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Ind. Eng. Chem. Res. 2003, 42, 3150-3155
Phase Behavior of the Reaction Medium of Limonene Oxidation in Supercritical Carbon Dioxide Marcos L. Corazza,† Lu ´ cio Cardozo-Filho,† Octa´ vio A. C. Antunes,‡ and ,§ Cla´ udio Dariva* Departamento de Engenharia Quı´mica, Centro de Tecnologia, Universidade Estadual de Maringa´ , Avenida Colombo, 5790, Bl. D90, 87020-900 Maringa´ -Pr, Brazil, Instituto de Quı´mica, Universidade Federal do Rio de Janeiro, Centro de Tecnologia, Bl. A, Lab. 641, Ilha do Funda˜ o, 21945-970 Rio de Janeiro-RJ, Brazil, and Departamento de Engenharia de Alimentos, Centro Tecnolo´ gico, Universidade Regional IntegradasCampus de Erechim, Avenida Sete de Setembro, 1621, 99700-000 Erechim-RS, Brazil
The main objective of this work is to investigate the phase behavior of the reaction medium for the limonene oxidation reaction at high pressures. For this purpose, the phase behavior of binary and ternary systems related to limonene oxidation in supercritical carbon dioxide was experimentally investigated. The experiments were carried out through the static synthetic method using a variable-volume view cell with two sapphire windows in the temperature range of 313-343 K at several overall compositions. The experimental data were modeled with the Peng-Robinson equation of state (PR-EOS) with the conventional quadratic mixing rule. The parameters fitted from binary systems were then used to simulate the phase behavior of ternary systems, and the results show that the PR-EOS is adequate to represent the experimental data. Introduction Supercritical fluid (SCF) technology has received great attention in the application and development of new processes and products.1,2 Some of these applications are the use of SCFs as a solvent in several chemical reactions,3-7 chromatographic analysis,8 polymer processing,9,10 pharmaceutical and food processing,9 coal and petroleum processing,11 and environmental applications.2,9,12 SCFs can also be useful as an alternative medium for chemical reactions because of their unique properties. Their gaslike transport properties and liquidlike densities can be easily manipulated by changing the pressure and temperature.4 Besides environmental advantages, one additional attractive feature of SCFs as a medium for chemical reactions is that they can lower masstransfer limitations and allow a better control of selectivity and reaction yield. Also, they permit advantageous combination of the reaction, separation, and purification steps.5,13 In this sense, the natural use of supercritical carbon dioxide (SCCO2) becomes an interesting possibility to replace organic solvents in many chemical reactions, like the catalytic oxidation of monoterpenes. The limonene oxidation is an alternative route to carvone production compared with extraction from natural products such as caraway seeds.14 Limonene oxidation is traditionally carried out in an organic solvent media (for example, dichloromethane, acetonitrile, and acetone among others), by using metal-Salen [metal(ethylene bis-salicylideneaminato)] as the catalyst and iodosobenzene as the oxidant, via a rebound mechanism.15 The main products obtained in the catalytic * To whom correspondence should be addressed. Tel.: +55 54-5209000. Fax: +55-54-5209090. E-mail: cdariva@ uricer.edu.br. † Universidade Estadual de Maringa ´. ‡ Universidade Federal do Rio de Janeiro. § Universidade Regional IntegradasCampus de Erechim.
oxidation of limonene are epoxides (1,2-limonene oxide), ketones (mainly carvone), and alcohols (mainly carveol).16 To improve the selectivity and yield of this reaction, SCCO2 can be used to replace organic solvents, totally or partially. The knowledge of the phase behavior of the reactional mixture places an important role on the selection of proper working regions and process scaleup. The literature presents some experimental studies of binary systems such as CO2/limonene,17-21 CO2/carvone,18 CO2/acetonitrile,22,23 and CO2/acetone.24 Regarding the catalytic limonene oxidation at low pressures, the influence of organic solvents on the reaction yield and selectivity is not well established at present. According to Gomes and Antunes,15,16 it is possible that the polar solvent can also participate in the stabilization of the catalyst. Thus, the addition of small amounts of organic solvents in the reaction medium might be necessary. In this work, vapor-liquid equilibrium (VLE) in the systems related to limonene oxidation (limonene, acetone, acetonitrile, dichloromethane, 1,2-limonene oxide, and carvone in CO2) were measured in the temperature range from 313 to 343 K. The binary results were correlated using the Peng-Robinson equation of state (PR-EOS) with the conventional quadratic mixing rule.25 The binary interaction parameters were then used to predict the experimental phase behavior of ternary systems. Experimental Section and Modeling Materials. (R)-(+)-Limonene (99%, Sigma), (S)-(+)carvone (98%, Aldrich), 1,2-limonene oxide (99%, Aldrich), dichloromethane (99%, Carlo Erba), acetonitrile (99%, Johnson Matthey), and acetone (99.5%, Nuclear) were used without further purification. CO2 is 99.9% in purity in the liquid phase (AGA). The critical properties of pure compounds are presented in Table 1. When
10.1021/ie021040+ CCC: $25.00 © 2003 American Chemical Society Published on Web 05/21/2003
Ind. Eng. Chem. Res., Vol. 42, No. 13, 2003 3151 Table 1. Critical Properties and Acentric Factors of Pure Components26 compound
MW/(g/gmol)a
Tc/K
Pc/bar
ω
CO2 limoneneb carvoneb 1,2-limonene oxideb dichloromethane acetone acetonitrile
44.01 136.24 150.21 152.13 84.93 58.08 41.05
304.21 657.16 712.76 661.07 510.00 508.10 547.95
73.83 27.56 27.21 30.46 63.00 46.96 48.30
0.2236 0.3396 0.4930 0.4315 0.1990 0.3067 0.3270
a MW ) molecular weight. b Estimated by the Joback group contribution method.
experimental values were not available in the literature,26 they were predicted by the Joback group contribution method.26 Apparatus and Procedure. Phase equilibrium experiments (cloud points) were performed in a highpressure variable-volume view cell. The experimental apparatus used in this work is similar to the one used in previous investigations,27-31 and thus only a brief description is given. A schematic diagram of the apparatus is presented in Figure 1, which consists basically of a view cell with two sapphire windows for visual observations, an absolute pressure transducer (Smar LD 301), with an uncertainty of 0.12 bar, a portable programmer (Smar, HT 201) for the pressure data acquisition, and a syringe pump (ISCO 260D). The equilibrium cell has a maximum internal volume of 25 cm3 and contains a movable piston, which permits the pressure control inside the cell. Phase transitions were recorded visually as bubble or dew points by varying the pressure behind the piston using the syringe pump and CO2 as the pressurizing fluid. The cell was equipped with an electrical heater and a proportional-integralderivative controller (DIGI MEC mark, SHM 112 model). The controller was connected to a thermocouple (PT100, with an accuracy of 0.1 K), which was in direct contact with the fluid mixture inside the cell body. Depending on the desired overall composition, an amount of solute was weighed on a high-precision scale (Ohaus Analytical Standard, with 0.0001 g accuracy) and loaded into the cell. Then the cell and all lines were flushed with low-pressure CO2 to remove residual air. Afterward, the solvent was pumped into the cell in order to reach the preestablished overall composition. The amount of solvent charged was monitored by the change in the volume of the transfer vessel of the pump. Then, the cell content was kept in continuous agitation with the help of a magnetic stirrer and a Teflon-coated stirring bar. After the desired temperature was reached, the cell pressure was increased by applying pressure on the back of the piston with the syringe pump until observation of a single phase. At this point, the system was allowed to stabilize at least 30 min and the cell pressure was decreased slowly until incipient formation of a new phase. The equilibrium pressure was then recorded, after repetition of the experimental procedure at least four times. After completion of the measurement at a given temperature, the cell temperature was established at a new value and the experimental procedure was repeated. Through replicate measurements and the experience with the present apparatus, the uncertainty in pressure values is lower than 0.70 bar. Concerning the uncertainty in temperature, the arrangement of this work provided a temperature control with a precision better than 0.5 K. With regard to phase composition, the mass
of carbon dioxide was carefully accounted for by the volume decay in the syringe pump. It was estimated that the uncertainty was around 0.001 in molar fraction basis. Modeling. The VLE experimental data were modeled with the PR-EOS with the classic quadratic mixing rules (two temperature-independent adjustable parameters: kij and lij).26 The binary interaction parameters were estimated through the maximum likelihood method coupled to a bubble- or dew-point algorithm for calculation of VLE according to the Asselineau formulation.32,33 The experimental results of binary systems CO2 + carvone, CO2 + limonene oxide, CO2 + dichloromethane, and CO2 + acetonitrile used to obtain the EOS binary parameter were discussed in a previous work.31 Two parameter estimation procedures were employed: isothermal parameter estimation and a global fitting procedure. The results indicated that the global fitting procedure produce a fitting quite similar to the isothermal parameter estimation for all binary systems investigated. To exemplify, Figures 2 and 3 present P-xy diagrams for the CO2 + carvone system obtained using isothermal and global fitting, respectively. Accordingly, throughout this work, a global parameter regression of the experimental data was accomplished using the PREOS, leading to the parameter values shown in Table 2. For the modeling of the ternary systems, only binary information was taken into account. To carry out the predictions, the algorithm described by Asselineau32,33 for VLE calculations was employed, which comprises the following main equations:
NLxi + NVyi - zi ) 0; i ) 1, 2, ..., nc
(1)
yiφˆ iV - xiφˆ iL ) 0; i ) 1, 2, ..., nc
(2)
nc
∑ i)1
nc
xi -
yi ) 0 ∑ i)1
NL + NV - 1 ) 0
(3) (4)
where NL and NV are the mole numbers of the liquid and vapor phases, respectively; xi and yi are the mole fractions of component i in the liquid and vapor phases, respectively. φˆ iV and φˆ iL are the fugacity coefficients of component i in the vapor and liquid phases, respectively. The following constraints must be considered: nc
xi ) 1 ∑ i)1
nc
and
yi ) 1 ∑ i)1
(5)
The fugacity coefficients of the vapor and liquid phases were evaluated from the PR-EOS.25 The equation system (eqs 1-4) has been solved by applying Broyden’s method.34 Results and Discussion In all tables reporting the experimental data, the pressure values are, in fact, average values of at least four replicate runs, and the standard deviations presented reflect experimental errors of these replicates. Table 3 presents the experimental results of VLE of the ternary system CO2 + limonene + acetonitrile (bubblepoint phase transition). The mass fraction of organic
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Figure 1. Schematic diagram of the experimental apparatus. Table 2. Binary Interaction Parameters of the PR-EOSa system CO2 + 1,2-limonene oxide CO2 + dichloromethane CO2 + acetonitrile CO2 + acetoneb CO2 + carvone CO2 + limonene
kij ( σ (×102) lij ( σ (×102) DP/bar 5.22 ( 0.56 2.95 ( 0.20 8.64 ( 0.91 1.28 5.56 ( 0.56 9.39 ( 0.34
1.04 ( 0.12 -6.61 ( 0.24 1.65 ( 0.14
1.45 0.74 1.06
-4.99 ( 0.41 -2.81 ( 0.28
0.94 1.24
a ∆P ) ∑NEP|Pexp - Pcalc|/NEP. NEP is the number of experii)1 mental points. b Taken from Adrian et al.24
Table 3. Experimental Results of VLE for the CO2 (1) + Limonene (2) + Acetonitrile (3) System mass fraction
Figure 2. P-xy diagram (VLE) for the system CO2 (1) + carvone (2). PR-EOS binary parameters estimated isothermally.
w1
w2
pressure ( σ/bar T ) 313 K
T ) 323 K
T ) 333 K
T ) 343 K
0.498 0.293 56.46 ( 0.36 67.67 ( 0.29 77.50 ( 0.87 87.71 ( 0.19 0.619 0.207 64.86 ( 0.33 77.28 ( 0.19 89.49 ( 0.47 100.92 ( 0.24 0.720 0.091 66.40 ( 0.37 80.18 ( 0.57 94.13 ( 0.99 104.93 ( 0.35
Figure 3. P-xy diagram (VLE) for the system CO2 (1) + carvone (2). PR-EOS binary parameters estimated using a global fitting procedure.
Figure 4. P-T diagram for the system CO2 (1) + limonene (2) + acetonitrile (3). Acetonitrile content fixed around 20 wt %.
solvent was kept constant around 20 wt % in order to study the influence of the limonene concentration on the phase behavior of the mixture. Figure 4 presents the experimental results in the P-T representation. For comparison, in this figure is also depicted the binary CO2 + acetonitrile system. It can
be observed that with an increase of the limonene content in the reactional medium the bubble-point phase transitions are shifted to lower pressures. Figure 4 also presents the prediction of PR-EOS for this ternary system obtained using the binary interaction parameters presented in Table 2. The interaction parameter
Ind. Eng. Chem. Res., Vol. 42, No. 13, 2003 3153 Table 4. Experimental Results of VLE for the CO2 (1) + Limonene (2) + Acetone (3) System mass fraction
pressure ( σ/bar
w1
w2
T ) 313 K
T ) 323 K
0.431 0.453 0.631 0.805
0. 392 0.435 0.250 0.080
53.03( 0.52 59.31 ( 0.40 63.65 ( 0.32 66.95 ( 0.14
63.37 ( 0.71 69.82 ( 0.23 77.18 ( 0.24 82.09 ( 0.08
T ) 333 K
T ) 343 K
72.39 ( 0.36 81.60 ( 0.31 81.99 ( 0.65 92.13 ( 0.58 91.24 ( 0.88 101.37 ( 0.45 95.18 ( 0.48 105.48 ( 0.45
Figure 6. P-T diagram for the system CO2 (1) + limonene (2) + dichloromethane (3). Limonene content fixed around 25 wt %. Table 6. Experimental Results of VLE for the CO2 (1) + Limonene (2) + Carvone (3) System mass fraction
Figure 5. P-T diagram for the system CO2 (1) + limonene (2) + acetone (3). Acetone content fixed around 12 wt %. Table 5. Experimental Results of VLE for the CO2 (1) + Limonene (2) + Dichloromethane (3) System mass fraction
w2
0.703 0.703 0.700 0.901 0.900 0.900
0.144 0.067 0.222 0.047 0.078 0.022
a
pressure ( σ/bar T ) 313 K
T ) 323 K
79.84 ( 0.37 99.11 ( 0.48 81.05 ( 0.59 103.52 ( 0.38 78.04 ( 0.25 96.02 ( 0.37 82.51 ( 0.18 102.13 ( 0.22 82.16 ( 0.32 99.71 ( 0.09 82.73 ( 0.19 102.79a ( 0.20
T ) 333 K
T ) 343 K
120.81 ( 0.49 124.94 ( 0.25 116.39 ( 0.49 121.61a ( 0.49 117.53a ( 0.25 123.21a ( 0.30
138.67 ( 0.32 145.61 ( 0.57 133.74 ( 0.20 139.16a ( 0.36 132.19a ( 0.20 141.31a ( 0.53
Dew point; all others are bubble point.
pressure ( σ/bar
w1
w2
T ) 313 K
T ) 323 K
T ) 333 K
T ) 343 K
0.515 0.715 0.851 0.438 0.636 0.787 0.395 0.544 0.724
0.424 0.224 0.099 0.447 0.251 0.111 0.404 0.242 0.083
72.19 ( 0.24 75.92 ( 0.30 80.50 ( 0.40 66.09 ( 0.18 71.15 ( 0.57 73.94 ( 0.62 62.99 ( 0.29 66.06 ( 0.38 69.44 ( 0.44
86.11 ( 0.17 92.47 ( 0.48 95.47 ( 0.23 79.42 ( 0.96 85.67 ( 0.38 89.10 ( 0.46 74.69 ( 0.24 78.85 ( 0.37 83.69 ( 0.82
100.92 ( 0.14 108.11 ( 0.33 108.78 ( 0.13 91.61 ( 0.48 100.93 ( 0.25 104.61 ( 0.39 86.75 ( 0.06 91.05 ( 0.29 97.08 ( 0.14
115.19 ( 0.20 123.59 ( 0.42 122.18a ( 0.39 103.83 ( 0.83 115.49 ( 0.40 117.39 ( 0.63 98.35 ( 0.62 104.87 ( 0.13 108.67 ( 0.30
a
w1
Dew point; all others are bubble point.
between limonene and acetonitrile was fixed equal to zero. It can be noted that the model predicts reasonably well the phase behavior of this ternary system. Table 4 presents the experimental results (bubblepoint phase transition) for the ternary system CO2 + limonene + acetone, keeping the acetone content around 10 wt %. Figure 5 presents the P-T diagram along with the PR-EOS prediction using the binary interaction parameters listed in Table 2. One can observe from this figure that the model predicts qualitatively well the phase behavior of the ternary system. It is important to mention that the interaction parameter between limonene and acetone was fixed equal to zero. As is observed for the CO2 + limonene + acetonitrile system, an increase in the limonene content also leads to a decrease in the pressure transitions. Table 5 presents the VLE experimental results for the system CO2 + limonene + dichloromethane (bubblepoint phase transition). With this system, our goal is to investigate the influence of both limonene and organic solvent contents on the phase behavior of the system. Figure 6 depicts the experimental results for the CO2 + limonene + dichloromethane system, keeping the limonene content around 25 wt %. Also in this figure are plotted the binary CO2 + limonene VLE equilibrium
Figure 7. P-T diagram for the system CO2 (1) + limonene (2) + carvone (3) system. CO2 content fixed around 70 wt %.
data. The results show that when the organic solvent content is increased, the pressure transitions are shifted toward lower pressures. This fact can be attributed to the higher solvent power of organic solvents when compared to carbon dioxide. The PR-EOS predictions for this ternary system, keeping the interaction parameter between limonene and dichloromethane equal to zero, can be considered satisfactory. An inspection of Table 5 reveals that the same trend regarding the limonene content can be observed: when limonene is increased (decreasing the CO2 content), the pressure transition values were decreased. Table 6 presents the experimental results for the system CO2 + limonene + carvone, in which bubble and dew points were observed. Figure 7 depicts the phase behavior for this ternary system, keeping the carbon dioxide content around 70% in mass basis. In this figure are also depicted the binary systems CO2 + limonene and CO2 + carvone, and it can be observed that the
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Figure 8. P-T diagram for the system CO2 (1) + limonene (2) + carvone (3) system. CO2 content fixed around 90 wt %.
experimental data of the ternary system are located between the binary VLE boundary limits. Also, the PREOS provides a good description of the phase behavior of the ternary system using the binary parameters obtained from the global fitting procedure (Table 2), setting the binary interaction parameter between limonene and carvone equal to zero. Figure 8 presents the experimental data maintaining the CO2 content around 90% in mass basis. It can be observed that bubble- (BP) and dew-point (DP) transitions were identified. The upper curve is related to the binary system CO2 + carvone. One can note from this figure that, as the limonene content is increased, the phase transition pressure values are decreased. The dashed lines in this figure are the PR-EOS predictions of the dew-point transitions, and the full lines are the predictions of the bubble-point transitions. It can be noted that PR-EOS, far from critical points, predicts qualitatively well the experimental results. Solubility of Catalyst and Oxidant in SCCO2 + Cosolvent. To check whether the catalyst metal-Salen (Ni-Salen) and oxidant (iodozylbenzene) affect the phase behavior of the reaction mixture, some solubility tests in SCCO2 have been carried out at pressures up to 250 bar. We have verified, however, in both cases the presence of a solid phase, indicating that the catalyst and oxidant present very small solubility in SCCO2. To verify the influence of the catalyst and oxidant on the phase behavior of the reactional mixture, some comparative tests were carried out using the ternary system CO2 + limonene + dichloromethane and adding a small amount of Ni-Salen and iodozylbenzene. The results are depicted in Figure 9 in a P-T diagram, from where one can note that the presence of the catalyst or the oxidant did not affect significantly the phase behavior of the ternary system, leading to a perturbation around 3.0 bar on the pressure transitions. Simulation of the Phase Behavior of Multicomponent Systems According to Stradi et al.,4 the main objective of performing binary system measurements is to provide information that permits the prediction of the phase behavior of multicomponent systems. The information collected may be used as a guide to system operation, indicating experimental regions where the reaction can be processed. In this section, the phase behavior of limonene oxidation systems is simulated using the PREOS with the binary parameters presented in Table 2.
Figure 9. Effect of the catalyst and oxidant addition on the phase behavior of the ternary system CO2(1)+limonene(2)+dichloromethane(3). Composition in free basis of catalyst and oxidant: w1 ) 0.8987 wt %, w2 ) 0.0411 wt % and w3 ) 0.0602 wt %.
Figure 10. Phase behavior simulation of limonene oxidation at 343 K in SCCO2 with the addition of 0.10 mol % of dichloromethane and acetonitrile (0.10 mol %).
These predictions were performed by considering a hypothetical reaction course where limonene is totally converted to carvone and 1,2-limonene oxide. The CO2 composition was kept constant at 0.85 using acetonitrile or dichloromethane as the organic solvent (0.10 mol %). To investigate the phase behavior related to the selectivity of the reaction, three possible kinetics were considered: 50% carvone and 50% 1,2-limonene oxide are produced simultaneously (path A); only carvone is produced (path B); and only 1,2-limonene oxide is produced (path C). All binary interaction parameters between solutes (organic solvent, reactants, and products) were fixed to zero. Figure 10 presents the calculated values for the phase behavior of the multicomponent mixture at 343 K. Through these simulations of pressure transitions (liquid-vapor transition), one can observe that during the limonene oxidation reaction in SCCO2, at fixed temperature and pressure conditions, the formation of a second phase into the reaction system (vapor phase specifically) can occur, when limonene is converted to products. This aspect can be very important when a reaction mechanism and kinetics are under consideration and may be used to select adequate regions to conduct the reaction. The influence of the organic solvent is also depicted in this figure, from where one can note that acetonitrile is a better solvent for this reaction than dichloromethane.
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Conclusions In this work the phase behavior of ternary systems related to the reactional mixture of the catalytic limonene oxidation in SCCO2 is reported in the temperature range from 313 to 343 K. The VLE observed were BP and DP types. It was observed that when the concentration of limonene is increased in ternary systems, keeping constant the content of the organic solvent, pressure transitions are decreased. The same effect occurred when the composition of the organic solvent was increased as a result of the enhancement of the solvent mixture power. It was also observed that the addition of Ni-Salen or iodozylbenzene did not affect significantly the phase behavior of the CO2 + limonene + dichloromethane system. PR-EOS with the classical quadratic mixing rules provided a satisfactory representation of the binary experimental information. The model was used to predict the phase behavior of the ternary systems. The results showed that the PREOS was capable of satisfactorily representing the experimental data. Acknowledgment The authors express their gratitude to CAPES, FAPERGS, CNPq, and PRONEX for financial support. Literature Cited (1) Hauthal, W. H. Advances with supercritical fluids [review]. Chemosphere 2001, 43, 123-135. (2) Marr, R.; Gamse, T. Use of supercritical fluids for different processes including new developmentssa review. Chem. Eng. Process 2000, 39, 19-28. (3) Chateauneuf, J. E.; Nie, K. An investigation of a FriedelCrafts alkylation reaction in homogeneous supercritical CO2 and under subcritical and split phase reaction conditions. Adv. Environ. Res. 2000, 4, 307-312. (4) Stradi, B. A.; Kohn, J. P.; Stadtherr, M. A.; Brennecke, J. F. Phase behavior of reactants, products and catalysts involved in the allylic epoxidation of trans-2-Hexen-1-ol to (2R,3R)-(+)-3Propyloxiranemethanol in high-pressure carbon dioxide. J. Supercrit. Fluids 1998, 12, 109-122. (5) Kemmere, M.; Vries, T.; Vorstam, M.; Keurentjes, J. A novel process for the catalytic polymerization of olefins in supercritical carbon dioxide. Chem. Eng. Sci. 2001, 56, 4197-4204. (6) Leitner, W. Homogeneous catalysts for application in supercritical carbon dioxide as a ‘green’ solvent. C. R. Acad. Sci. Paris/Chemistry 2000, 3, 595-600. (7) Francio`, G.; Wittmann, K.; Leitner, W. Highly efficient enantioselective catalysis in supercritical carbon dioxide using the perfluoroalkyl-substituted ligant (R,S)-3-H2F6-binaphos. J. Organomet. Chem. 2001, 621, 130-142. (8) Carrilho, E.; Tavares, M. C. H.; Lanc¸ as, F. M. Supercritical fluid in analytical chemistry. I. Supercritical fluid chromatography: Thermodynamic definitions. Quim. Nova 2001, 24, 509-515. (9) McHugh, M. A.; Krukonis, V. Supercritical Fluid Extraction: Principles and Practice, 2nd ed.; Butterworth-Heinemann: Oxford, U.K., 1994. (10) Pratt, J. A.; McHugh, M. A. Supercritical-Fluid Fractionation of Poly(ethylene-co-acrylic acid). J. Supercrit. Fluids 1996, 9, 61-66. (11) Dariva, C.; Oliveira, J. V.; Vale, M. G. R.; Carama˜o, E. B. Supercritical fluid extraction of high-ash Brazilian Coal. Fuel 1997, 76, 585-591. (12) Sihvonen, M.; Ja¨rvenpa¨a¨, E.; Hietaniemi, V.; Huopalahti, R. Advances in supercritical carbon dioxide technologies. Trends Food Sci. Technol. 1999, 10, 217-222. (13) Camy, S.; Condoret, J. Dynamic modeling of a fractionation
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Received for review December 30, 2002 Revised manuscript received April 16, 2003 Accepted April 21, 2003 IE021040+
