Density and Conversion in Biodiesel Production with Supercritical

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Ind. Eng. Chem. Res. 2010, 49, 7666–7670

Density and Conversion in Biodiesel Production with Supercritical Methanol A. Velez, P. Hegel, G. Mabe, and E. A. Brignole* Planta Piloto de Ingenierı´a Quı´mica, PLAPIQUI UniVersidad Nacional del Sur, CONICET Camino La Carrindanga Km 7, CC 717, 8000 Bahı´a Blanca, Argentina

The densities of reacting mixtures of vegetable oils + methanol were measured by loading a closed vessel with a known amount of an oil/alcohol mixture of a given molar ratio. For each studied condition, the mixture was brought to a temperature at which the system became homogeneous. The isochoric (constant-volume) relation between temperature and pressure at this particular density and composition was recorded. In the temperature range of 550-630 K, a high conversion of the oil to the corresponding fatty esters and glycerin was obtained. The densities of sunflower oil with methanol for two different alcohol/oil ratios, namely, 40:1 and 25:1, were measured. Densities of coconut oil + methanol and methyl oleate + methanol mixtures were also measured. The effect of global density on conversion was studied at long reaction times (3-4 h). Strong evidence of the phase transitions of the reacting mixtures, from heterogeneous to homogeneous, was obtained from the experimental results. 1. Introduction Since the early study of Saka and Kusdiana1 on the noncatalytic transesterification of vegetable oils with supercritical alcohols, a large collection of reaction data for different vegetable oils with methanol or other alcohols has been reported in the open literature.2-8 Recent reviews on noncatalytic transesterification have been presented by Pinnarat and Savage9 and Wen et al.10 In general, most of these experiments have been carried out in batch reactors using constant-volume vessels. Whereas the noncatalytic transesterification process takes place at around 600 K, the reaction starts above 500 K.2 Because of the uncertainties associated with the duration of the heating and cooling periods in this temperature range, the determination of reaction times in previous experiments4,6 has been only approximate. Some authors1,7 have reduced this uncertainty by using a rather small cell and a highly efficient heat-transfer medium (melted tin or mixed salts baths). However, in these experiments, a direct measurement of the operating pressure was not possible. For this reason, Song et al.7 assumed that the pressure inside the batch reactor was dependent on only the methanol density. In this way, pressures of about 400 bar were estimated. The use of a continuous reactor is an attractive alternative to obtain more quantitative information regarding the experimental reaction time and better temperature and pressure control. The computation of the residence time in a continuous reactor depends on the mixing intensity and on the knowledge of the reacting system density. However, no information is available in the open literature about the densities of these mixtures under reacting conditions. For example, Minami and Saka11 used an approximate approach to obtain the residence time based on the oil and methanol pure-component values. In another study, the methodology for computing the residence time5 is not given. In the present study, the densities of mixtures of vegetable oils with supercritical methanol were measured under a given set of reaction conditions. Such information is valuable for the computation of the reactor volumes and for the sizing of different process units such as heat exchangers, high-pressure pumps, and compressors. * To whom correspondence should be addressed. Tel.: + 54 291 4861700. E-mail: [email protected].

The mixtures under consideration are very asymmetrical in size and of different chemical natures, because they are composed of vegetable oils, methanol, glycerin, and fatty acid esters. These facts make the prediction of densities and phase transitions particularly unreliable under near-critical conditions. The determination of the boundaries of the homogeneous region is of particular interest because operating in the supercritical region gives higher reaction rates.2 Pinnarat and Savage9 pointed out the need for information on the critical points of the mixture as the reaction progresses in order to ensure that the reaction temperature remains above these values throughout. Phase transitions were obtained by direct observation by Hegel et al.4 with and without the use of cosolvents, but the effect of the global density (mass of reactants divided by reactor volume) on the phase transitions was not clearly determined. These authors also claimed that the operating global densities were not reported in most previous studies of supercritical methanol transesterification in batch reactors. Previous studies on transesterification with supercritical alcohols2-8 have covered a wide range of conditions: temperatures from 500 to 650 K, pressures from 8 to 40 MPa, and several mixtures of different vegetable oils with alcohols have been used. Considering the ranges of pressures and temperatures to be investigated and the need for information on the conditions of mixture phase transitions from heterogeneous to homogeneous, an isochoric method (constant-volume study) was selected for this study. The isochoric technique is an attractive method for obtaining precise densities and phase transitions and has been used by several researchers.12-14 For this purpose, a closed constant-volume cell with temperature and pressure sensors was used. In this way, the variations of pressure with temperature of mixtures of known global density were measured over the desired experimental range. Valuable information on the system behavior was collected from these experiments, including (i) the isochoric pressure-temperature relationship for the given density under homogeneous conditions, (ii) the span of the region of liquid-vapor (LV) equilibrium, (iii) the pressure and temperature of the phase transition of the mixtures, (iv) the vapor pressure of mixtures of vegetable oil + methanol during the heating process of the reacting mixtures, (v) the effect of system density on ester conversion at long reaction times

10.1021/ie100670r  2010 American Chemical Society Published on Web 07/19/2010

Ind. Eng. Chem. Res., Vol. 49, No. 16, 2010

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Figure 2. Comparison of experimental values with NIST data for methanol in the liquid-vapor region and in the supercritical isochoric region of 0.28 g/cm3 density: ([) experimental data, (s) NIST data for methanol (density ) 0.28 g/cm3), (*) critical point of pure methanol.

Figure 1. Scheme of the isochoric apparatus: (1) constant-volume cell, (2) electric oven, (3) aluminum foil, (4) temperature sensor, (5) process control equipment, (6) silicone line, (7) pressure gauge.

(3-4 h), and (vi) the observation of linear isochoric lines for both the homogeneous region and the heterogeneous LV region. The experimental results indicate that it is possible to obtain phase transition points from the intersection of the isochoric lines. It is thus possible to locate the boundaries between the liquid-vapor and homogeneous regions for the reacting systems. Also, the information on volumetric and phase transition points obtained from these mixtures can be used to verify the predictive capabilities of equations of state. 2. Experimental Section Materials. The compounds used in the present experiments were methanol (99.8% from Anedra), methyl oleate (>70%; 81.3% C18 esters, 15.5% C18 esters), and methyl heptadecanoate (99% from Sigma-Aldrich), sunflower oil with high oleic acid content (Ecoop, S.A., Buenos Aires, Argentina), and coconut oil (Parafarm, Buenos Aires, Argentina). Experimental information was obtained in the constant-volume cell for pure methanol and for the following mixtures: sunflower oil + methanol, coconut oil + methanol, and methyl oleate + methanol. In the case of the vegetable oil + alcohol mixtures, there were different degrees of conversion to fatty esters and glycerin depending on the temperature and global density of each experiment. Experimental Apparatus. A scheme of the experimental apparatus is given in Figure 1. It consisted of a constant-volume cell with temperature and pressure sensors. The cell was a stainless steel tube (closed at both ends) of 3/8-in. nominal diameter and 64-cm length with a volume at room temperature of 26.1 cm3. This cell was used in all of the density-related experiments. The pressure was measured with a Dynisco (Franklin, MA) melt pressure gauge PG4 series suitable for measurements at elevated temperatures with an error margin of 2%. Temperature was measured with a PT-100 platinum

resistance thermometer that guarantees an accuracy of within 0.1 K. Temperature values were recorded with a process control unit (Eurotherm Chessell 6100 E). The cell was covered with aluminum foil to isolate it from the electric oven radiation. The cell was placed in an electric oven with temperature control. The cell temperature was increased slowly to a desired temperature level and was kept at that level until no variation in the system pressure was observed. The accuracy of the pressure measurements obtained from these experiments was checked against the vapor pressure of methanol. The comparison between the experimental measurements and data for methanol from the National Institute of Standards and Technology (NIST) tables15 is given in Figure 2. For this calibration, the cell was charged with the critical density of methanol. In the supercritical region, the experiments with pure methanol yielded the isochoric pressure vs temperature line at the methanol critical density. The intersection between the isochoric line and the vapor pressure curve corresponds to the critical point of the substance investigated. The experimental values were in good agreement with the values from the NIST tables. Measurements of the conversion of oil to methyl esters were obtained at different global densities by gas chromatography. The conversion is given as the weight percentage of vegetable oil converted to ester. The methanol present in the reaction products was separated in a vacuum oven at 363 K and 25 mmHg. After the methanol has been extracted, the biodiesel and glycerin become practically immiscible and can easily be recovered. The biodiesel samples were analyzed with a gas chromatograph (Varian Star 3400 CX, Palo Alto, CA) to determine their ester contents. The equipment was assembled with a flame ionization detector (FID) and capillary column (J&W Scientific, Folson, CA; model DB-5HT, 15-m length, 0.320-mm inner diameter, and 0.10-µm film thickness). The chromatography conditions were selected according to BS EN 14103.16 Hydrogen was used as the gas carrier at a flow rate of 1.5 mL/min and at a split flow rate of 75 mL/min. The injector and detector temperatures were 633 and 643 K, respectively. The oven temperature program was initially set at 368 K for 5 min and included a ramp of 30 K/min to 453 K, a ramp of 30 K/min to 503 K, and a ramp of 25 K/min to 623 K, where the temperature was maintained for 15 min. Tetradecane was used as the internal standard, and methyl heptadecanoate was used as the reference ester for the calibration curve. 3. Results and Discussion A pressure vs temperature variation curve for a typical experiment is shown in Figure 3 for the system sunflower oil + methanol at a density of 0.37 g/cm3 and a methanol/oil molar

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Figure 3. Pressure vs temperature for methanol/sunflower oil; molar ratio ) 40:1, global density ) 0.37 g/cm3. (0,b) LLV and LV region, (∆) singlephase region.


ratio of 40. It should be pointed out that the measurements were made under reaction conditions and that the degree of conversion changes during the experiment. The pressure vs temperature line has three different sections. In the first section, at the lower temperature range, the liquid phase of the methanol + oil system is heterogeneous and presents liquid-liquid-vapor (LLV) equilibrium. Under these conditions, there is only one degree of freedom in a binary system, and the vapor pressure line is unique at any system density or methanol/oil ratio. The complete miscibility in the liquid phase of the vegetable oil with methanol is achieved at temperatures close to the critical values of methanol.17 At higher temperatures, a linear relation of pressure vs temperature with a lower slope than the vapor pressure line is observed. In this region, depending on the global density of the mixture, either the liquid phase or the vapor phase increases up to a point at which the whole mixture becomes homogeneous. At this point, a high conversion of the sunflower oil to fatty esters and glycerin has already taken place, because the reaction starts above 500 K.2 The homogeneous phase at this point is of almost constant composition and displays a typical linear relationship of a single-phase isochoric pressure variation with temperature. The transition between the two linear sections takes place at a temperature of 573 K and a pressure of 145 bar for a density of 0.37 g/cm3. These conditions are considered to be the dew- or bubble-point values for the reaction product mixture. The linear relation of the third section is observed up to the highest temperatures studied (around 633 K). Similar behavior can be observed in Figures 4 and 5 for the global densities 0.44 and 0.51 g/cm3, respectively. At higher densities, the length of the second section is reduced, and the transition points to a homogeneous mixture are achieved at lower temperatures. The transition to homogeneous conditions at lower



Figure 7. Pressure vs temperature for methanol/sunflower oil; molar ratio ) 40:1, global density ) 0.29 g/cm3. (0,b) LLV and LV region.

temperatures is the result of two effects: (i) at higher densities, the solvent power of the gas phase increases, and (ii) the conversion to esters is higher, as discussed later. When using a methanol/oil ratio of 25 instead of 40 for a density of 0.37 g/cm3, the length of the intermediate LV section is increased, and homogeneous conditions are achieved at a higher temperature (Figure 6). This behavior can be explained by the lower conversion to esters that is obtained at smaller molar ratios and by the higher pressure and temperature required to dissolve the heavy substrate in a smaller amount of methanol. The experiments carried out at the lowest global density studied (0.29 g/cm3) exhibited a different behavior. In this case, the second section was observed up to the highest temperature studied (623 K and 160 bar of pressure), and no phase transition was observed (Figure 7). Therefore, the system seemed to remain in the vapor-liquid region throughout the temperature and pressure range studied. However, it is possible that what we are observing is a phenomenon already discussed by Barrufet and Eubank18 of the colinearity of the heterogeneous and


Figure 8. Pressure vs temperature for methanol/coconut oil; molar ratio ) 40:1, global density ) 0.37 g/cm3. (0,b) LLV and LV region, (∆) singlephase region.

Figure 9. Pressure vs temperature for methanol/methyl oleate ) 12:1, global density ) 0.37 g/cm3. (0,b) LLV and LV region, (∆) single-phase region.

homogeneous isochoric lines at the cricondertherm (maximumtemperature) point of the phase envelope of a mixture. Therefore, near this point, even if there is a transition, it might not be possible to detect it using the isochoric technique. Continuing with the isochoric experiments, the system coconut oil + methanol was studied to observe the effect of the oil molecular weight (730) on the system density and phase transitions (Figure 8). The results for coconut oil working with the same density (0.37 g/cm3) and methanol/oil molar ratio (40) as used in the case of methanol/sunflower oil indicate a reduction in the heterogeneous vapor-liquid section. The phase transition temperature of the mixture decreases from 598.5 to 571 K when compared to the sunflower oil/methanol case, at the same density and molar ratio. This result can be expected considering the lower molecular weights of coconut oil and the esters derived from the coconut oil. Finally, experiments with methanol + methyl oleate (Figure 9) were carried out to study the behavior of a system in which no chemical reaction is taking place. The behavior of the nonreacting system is similar to that observed in the methanol/oil systems. Again, in the supercritical methanol region, a linear pressure vs temperature region is obtained, and a change of slope is observed at the transition point to a singlephase condition. The data obtained on pressure-temperature variations in the homogeneous isochoric region were used to plot (Figure 10) the reacting mixture densities as a function of pressure at various temperatures. This type of plot, for the oil/methanol molar ratio of 40, is helpful for the estimation of densities under reacting conditions. This molar ratio has been proposed by several researchers1,2,4,8 as an optimum value for supercritical transesterification with methanol. In Figure 11, the densities of the

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Figure 10. Densities of homogeneous reacting mixtures of methanol + sunflower oil (molar ratio ) 40) as a function of temperature and pressure: ([) 553, (9) 563, (2) 573, (b) 583, (]) 593, (0) 603, (4) 613, and (O) 623 K.

Figure 11. Densities of homogeneous reacting mixtures of methanol + sunflower oil, molar ratio ) 40 (open symbols) and pure methanol (solid symbols, NIST data) as a function of temperature and pressure: (]) mixture at 553 K, (4) mixture at 583 K, (O) mixture at 623 K, (9) methanol at 553 K, (2) methanol at 563 K, (b) methanol at 573 K, ([) methanol at 583 K.

Figure 12. Densities of homogeneous reacting mixtures of methanol + sunflower oil (molar ratio ) 25) as a function of temperature and pressure: ([) 553, (9) 563, (2) 573, (b) 583, (]) 593, (0) 603, (4) 613, and (O) 623 K.

reacting mixtures (molar ratio of 40) and pure methanol (NIST15) are compared to highlight the large difference in densities of the reacting mixture with respect to pure methanol. In Figure 12, the reacting mixture density for a methanol/oil ratio of 25 is presented. The temperatures and pressures of the phase transitions to homogeneous conditions for the different systems and conditions studied are presented in Table 1. The experimental results on the phase transitions indicate that, at a similar global density (0.37 g/cm3) and methanol/oil molar ratio, a decrease in the molecular weight of the oil (coconut oil) decreases the temperature and pressure needed to obtain homogeneous conditions. On the other hand, the results for the methyl oleate + methanol

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Table 1. Temperature and Pressure Values at the Transition to Homogeneous Conditions for the Systems Studied system (at initial conditions)

methanol/sunflower oil methanol/sunflower oil methanol/sunflower oil methanol/sunflower oil methanol/coconut oil methanol/methyl oleate methanol/sunflower oil methanol/sunflower oil methanol/sunflower oil methanol/sunflower oil

phase transition

ester yield methanol/oil density temperature pressure at 633 K (K) (bar) (wt %) ratio (g/cm3)

40 40 40 40 40 12.37 25 25 25

0.29 0.37 0.44 0.51 0.37 0.37 0.29 0.37 0.44 0.51

598.5 589 548 571 598 623 606 578

146 166 112 116 165 148 153 147

81.3 86.7 94.4 95.4 90.0 73.2 74.3 86.2 90.6

transition at this density are close to those obtained for the sunflower oil + methanol transition, indicating that a high degree of transesterification has taken place in the second system, at the conditions of the phase transition. When the methanol/oil molar ratio is reduced from 40 to 25, an increase in the temperature and pressure required to reach homogeneous conditions is observed for all of the sunflower oil + methanol mixtures studied. All of the isochoric experiments for the different mixtures and densities were carried out up to temperatures close to 633 K. At the end of each experiment, the conversion to methyl esters was measured. In these experiments, the total reaction time was 3-4 h. Therefore, these results can be considered values of conversion at equilibrium at this temperature. The effect of the global density on the conversion for a methanol/ oil ratio of 40 is demonstrated in Table 1. From these data, it is clear that higher densities favor greater conversion efficiencies at very long reaction times. These data also suggest that a density of at least 0.44 g/cm3 is necessary to achieve high conversion levels. The conversion to methyl esters was measured at the beginning and at the end of the first and second linear sections to follow the degree of conversion along an experimental run. The measured conversions were 27%, 80.3%, and 94.4% for a density of 0.44 g/cm3. At these points, the temperatures were 533, 593, and 633 K, respectively. These results indicate the need to operate above 600 K to achieve high conversions. Conclusions A simple technique has been implemented to obtain density data for reacting mixtures of vegetable oils + methanol under heterogeneous and homogeneous conditions. The effects of the methanol/oil molar ratio, molecular weight, pressure, and temperature on the system density were studied. In addition, the phase transitions from the heterogeneous to the homogeneous region were determined from the intersection between the linear isochoric lines. The results obtained are valuable for obtaining reactant residence times in continuous reactors and confirm the importance of system density in the achievement of high conversions.

Acknowledgment The authors thank the Argentinean National Research Council (CONICET) and FONCYT-ANPCYT of Argentina for the financial support of this study. Supporting Information Available: Tabulated information on the densities of sunflower oil/methanol mixtures as a function of pressure, temperature, and molar ratio. This information is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Saka, S.; Kusdiana, D. Biodiesel Fuel from Rapeseed Oil as Prepared in Supercritical Methanol. Fuel 2001, 80, 225–231. (2) Kusdiana, D.; Saka, S. Kinetics of Transesterification in Rapeseed Oil to Biodiesel Fuel as Treated in Supercritical Methanol. Fuel 2001, 80, 693–698. (3) Cao, W.; Han, H.; Zhang, J. Preparation of Biodiesel from Soybean Oil Using Supercritical Methanol and Co-solvent. Fuel 2005, 84, 347–351. (4) Hegel, P.; Mabe, G.; Pereda, S.; Brignole, E. A. Phase Transitions in a Biodiesel Reactor Using Supercritical Methanol. Ind. Eng. Chem. Res. 2007, 46, 6360–6365. (5) He, H.; Wang, T.; Zhu, S. Continuous Production of Biodiesel Fuel from Vegetable Oil Using Supercritical Methanol Process. Fuel 2007, 86, 442–447. (6) Madras, G.; Kolluru, C.; Kumar, R. Synthesis of biodiesel in supercritical fluids. Fuel 2004, 83, 2029–2033. (7) Song, E.; Lim, J.; Lee, H.; Lee, Y. Transesterification of RBD Palm Oil Using Supercritical Methanol. J. Supercrit. Fluids 2008, 44, 356–363. (8) Demirbas, A. Biodiesel from Vegetable Oils via Transesterification in Supercritical Methanol. Energy ConVers. Manage. 2002, 43, 2349–2356. (9) Pinnarat, T.; Savage, P. E. Assessment of Noncatalytic Biodiesel Synthesis Using Supercritical Reaction Conditions. Ind. Eng. Chem. Res. 2008, 47, 6801–6808. (10) Wen, D.; Jiang, H.; Zhang, K. Supercritical Fluids Technology for Clean Biofuel Production. Prog. Nat. Sci. 2009, 19, 273–284. (11) Minami, E.; Saka, S. Kinetics of Hydrolysis and Methyl Esterification for Biodiesel Production in Two Step Supercritical Methanol Process. Fuel 2006, 85, 2479–2483. (12) Yurttas, L.; Jolste, J. C.; Hall, K. R. Semiautomated Isochoric Apparatus for PVT and Phase Equilibrium Studies. J. Chem. Eng. Data 1994, 39, 418–423. (13) Duarte-Gaza, H. A.; Magee, J. W. Isochoric p-F-T Measurements {(x)CO2 + (1- x)C2H6, x ≈ 0.25, 0.49, 0.74} from (220 to 400 K) at Pressure to 35 MPa. J. Chem. Eng. Data 2001, 46, 1095–1106. (14) Zhou, J.; Patil, P.; Ejaz, S.; Atilhan, M.; Holste, J. C.; Hall, K. R. (p, Vm, T) and Phase Equilibrium Measurements for a Natural Gas-Like Mixture Using an Automated Isochoric Apparatus. J. Chem. Thermodyn. 2006, 38 (11), 1489–1494. (15) NIST Chemistry WebBook; NIST Standard Reference Database Number 69; National Institute of Standards and Technology (NIST): Gaithersburg, MD, 2005; available at http://webbook.nist.gov/chemistry/ (accessed January 2010). (16) BS EN 14103:2003, Fat and oil deriVatiVes - Fatty acid methyl esters (FAME) - Determination of Ester and Linolenic Acid Methyl Ester Contents; European Committee for Standardization (CEN): Brussels, Belguim, 2003. (17) Tang, Z.; Du, Z.; Min, E.; Gao, L.; Jiang, T.; Han, B. Phase Equilibria of Methanol-Triolein System at Elevated Temperature and Pressure. Fluid Phase Equilib. 2006, 239, 8–11. (18) Barrufet, M. A.; Eubank, P. T. New Physical Constraints for Fluid Mixture Equations of State and Mixing Combining Rules. Fluid Phase Equilib. 1987, 37, 223–240.

ReceiVed for reView March 18, 2010 ReVised manuscript receiVed June 7, 2010 Accepted July 2, 2010 IE100670R

Density and Conversion in Biodiesel Production with Supercritical

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