusion, the proposed correlation can be used to estimate used frying oil solubilities in CO2cosolvent mixtures (methanol, ethanol, acetone, and hexane) at operational conditions far from the low pressure region. The model provides a good agreement between experimental and calculated solubilities as a result of the stronger solutecosolvent interaction effects embedded, compared to the density effect that its addition also implies. 4.5. Composition of the Extracts. In order to determine the technical feasibility of the high pressure extraction technology for the selective separation of triglycerides from used frying oil, it is important to know the oil solubilities, but also the composition of the oil recovered. For this reason, the mass percentages of polar compounds with molecular weights higher and lower than that of triglycerides (HMWC and LMWC, respectively) in the oil dissolved in each solvent (pure and modified CO2) have been determined. Results obtained are commented on as follows. 9321
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Industrial & Engineering Chemistry Research 4.5.1. Pure CO2 Extracts. The dissolution of polar compounds using an apolar solvent like CO2 can be explained if the molecular weights of the dissolved polar compounds are smaller than that of apolar triglycerides,9 and taking into account that CO2 has a quadrupolar moment of 8.32 1026 esu,35 which may facilitate certain interactions with polar substances, such as attractive dipoledipole forces. In this sense, it can be observed (Table 1) that the HMWC fraction (% HMWC = % TPC % LMWC) is poorly solved in the pure solvent, with an average percentage in the extract of about 4%. However, the LMWC fraction is more easily solved, since its average concentration is around 12% (Table 1). Other authors have reported similar results when studying the solubility behavior of ternary systems of lipids in supercritical CO2.4,25 4.5.2. CO2 + Cosolvent Extracts. It may be expected that when alcohols or acetone are added to CO2, those polar components with molecular size similar to or smaller than triglycerides (LMWC) will be dissolved more easily than triglycerides because of the ability of the alcohols to participate in hydrogen bonding, as either donor or acceptor molecules.29 Cosolvent addition may also increase the solubility of the HMWC fraction, but a smaller effect is expected, precisely because the high molecular weight of the components of this fraction may hinder their solubilization. In this work, at all experimental conditions, it has been found that the larger polar compound percentages correspond to oils obtained using ethanol (2228%), followed by methanol (1827%), acetone (1521%), and hexane (1214%), as shown in Table 1. The larger selectivities of alcohols for components of the polar fraction are due to their larger capacity to form hydrogen bonds. Further, the larger percentage of polar compounds in the oil dissolved when ethanol is used as cosolvent (vs methanol) may be imputed to its greater capacity to accept hydrogen bonds, since the β parameter of the alcohols increases with alcohol chain length.28 On the other hand, it seems that the increase in the percentage of polar compounds is almost exclusively due to the increase in the percentage of the LMWC fraction in the extract (with values ranging from 11 to 23%), since the proportion of the HMWC fraction remains almost unaltered (4%) in relation to the experiments with pure CO2. In this way, the previous hypothesis about the easier extraction of low molecular weight polar solutes is proved. In relation to the effect of cosolvent concentration, the percentage of polar compounds in the dissolved oil increases with cosolvent concentration (Table 1), probably because polar solutecosolvent interactions also increase this way.38,39 Regarding the alcohol results, the fact that this effect is less pronounced with ethanol than with methanol can be due to that ethanol molecules, even at supercritical conditions, show a larger tendency to self-combine at high concentrations.40 4.5.3. Comparison between CO2 and CO2 + Cosolvent Extracts. The separation of HMWC and triglycerides seems effective and almost independent of the operating conditions with both pure and combined CO2. The separation of LMWC is more difficult since coextraction of LMWC and triglycerides is almost unavoidable.7,9 However, a proper selection of extraction schemes and operational variables could lead to lipid fractions with polar content close to that of fresh frying oil. In short, in the experimental range investigated the highest used frying oil solubility (14.1 g of oil/kg of solvent) is attained at 40 MPa and 298 K when pure CO2 is used. The composition of the oil dissolved at these conditions (Table 1) is halfway between the compositions of untreated used frying oil (70.1% triglycerides
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and 29.9% polar material) and fresh oil (92.7% triglycerides and 7.3% polar material). Regarding the composition of the oil dissolved by CO2 modified with alcohols or acetone, it contained larger percentages of polar components. However, at conditions leading to similar oil solubilities, the oil dissolved by CO2 + hexane presented lower polar percentages (Table 1) than that of the CO2 dissolved oil.
5. CONCLUSIONS Within the analyzed intervals of pressure and temperature, the oil solubility increases with isobaric decreases of temperature and with isothermal increases of pressure, no matter the cosolvent studied. This result is closely related to the density of the mixed solvent, since its variation with pressure and temperature is similar to that found for oil solubility. The oil solubility is always higher in the presence of cosolvents, with this effect being larger for ethanol. The result is related to the capacity of the cosolvents to interact with used frying oil components through hydrogen bonding (alcohols or acetone) or dipoleinduced dipole interactions (hexane). Nevertheless, results attained indicate that the effect of hydrogen bonding is larger than that of the other interaction. Likewise, the oil solubility is larger at the highest cosolvent concentration. This is so because the higher the concentration of cosolvent is, the larger the density of the mixed solvent and the number of cosolvent molecules available to specifically interact with the solute are. Both factors favor oil solubility. Regarding data modeling, the solubility of used frying oil in pure CO2 can be related to the experimental conditions by means of semiempirical Chrastil-type equations. In the case of mixed solvents, since the variation of solubility with operating conditions cannot be explained exclusively by the values of pressure, temperature, and the modification of the density provoked by the addition of cosolvent to the primary solvent, the Chrastil model shows limitations. Its modification by the introduction of an additional term considering the cosolvent concentration in the mixed solvent leads to sensibly better predictions with higher statistical significance with respect to parameters and residuals. Finally, it should be noticed that the separation of HMWC and triglycerides from used frying oil is effective and almost independent of the operating conditions with both pure and combined CO2. However, the coextraction of LMWC and triglycerides is almost unavoidable. Nevertheless, it has been shown that a proper selection of extraction schemes and operational variables may lead to lipid fractions with polar content close to that of fresh frying oil.9 ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors thank the Regional Government of Castilla-La Mancha (project with reference PBI-05-048) for the financial support of this work. ’ REFERENCES (1) 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. 9322
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Industrial & Engineering Chemistry Research (2) 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. (3) Perrut, M.; Majewski, W. Procede de fractionnement d’une huile de cuisson. WO Patent 00/78902 A2, 2000. (4) 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. (5) Sesti-Osseo, L.; Caputo, G.; Gracia, I.; Reverchon, E. Continuous Fractionation of Used Frying Oil by Supercritical CO2. J. Am. Oil. Chem. Soc. 2004, 81, 879. (6) Rincon, J.; Martínez, F.; Rodríguez, L.; Ancillo, V. Recovery of triglycerides from used frying oil by extraction with liquid and supercritical ethane. J. Supercrit. Fluids 2011, 56, 72. (7) Rincon, J.; Camarillo, R.; Martínez, F.; Rodríguez, L; Ancillo, V. Correlating the Solubilities of Used Frying Oil in High-Pressure Propane, Carbon Dioxide, and Ethane. Ind. Eng. Chem. Res. 2011, 50 (2), 1028. € st€undag, O.; Temelli, F. Solubility behaviour of ternary (8) G€u)c l€u-U systems of lipids, cosolvents and supercritical carbon dioxide and processing aspects. J. Supercrit. Fluids 2005, 36, 1. (9) Rincon, J.; Camarillo, R.; Rodríguez, L.; Ancillo, V. Fractionation of used frying oil by supercritical CO2 and cosolvents. Ind. Eng. Chem. Res. 2010, 49, 2410. (10) Chrastil, J. Solubility of Solids and Liquids in Supercritical Gasses. J. Phys. Chem. 1982, 86, 3016. (11) Adachi, Y. B.; Lu, C.-Y. Y. Supercritical Fluid Extraction with Carbon Dioxide and Ethylene. Fluid Phase Equilib. 1983, 14, 147. (12) del Valle, J. M.; Aguilera, J. M. An Improved Equation for Predicting the Solubility of Vegetable Oils in Supercritical CO2. Ind. Eng. Chem. Res. 1988, 27, 1551. (13) Lucas, A.; Gracia, I.; García, M. T.; Rincon, J. Solubility Determination and Model Prediction of Olive Husk Oil in Supercritical Carbon Dioxide and Cosolvents. Ind. Eng. Chem. Res. 2007, 46 (14), 5061. (14) Luque de Castro, M. D.; Valcarcel, M.; Tena, M. T. Extraccion con Fluidos Supercríticos; Reverte: Barcelona, 1993. (15) Sparks, D. L.; Hernandez, R.; Estevez, L. A. Evaluation of densitybased models for the solubility of solids in supercritical carbon dioxide and formulation of a new model. Chem. Eng. Sci. 2008, 63 (17), 4292. (16) Foster, N. R.; Singh, H.; Yun, S. L. J.; Tomasko, D. L; Macnaughton, S. J. Polar and Nonpolar Effects on the Solubility of Cholesterol in Supercritical Fluids. Ind. Eng. Chem. Res. 1993, 32, 2849. (17) Singh, H.; Yun, S. L. J.; Macnaughton, S. T.; Tamasko, D. L.; Foster, N. R. Solubility of cholesterol in supercritical ethane and binary gas mixtures containing ethane. Ind. Eng. Chem. Res. 1993, 32, 2841. (18) Peng, D. Y.; Robinson, D. B. A New Two-Constant Equation of State. Ind. Eng. Chem. Fundam. 1976, 15, 59. (19) Fritsch, C. W. Measurements of frying fat deterioration: a brief review. J. Am. Oil Chem. Soc. 1981, 5, 272. (20) Dobarganes, M. C.; Perez-Camino, M. C.; Marquez-Ruíz, G. High performance size exclusion chromatography of polar compounds in heated and non-heated fats. Fat Sci. Technol. 1988, 90, 308. (21) Sesti-Osseo, L.; Capolupo, G.; Reverchon, E. Continuous Fractionation of Fried Oil by Supercritical CO2 and Hexane. Presented at the 10th European Meeting on Supercritical Fluids, Colmar, France, Dec 1215, 2005. (22) Stahl, E.; Quirin, K. W.; Gerard, D. Dense Gases for Extraction and Refining; Springer-Verlag: Berlin, 1987. € st€undag, O.; Temelli, F. Solubility behavior of ternary (23) G€u)c l€u-U systems of lipids in supercritical carbon dioxide. J. Supercrit. Fluids 2006, 38, 275. (24) 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. € st€undag, O.; Temelli, F. Correlating the solubility (25) G€u)c l€u-U behavior of fatty acids, mono-, di-, and triglycerides, and fatty acid esters in supercritical carbon dioxide. Ind. Eng. Chem. Res. 2000, 39, 4756. (26) Perry, R. H.; Green, D. W.; Maloney, J. O. Manual del Ingeniero Químico; McGraw-Hill: Mexico City, Mexico, 1992.
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(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 Cosolvent. Ind. Eng. Chem. Res. 1993, 32, 1471. (29) Burrel, H. The challenge of the solubility parameter concept. J. Paint Technol. 1955, 40, 1968. (30) Barton, A. F. M. CRC Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press: Boca Raton, FL, USA, 1985. (31) 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. (32) 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. (33) Lemert, R. M.; Johnston, K. P. Chemical complexing agents for enhanced solubilities in supercritical fluid carbon dioxide. Ind. Eng. Chem. Res. 1991, 30, 1222. (34) Walsh, J. M.; Donohe, M. D. Hydrogen Bonding in Entrainer Cosolvent Mixtures a Parametric Analysis. Fluid Phase Equilib. 1989, 52, 397. (35) Coriani, S.; Halkier, A.; Rizzo, A.; Ruud, K. On the molecular electric quadrupole moment and the electric-field-gradient-induced birefringence of CO2 and CS2. Chem. Phys. Lett. 2000, 326, 269. (36) King, J. W.; List, G. R.; Heaking, A. J. Equilibrium and Rate Data for the Extraction of Lipids Using Compressed Carbon Dioxide. Sep. Sci. Technol. 1987, 22, 1103. (37) Eckert, C. A.; Van Alsten, J. G. Effect of Entrainers and Solute Size and Polarity in Supercritical Fluid Solutions. J. Chem. Eng. Data 1993, 38, 605. (38) Zhong, M.; Han, B.; Yan, H. Solubility of stearic acid in supercritical CO2 with cosolvents. J. Supercrit. Fluids 1997, 10, 113. (39) Catchpole, O. J.; Perry, N. B.; da Silva, B. M. T.; Grey, J. B.; Smallfield, B. M. Supercritical extraction of herbs I: Saw Palmetto, St. John’s Wort, Kava Root, and Echinacea. J. Supercrit. Fluids 1998, 22, 129. (40) 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.
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