Ind. Eng. Chem. Res. 2006, 45, 3693-3696
3693
Liquid-Liquid Phase Equilibrium in Glycerol-Methanol-Methyl Oleate and Glycerol-Monoolein-Methyl Oleate Ternary Systems Devender S. Negi, Felix Sobotka, Tobias Kimmel, Gu1nter Wozny, and Reinhard Schoma1 cker* Institut fu¨r Chemie and Fachgebiet Dynamik und Betrieb technischer Anlagen, Technische UniVersita¨t Berlin, Berlin 10623, Germany
In the present work, the liquid-liquid phase equilibrium data for the glycerol-methanol-methyl oleate and glycerol-monoolein-methyl oleate ternary systems were determined experimentally and compared with predictions of the UNIFAC and UNIFAC-Dortmund models. Liquid two-phase multicomponent systems consisting mainly of these chemical species are found in some industrially important transesterification reactions. For the glycerol-methanol-methyl oleate ternary system at 60 °C, the predictions of UNIFAC and UNIFACDortmund are in good agreement with the experimental results. For the ternary glycerol-monoolein-methyl oleate system at 135 °C, the predictions of UNIFAC and UNIFAC-Dortmund deviate significantly from the experimental data and are only qualitatively correct. The reliability of the experimental data was determined through Othmer-Tobias plots. Introduction Transesterification reactions whereby two liquid phases coexist are common in the production of some important chemicals, e.g., biodiesel and mono- and diglycerides, among others. Mono- and diglycerides are important food emulsifiers,1 and biodiesel, as the name suggests, is a potential alternative to petroleum-based diesel fuel. The reactions employed for their production involve an alcohol phase (mostly glycerol and/or methanol) and a fatty phase (fats, fatty acids, or fatty acid methyl esters). In the literature on processes for monoglyceride production, the published information on the solubility of glycerol in the fatty phase is mostly qualitative and sometimes not consistent.2 To our knowledge, the effect of glycerol solubility on the reaction kinetics has not been investigated. During investigations of transesterification reactions between fatty acid methyl esters and glycerol in our laboratory,3 it was found that the glycerol concentration in the fatty phase increases with conversion, and the reaction was observed to accelerate after a slow start. Similar observations of transesterification reaction systems have been reported by other investigators,4-7 although they gave different explanations for their observations. In liquid two-phase reacting systems, the produced components can, in some cases, affect the reactant-phase miscibility significantly, as was found in the present study with the glycerol-monooleinmethyl oleate system. This changing phase behavior will affect the reaction kinetics in these systems. The main route for biodiesel production involves the reaction of triglycerides (fats) with excess methanol (generally at a molar ratio of 1:6). This process is also sometimes called methanolysis. Higher temperatures give higher reaction rates, but at atmospheric pressure, the reaction temperature is limited by the boiling point of the alcohol. Methanolysis is generally carried out at 40-70 °C. The product mixture in methanolysis comprises mainly the product fatty acid methyl esters (biodiesel) and glycerol and the leftover methanol. Mono- and diglycerides appear as intermediate products in this process. This process shows a complex phase behavior, which has also not been * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +49 30 31424973. Fax: +49 30 31421595.
investigated in kinetic studies. Only recently has a research group published its experimental results on the phase behavior in a biodiesel production process.8 Monoglyceride production is generally carried out by basecatalyzed fat glycerolysis at 200-260 °C. A similar method is the base-catalyzed glycerolysis of fatty acid methyl esters (FAMEs) that can be carried out at lower temperatures (120200°C) and has certain advantages over fat glycerolysis.2,9 The FAME glycerolysis for the production of monoglycerides employs the reverse reaction of that used for biodiesel production, i.e., the reaction between glycerol and FAME to give mono-, di-, and triglycerides and methanol. Knowledge about the phase equilibrium in these systems is essential for a better understanding of the process and improvement of the reaction rate, the selectivity of the desired product, and the separation process for the product mixture. Moreover, simulations for developing new processes for such systems require an activity coefficient model that can adequately describe these multiphase systems. Some authors have used UNIFAC for describing similar systems,10 while others have used UNIQUAC and NRTL (with missing parameters estimated by UNIFAC) for the process simulation of biodiesel production.11 However, information on the experimental implementation of the biodiesel production process is not available. Here, we have experimentally investigated the phase equilibria of the glycerol-methanol-methyl oleate system at 60 and 135 °C and of the glycerol-monoolein-methyl oleate system at 135 °C. Monoolein is the monoglyceride produced by the glycerolysis of methyl oleate. The experimentally obtained data have been compared with predictions by UNIFAC and UNIFACDortmund. Theoretical Section Two liquid phases at equilibrium can be described by the expression
xi′γi′ ) xi′′γi′′ In the absence of experimental data, the activity coefficients, γi, can be calculated by predictive methods such as UNIFAC,12 which gives good results for many multicomponent systems. It
10.1021/ie051271r CCC: $33.50 © 2006 American Chemical Society Published on Web 04/13/2006
3694
Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006
Table 1. Group Distributions for UNIFAC-LLE, UNIFAC-VLE, and UNIFAC-Dortmund oleatea-c
methyl glycerola,b glycerolc methanola methanolb,c monooleina,b monooleinc a
CH3
CH2
CH
C2H2
OH/OH-p
OH-s
CH3OH
CH3COO
CH2COO
1 0 0 1 0 1 1
14 2 2 0 0 15 15
0 1 1 0 0 1 1
1 0 0 0 0 1 1
0 3 2 1 0 2 1
0 0 1 0 0 0 1
0 0 0 0 1 0 0
1 0 0 0 0 0 0
0 0 0 0 0 1 1
UNIFAC-LLE. b UNIFAC-VLE. c UNIFAC-Dortmund
is known, however, that the model predictions can sometimes deviate widely from reality, and therefore, experimental data are needed for validation of model predictions; such data can also be used for the estimation of missing parameters of thermodynamic models. Details of UNIFAC can be found in the literature.12,13 UNIFAC parameters were introduced originally for vaporliquid equilibrium (VLE) data prediction. Later, Magnussen et al.14 introduced parameters for liquid-liquid equilibrium (LLE) data prediction. The main limitations of this model, relevant to the present system, are that the recommended temperature range for the use of LLE parameters is small (10-40 °C) and the activity coefficients at infinite dilution are often unsatisfactory, especially when molecules of very different sizes are involved. Furthermore, predictions with systems containing alcohols are also sometimes incorrect. UNIFAC-Dortmund was introduced by Weidlich and Gmehling15 to overcome the disadvantages of UNIFAC, including those mentioned above. UNIFAC-Dortmund uses a single set of parameters for both LLE and VLE predictions, and the parameters are applicable in a temperature range of around 25-150 °C. Also, secondary and tertiary OH groups were introduced to improve the predictions with multicomponent systems involving secondary and tertiary alcohols. However, groups for polyalcohols such as glycerol are still not included in UNIFAC. This work is also an attempt to test the model predictions using the recommended alcohol grouping for glycerol. The data presented here can be used for the estimation of new parameters involving glycerol. The group distributions of molecules present in the investigated system according to UNIFAC-LLE, -VLE, and -Dortmund are given in Table 1. The UNIFAC parameters used were taken from literature.14,16,17 The phase equilibrium equations were solved using a MATLAB program. The LLE parameters were used here outside the recommended range for comparison of UNIFAC-LLE with experimental results and with the predictions of UNIFACDortmund. Experimental Section Materials and Analytic Apparatus. The chemicals used for the liquid-liquid phase equilibrium experiments were glycerol (>99%, Sigma), methanol (>99.9%, Roth), and R-monoolein (>99%, Fluka). For experiments with glycerol-monooleinmethyl oleate, pure methyl oleate (99%, Aldrich) was used. For experiments with the glycerol-methanol-methyl oleate system, technical-grade methyl oleate (>75%, Lancaster) was used. The analysis of the supplied technical-grade methyl oleate showed that the actual methyl oleate (C18:1) content was 85 ( 1%, and the overall content of C18 (C18:0-C18:3) was 97 ( 1%. The samples were analyzed by gas chromatography (GC). For sample preparation, 1,4-dioxane (>99.8%, Roth) was used as the solvent. N,O-Bis-(trimethylsilyl)trifluoroacetamide (BSTFA) (98%, ABCR) was used as a silylating agent. Hexadecane (>98%, Fluka) was used as an internal standard. For calibration,
the pure chemicals mentioned above were used. The analysis was done using a Hewlett-Packard 5890 series-II GC with a capillary column and flame ionization detector. The GC column was a DB5-HT column (J & W) with a length of 30 m, an inner diameter of 0.32 mm, and a film thickness of 0.1 µm. A deactivated fused silica tubing (3 m × 0.53 mm) coated with cyanophenylmethyl (Varian) was used as the precolumn. Nitrogen gas was used as the carrier. Experimental Setup and Procedure. A jacketed glass vessel with a stirrer was used to carry out the LLE experiments. The experiments with the glycerol-methanol-methyl oleate system were performed in a glass vessel of 50-mL volume. For the glycerol-monoolein-methyl oleate system, a similar setup was used, but with a smaller volume of 5 mL, as pure monoolein and methyl oleate are rather expensive. The procedure used was same in both cases: The two immiscible components (glycerol and methyl oleate) were added into the vessel in a specific molar ratio, and the third component (methanol or monoolein) was added in steps to obtain phase compositions for different tie lines. The jacketed glass vessels were heated to the desired temperature by a thermostatically controlled oil bath (Haake, B5/F6). The temperature was measured using a Pt-100 probe. The phases were stirred at a high speed under well-dispersed conditions for 150 min and allowed to separate until both phases were clear. This took a few minutes to a few hours. Samples of 10-µL volume were withdrawn from both phases with a clean GC syringe (Ito Corporation). Before being used to withdraw an actual sample, the syringe was rinsed four times with the liquid. The greatest possible care was taken not to disturb the phases during sample withdrawal. In the experiments with methanol, after the stirring was stopped, the methanol condensed on the vessel lid was not allowed to fall back into the liquid. Also, greater care had to be taken during experiments with the smaller vessel for stirring and sample withdrawal. Analysis. The samples were collected in 2-mL GC vials and diluted with 1 mL of 1,4-dioxane. The solvent dioxane contained 0.383% hexadecane. The samples were silylated with BSTFA and then injected into the GC column. The inlet temperature of the GC column was kept constant at 40 °C. The total time required to analyze one sample was 61.5 min. The temperature program ramped in four steps: First, the initial column temperature was kept constant at 38 °C for 15 min. Second, the temperature was raised at a rate of 1 °C/min to 46 °C. Third, the temperature was raised to 370 °C at a rate of 16 °C/min and held at 370 °C for 10 min. Finally, the column was cooled to 40 °C at a rate of 40 °C/min. The chromatogram thus obtained was analyzed for the composition of the sample. The mass of a component i in the injected sample, wi,, is proportional to the area of the GC peak
wi ∝ fiAi where fi is the response factor of component i as obtained by calibration.
Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006 3695
Figure 1. Ternary diagram for glycerol-methanol-methyl oleate at 60 °C. Comparison of experimental data with UNIFAC.
Figure 2. Ternary diagram for glycerol-monoolein-methyl oleate at 135 °C (except point 0, which represents the fatty phase at 21 °C). Comparison of experimental data with UNIFAC models.
Table 2. Compositions of the Points on the Tie Lines in the Ternary Diagram for Glycerol-Methanol-Methyl Oleate at 60 and at 135 °Ca fatty phase
b
glycerol phase
x′MeOH
x′G
x′ME
x′′MeOH
x′′G
x′′ME
0.043 0.098 0.163 0.267 0.303 0.016b
0.002 0.008 0.007 0.008 0.008 0.018b
0.955 0.894 0.830 0.725 0.689 0.966b
0.105 0.331 0.446 0.598 0.669 0.020b
0.895 0.669 0.554 0.402 0.330 0.980b
0.000 0.000 0.000 0.000 0.001 0.000b
a MeOH ) methanol, G ) glycerol, ME ) methyl ester (methyl oleate). Data at 135 °C.
The mole fraction of component i in the injected sample can be calculated as
xi )
Figure 3. Othmer-Tobias plot for the experimental data from the glycerolmethanol-methyl oleate system at 60 °C, where a is the mole fraction of methyl oleate in the fatty phase and b is the mole fraction of glycerol in the glycerol phase.
wi/Mi
∑(wi/Mi)
where Mi is the molecular weight of component i. The measured composition from the analysis of three samples taken from each phase at fixed experimental conditions was reproducible within (0.5 mol %. Based on the calibration, an error of within (1.0 mol % in the measured data was estimated. Results and Discussion Comparison of Experimental Results with Predicted Data. The two presently investigated systems are discussed below separately. System 1: Glycerol-Methanol-Methyl Oleate. Figure 1 shows a comparison of the experimental data with the predictions of UNIFAC. At 60 °C, the model predicts well the immiscibility between glycerol and methyl oleate. Also, the predictions of the methanol concentrations in both phases are good. Similar results were obtained using LLE/VLE parameters, as well as UNIFAC-Dortmund. The tie-line data are provided in Table 2. Table 2 also includes data for a single tie line obtained experimentally at 135 °C. These data show that temperature has little effect on the glycerol-methyl ester solubility in the range studied. For the data obtained at 135 °C and 1 atm, only the liquid phases were considered. For the complete description of this (VLLE) system at 135 °C, the vapor phase also has to be investigated, which was not within the scope of the present work. System 2: Glycerol-Monoolein-Methyl Oleate. Figure 2 shows the experimental data and the calculated component mole fractions for the two phases at equilibrium at 135 °C. The predictions for the glycerol phase are in good agreement with
Figure 4. Othmer-Tobias plot for the experimental data from the glycerolmonoolein-methyl oleate system at 135 °C, where a is the mole fraction of methyl oleate in the fatty phase and b is the mole fraction of glycerol in the glycerol phase.
the experimental values. The predictions for the fatty phase are only qualitatively correct, in the sense that they show increasing glycerol solubility in FAME upon addition of monoglyceride. None of the methods could provide acceptable results. The best trend was shown by UNIFAC-Dortmund. UNIFAC predicted a much lower solubility of glycerol in the ester phase. Figure 2 also includes one experimental point showing the composition of the fatty phase 5 days after the liquids had been cooled to room temperature. Reliability of Experimental Data. Othmer and Tobias18 introduced a correlation (tie-line correlation) that can be used to test the reliability of experimental data in a ternary system at LLE. Figures 3 and 4 show that the experimentally obtained data for the two ternary systems give a good linear fit with the tie-line correlation, indicating the consistency of the experimental data. The experimental tie-line data are given in Tables 2 and 3.
3696
Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006
Table 3. Compositions of the Points on the Tie Lines in the Ternary Diagram for Glycerol-Monoolein-Methyl Oleate at 135 °Ca fatty phase
glycerol phase
x′MG
x′G
x′ME
x′′MG
x′′G
x′′ME
0.234 0.375 0.424 0.429
0.100 0.258 0.386 0.456
0.666 0.367 0.190 0.115
0.004 0.005 0.006 0.006
0.990 0.987 0.983 0.981
0.006 0.008 0.011 0.013
a MG ) monoglyceride (monoolein), G ) glycerol, ME ) methyl ester (methyl oleate).
Concluding Remarks: Relevance to Systems Involving Mono- and Diglycerides and Biodiesel Production. This study was done as part of a series of kinetic studies on base-catalyzed FAME glycerolysis to produce mono- and diglycerides. The information obtained from the above ternary diagrams sheds light on some of the lesser-known facts about glycerol solubility in the fatty phase in the reaction system. The glycerol-FAME system represents the starting component mixture in FAME glycerolysis. In this work, it is shown that, even though the reactants FAME and glycerol are nearly immiscible, the miscibility of glycerol in the fatty phase increases by a large amount as the fraction of monoglyceride in the liquid mixture increases. A separate study indicated that the catalyst (NaOCH3 in our case) does not have a significant effect on glycerol solubility in FAME. We also investigated other aspects, such as mass-transfer limitations and the effect of emulsifying properties of monoglyceride on the reaction rate in FAME glycerolysis, and we found that the changing glycerol solubility plays a key role in increasing the reaction rate after a slower start. In the temperature range studied, the mutual solubility of glycerol and the FAME does not change much; rather, the monoglyceride concentration has a much stronger effect in dissolving glycerol in FAME. A similar effect can be expected in biodiesel production, where the product FAME leads to an increased mutual solubility of methanol and the fatty phase. Methanol, the second important component that could be suspected to change the mutual solubility of glycerol and FAME, is shown to have no such effect, even at temperatures much above the room temperature. In biodiesel production, the separation of glycerol from the fatty phase in the product mixture is generally done by gravity separation. The presence of excess methanol generally used is not expected to affect the separation of glycerol from the fatty phase that consists mainly of FAME. The UNIFAC models considered here were not found to be adequate for simulating the presently addressed transesterification processes involving fatty acid glycerides, although good predictions were shown by these models for the glycerolmethanol-methyl oleate system. Nomenclature wi ) mass of component i in the sample injected in the GC column mi ) mass fraction of component i in the sample xi ) mole fraction of component i in the sample fi ) response factor of component i for GC analysis Ai ) area of the GC peak of component i γi ) activity coefficient of component i Mi ) molar mass of component i
AbbreViations GC ) gas chromatography/chromatograph/chromatogram C18:n ) methyl esters with a fatty acid chain length of 18 carbon atoms and n unsaturated bonds LLE )liquid-liquid equilibrium VLE )vapor-liquid equilibrium VLLE ) vapor-liquid-liquid equilibrium NRTL ) nonrandom two liquid UNIFAC ) UNIQUAC functional-group activity coefficients UNIQUAC )universal quasichemical Acknowledgment The authors acknowledge the Berliner Graduiertenkolleg 827 “Transportvorga¨nge an bewegten Phasengrenzfla¨chen” and the Deutsche Forschungsgemeinschaft (DFG) for financial support. Help regarding the UNIFAC program from Prof. J. M. Prausnitz (UC Berkeley) and Dr. P. Li (TU Berlin) is acknowledged. Literature Cited (1) Henry, C. Monoglycerides: The Universal Emulsifier. Cereal Foods World 1995, 40, 734. (2) Sonntag, N. Glycerolysis of Fats and Methyl Esters-Status, Review and Critique. J. Am. Oil Chem. Soc. 1982, 50, 795. (3) Kimmel, T. Kinetic Investigation of the Base-Catalyzed Glycerolysis of Fatty Acid Methyl Esters. Ph.D. Thesis, Institut fu¨r Chemie, T. U. Berlin, Berlin, Germany, 2004. (4) Noureddini, H.; Zhu, D. Kinetics of Transesterification of Soybean Oil. J. Am. Oil Chem. Soc. 1997, 74, 1457. (5) Lotero, E.; Liu, Y.; Lopez, D. E.; Suwannakarn, K.; Bruce, D. A.; Goodwin, J. G., Jr. Synthesis of Biodiesel via Acid Catalysts. Ind. Eng. Chem. Res. 2005, 44, 5353. (6) Freedman, B.; Butterfield, R. O.; Pryde, E. H. Transesterification Kinetics of Soybean Oil. J. Am. Oil Chem. Soc. 1986, 63, 1375. (7) Bossaert, W. D.; De Vos, D. E.; Van Rhijn, W. M.; Bullen, J.; Grobet, P. J.; Jacobs P. A. Mesoporous Sulfonic Acids as Selective Heterogeneous Catalysts for the Synthesis of Monoglycerides. J. Catal. 1999, 182, 156. (8) Cerce, T.; Peter, S.; Weidner, E. Biodiesel-Transesterification of Biological Oils with Liquid Catalysts: Thermodynamic Properties of OilMethanol-Amine Mixtures. Ind. Eng. Chem. Res. 2005, 44, 9535. (9) Jeromin, L.; Wozny, G.; Li, P. U.S. Patent 6,127,561, 2000. (10) Wollmann, G.; Gutsche, B.; Peukert, E.; Jeromin, L. Glycerinolyses Modellierung und Auslegung einer Reaktion mit einer Mischungslu¨cke der Reaktanten. Fat Sci. Technol. 1988, 90, 507. (11) Zhang. Y.; Dube, M. E.; McLean, D. D.; Kates, M. Biodiesel Production from Waste Cooking Oil: 1. Process Design and Technological Assessment. Bioresour. Technol. 2003, 89, 1. (12) Fredenslund A.; Jones, R. L.; Prausnitz, J. M. Group-Contribution Estimation of Activity Coefficients in Nonideal Liquid Mixtures. AIChE J. 1975, 21, 1086. (13) Fredenslund, A.; Gmehling, J.; Rasmussen, P. Vapor-Liquid Equilibria Using UNIFACsA Group Contribution Method; Elsevier: Amsterdam, 1977. (14) Magnussen, T.; Rasmussen, P.; Fredenslund, A. Parameter Table for Prediction of Liquid-Liquid Equilibria. Ind. Eng. Chem. Res. 1981, 20, 331. (15) Weidlich, U.; Gmehling, J. A Modified UNIFAC Model. 1. Prediction of VLE, he, and γ∞. Ind. Eng. Chem. Res. 1987, 26, 1372. (16) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th ed.; McGraw-Hill Inc.: New York, 1987. (17) Gmehling, J.; Li, J.; Schiller, M. A Modified UNIFAC Model. 2. Present Parameter Matrix and Results for Different Thermodynamic Properties. Ind. Eng. Chem. Res. 1993, 32, 178. (18) Othmer, D. F.; Tobias P. E. Tie Line Correlation. Ind. Eng. Chem. 1942, 34, 693.
ReceiVed for reView November 16, 2005 ReVised manuscript receiVed March 2, 2006 Accepted March 21, 2006 IE051271R
