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Variables Affecting the Induction Period during Acid-Catalyzed Transesterification of Canola Oil to FAME Fadi Ataya,† Marc A. Dubé,*,† and Marten Ternan†,‡ Department of Chemical Engineering, Centre for Catalysis Research and InnoVation, UniVersity of Ottawa, 161 Louis Pasteur St., Ottawa, ON, K1N 6N5, Canada, and EnPross Inc., 147 Banning Road, Ottawa, ON, K2L 1C5, Canada ReceiVed September 9, 2007. ReVised Manuscript ReceiVed NoVember 6, 2007
Experiments were conducted at ambient temperature to study mass transfer limitations, indicated by the presence of a triglyceride (TG) induction period, during the acid-catalyzed transesterification reaction. The effects of the water and free fatty acid (FFA) content, the molar ratio of alcohol to oil, and the alcohol type on the TG induction period were examined. Experimental results showed that the TG induction period (i) decreased with a decrease in the water concentration, (ii) decreased with an increase in the methanol (MeOH)to-oil molar ratio for a dispersed nonpolar-TG phase and continuous polar-MeOH phase, (iii) decreased with an increase in the alcohol carbon chain length, and (iv) increased with an increase in the FFA concentration. The addition of fatty acid methyl esters (FAMEs) to the reaction mixture was proposed as a strategy to decrease the TG induction period.
Introduction
1DG + 1CnH2n+1OH T 1MG + 1FAAE
(3)
Biodiesel is a biomass-derived fuel that is considered to be one of the most promising petroleum diesel fuel substitutes. The product of the transesterification of organic feedstocks such as vegetable oil and/or animal fat, biodiesel offers many “green” advantages. The benefits of biodiesel include biodegradability, reduction of most exhaust gas emissions, high flash point, high cetane number, and superior lubricity.1 It is nontoxic and essentially free of sulfur, aromatics, metals, and crude oil residues. Moreover, because biodiesel is plant-derived, its use significantly reduces greenhouse gas emissions over a full life cycle.2 Extensive reviews of the biodiesel production process have been published.2–4 The transesterification reaction consists of a number of consecutive reversible steps. When using 3 mol of an alkyl alcohol, 1 mol of triglyceride (TG) is converted sequentially to diglyceride (DG), monoglyceride (MG), and fatty acid alkyl ester (FAAE) and glycerol (GLY). The overall reaction sequence for the transesterification reaction is shown below:
1MG + 1CnH2n+1OH T 1GLY + 1FAAE
(4)
Stepwise Reactions: 1TG + 1CnH2n+1OH T 1DG + 1FAAE (2)
The influence of process variables on the overall biodiesel reaction, under both acidic and basic conditions, has been reported in several studies.5–9 FAAE yields increase with increased reaction time, temperature, molar ratio of alcohol to oil, and catalyst concentration, whereas conversion rates of the TG ester decrease with increased moisture and free fatty acid (FFA) content. The effect of the molar ratio of alcohol to oil is one of the most important variables affecting the biodiesel yield.5 The effect of temperature, rather than the type of alcohol used, was shown to dominate the rate of reaction and control the time required to achieve complete conversion to ester, for the reactions conducted at temperatures just below the boiling points of the alcohols.5,6 Moreover, the acid-catalyzed reaction has been shown to necessitate significantly longer reaction times than the base-catalyzed reaction to reach any specified conversion to biodiesel.5 For the case when methanol (MeOH) is the alcohol used, the two-phase reaction mixture is composed of several components (TG, DG, MG, GLY, fatty acid methyl ester (FAME), FFA, MeOH, and H2SO4 or NaOH) present in either one or both of the dispersed and/or continuous phases. The chemical potentials of the mixture components at equilibrium are expected to be equal in each phase, ensuring some mutual solubility, even
* Author to whom correspondence should be addressed. Phone: (613) 562-5920. E-mail:
[email protected]. † University of Ottawa. ‡ EnPross Inc. (1) Krawczyk, T. Biodiesel: Alternative Fuel Makes Inroads but Hurdles Remain. INFORM 1996, 7, 800. (2) Ma, F.; Clements, L. D.; Hanna, M. A. Biodiesel Production: A Review. Bioresour. Technol. 1999, 70, 1. (3) Lotero, E.; Liu, Y.; Lopez, D. E.; Suwannakarn, K.; Bruce, D. A.; Goodwin, J. G. Synthesis of Biodiesel via Acid Catalysis. Ind. Eng. Chem. Res. 2005, 44, 5353. (4) Kulkarni, M. G.; Dalai, A. K. Waste Cooking Oil - An Economic Source for Biodiesel: A Review. Ind. Eng. Chem. Res. 2006, 45, 2913.
(5) Freedman, B.; Pryde, E. H.; Mounts, T. L. Variables Affecting the Yields of Fatty Esters from Transesterified Vegetable Oils. J. Am. Oil Chem. Soc. 1984, 61, 1638. (6) Canakci, M.; Van Gerpen, J. Biodiesel production via acid catalysis. Trans. ASAE 1999, 42, 1203. (7) Zheng, S.; Kates, M.; Dubé, M. A.; McLean, D. D. Acid-catalyzed production of biodiesel from waste frying oil. Biomass Bioenergy. 2006, 30, 267. (8) Goff, M. J.; Bauer, N. S.; Lopes, S.; Sutterlin, W. R.; Suppes, G. J. Acid-Catalyzed Alcoholysis of Soybean Oil. J. Am. Oil Chem. Soc. 2004, 81, 415. (9) Freedman, B.; Butterfield, R. O.; Pryde, E. H. Transesterification Kinetics of Soybean Oil. J. Am. Oil Chem. Soc. 1986, 63, 1375.
Overall Reaction: 1TG + 3CnH2n+1OH T 1GLY + 3FAAE (1)
10.1021/ef7005386 CCC: $40.75 2008 American Chemical Society Published on Web 12/29/2007
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if slight. The reaction will not occur uniformly throughout the system; instead, reactants will diffuse toward the reaction zone in a two-phase system while the products will diffuse away from the reaction zone.10 Both mass transfer and chemical reaction occur in the reaction zone which will extend into one or both of the phases. The depth and position of the reaction zone will depend on the rate of reaction and the relative rates of mass transfer of the mixture components.10 The presence of a TG induction period at the beginning of the reaction has been documented in several studies,11–15 and significant mass transfer limitations have been reported even when the reaction mixture was stirred vigorously.11 Some of the variables affecting the TG induction period have been determined to be mixing or agitation, reaction temperature, catalyst concentration, and the presence of an interface between the phases. Mass transfer limitations, indicated by the presence of the TG induction period at the beginning of the reaction, are minimized with increased mixing as the diffusion limitations are diminished.12,13 Agitation increases the interfacial surface area available for the reaction14 and enhances the rate of mass transfer. The TG induction period observed for quiescent twophase reaction mixtures was either decreased and/or eliminated when the same two-phase reaction medium was agitated, indicating that, under some reaction conditions, mixing or enhanced mass transfer affects the TG induction period.14,15 Several techniques have been used to homogenize the twophase reaction system. Temperature improves the mutual solubility of the phases.16 Elevated temperatures aid the mass transfer, in part, through homogenization,12,13 and an improved reaction rate results from the increased solubility between the phases. Other methods employed for overcoming mass transfer include ultrasonic irradiation,17 the use of supercritical conditions,18 and the creation of a homogeneous medium with addition of a cosolvent.11,14,15 Cosolvent addition enhances the miscibility of the phases, resulting in a homogeneous mixture and a faster reaction rate.11 The addition of tetrahydrofuran (THF) and the formation of a single-phase medium resulted in the removal of the interface between the phases. The TG induction period observed for agitated two-phase reaction mixtures was eliminated for the agitated single-phase reactions as the interfacial mass transfer barriers were eliminated.14,15 The concentration of catalyst can also influence the mass transfer limited region, as the TG induction period can be decreased with an increased concentration of catalyst under ambient conditions.14 The reaction occurs at the interface, and maximiz(10) Hanson, C. Recent AdVances in Liquid-Liquid Extraction; Pergamon Press Ltd.: Oxford, U.K., 1971. (11) Boocock, D. G. B.; Konar, S. K.; Mao, V.; Sidi, H. Fast OnePhase Oil-Rich Processes for the Preparation of Vegetable Oil Methyl Esters. Biomass Bioenergy 1996, 11, 43. (12) Noureddini, H.; Zhu, D. Kinetics of Transesterification of Soybean Oil. J. Am. Oil Chem. Soc. 1997, 74, 1457. (13) Noureddini, H.; Harkey, D. W.; Gutsman, M. R. A Continuous Process for the Glycerolysis of Soybean Oil. J. Am. Oil Chem. Soc. 2004, 81, 203. (14) Ataya, F; Dubé, M. A.; Ternan, M. Single-Phase and Two-Phase Base-Catalyzed Transesterification of Canola Oil to Fatty Acid Methyl Esters at Ambient Conditions. Ind. Eng. Chem. Res. 2006, 45, 5411. (15) Ataya, F.; Dubé, M. A.; Ternan, M. Acid-Catalyzed Transesterification of Canola Oil to Biodiesel fuel under Single and Two-Phase Reaction Conditions. Energy Fuels 2007, 21, 2450. (16) Ma, F.; Clements, L. D.; Hanna, M. A. Biodiesel Fuel from Animal Fat. Ancillary Studies on Transesterification of Beef Tallow. Ind. Eng. Chem. Res. 1998, 37, 3768. (17) Stavarache, C.; Vinatoru, M.; Nishimura, R.; Maeda, Y. Conversion of Vegetable Oil to Biodiesel Using Ultrasonic Irradiation. Chem. Lett. 2003, 32, 716. (18) Saka, S.; Kusdiana, D. Biodiesel Fuel from Rapeseed Oil as Prepared in Supercritical Methanol. Fuel 2001, 80, 225.
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Figure 1. Physical model describing the reaction medium: (1A) The continuous phase is the nonpolar-TG phase, and the dispersed phase is the polar-MeOH phase. (1B) The continuous phase is the polar-MeOH phase, and the dispersed phase is the nonpolar-TG phase.
ing the interfacial surface area is expected to improve the performance of the two-phase biodiesel reaction.14 The process costs of the transesterification are high and are due primarily to the cost of virgin feedstocks.7 In order to reduce costs, waste frying oils or yellow grease may be used, but these contain significant amounts of FFA. The FFA will react with base catalysts to form soaps, thus significantly reducing yield and causing significant downstream product separation difficulties. The use of an acid catalyst overcomes this issue as the acid catalyzes the esterification of the FFA to methyl ester. Thus, no soap is produced and the problems related to yield and product recovery are circumvented. Figure 1 shows a physical model for the reaction system, based on the previously cited literature including our own previous work.14,15 Figure 1A shows a reaction medium composed of polar-MeOH phase droplets dispersed in a continuous nonpolar-TG phase.14,15 Figure 1B shows a reaction medium composed of nonpolar-TG phase droplets dispersed in a continuous polar-MeOH phase.14,15 The following analysis was developed for the reaction conditions described in Figure 1A, and a similar understanding can be realized for the reaction conditions described in Figure 1B. The feed components in the acid-catalyzed reaction (TG, DG, FFA, MeOH, H2O, and
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H2SO4) are expected to be distributed between the phases based on their octanol–water partition coefficients.15,20 TG and DG are expected to be present in the nonpolar-TG phase, whereas MeOH and H2O are expected to be present in the polar-MeOH phase.15 FFA is expected to be present in both phases and titration experiments revealed the presence of H2SO4 in both phases.15 DG and FFA are surface active agents, and are also expected to be present at the MeOH-TG interface in addition to being present in the bulk polar-MeOH and nonpolar-TG phases.15 Protons associated with the acid will extract water molecules from the bulk phases to form (HnO2n+1)+ species. These species and HSO4- ions are included in the FFA-H3O+ micelles. FFA will react faster than DG and TG, since esterification reactions are known to be faster than transesterification reactions.9,21 TG and DG reaction profiles show the presence of an induction period, for the two-phase reactions, prior to the initiation of the reaction, and the DG induction period is shorter than the TG induction period.14,15 On the other hand, the FFA reaction profiles show an almost instantaneous reaction of FFA.22 Water is present in the reaction mixture. Our analytical measurements have determined that the feed canola oil contains TG, DG, and FFA. Therefore, the equilibrium reaction (see eq 5) ensures the presence of a small amount of water in the nonpolar-TG phase.15 Both the feed MeOH and H2SO4 contain water as an impurity, and this ensures the presence of a small amount of water in the polar-MeOH phase.15 Moreover, emulsions are present in both the polar-MeOH phase and the nonpolar-TG phase.15 The emulsion in the polar-MeOH phase includes FFA-H3O+ micelles having acid cores that are shielded from contact with the MeOH.15 Similarly, the emulsion in the nonpolar-TG phase includes FFA-H3O+ micelles having acid cores that are shielded from contact with the TG molecules.15 TG + H2O T DG + FFA
(5)
The acid-catalyzed esterification and transesterification reactions have two-step mechanisms that involve contact between the protonated FFA, DG, or TG molecules and MeOH (eqs 6, 3, and 2, respectively). The first step in the reaction mechanism involves protonation of the carbonyl group of the ester by the acid catalyst, forming a resonance stabilized complex, thereby converting it into a strong electrophile.23 The second step involves the reaction of the protonated species with the weak nucleophile, MeOH.23 It is the protonated FFA, DG, or TG molecules that need to interact with MeOH for a reaction to occur.15,23 FFA + MeOH T FAME + H2O
(6)
The transesterification reaction between the protonated TG molecules and MeOH occurs at the MeOH-TG interface.14,15 The FFA-H3O+ micelles and the TG molecules of the nonpolar-TG phase collide frequently during agitation,15 and a proton species may contact and associate with a TG molecule at the FFA-H3O+ micelle-TG interface, forming a protonated TG molecule.15 Both the reaction and phase FFA equilibria and (19) Dubé, M. A.; Zheng, S.; McLean, D. D.; Kates, M. A Comparison of Attenuated Total Reflectance-FTIR Spectroscopy and GPC for Monitoring Biodiesel Production. J. Am. Oil Chem. Soc. 2004, 81, 599. (20) Allen, D.; Shonnard, D. Green Engineering: EnVironmentally Conscious Design of Chemical Processes; Prentice Hall PTR: Upper Saddle River, NJ, 2001. (21) Sendzikiene, E.; Makareviciene, V.; Janulis, P.; Kitrys, S. Kinetics of free fatty acids esterification with methanol in the production of Biodiesel fuel. Eur. J. Lipid Sci. Technol. 2004, 106, 831. (22) Liu, J.; Lotero, E.; Goodwin, J. G. Effect of water on sulfuric acidcatalyzed esterification. J. Mol. Catal. 2006, 245, 132.
Figure 2. Agitated two-phase reaction. Conversion of TG (XTG) and yields of DG (YDG), MG (YMG), and FAME (YFAME), versus time at 3 wt % H2SO4 using a 1-to-6 molar ratio of oil to MeOH.
the reaction of the DG molecules at the MeOH-TG interface should increase the number of contact or reaction sites for the MeOH and the protonated TG species at the MeOH-TG interface, and the reaction would occur wherein the increased number of protonated TG molecules formed during the TG induction period would result in the increase in the TG conversion seen in Figure 2. Therefore, the TG induction period is limited by the number of protonated TG molecules present in the nonpolar-TG phase and the number of contact or reaction sites present at the MeOH-TG interface which are influenced by the rates of mass transfer and chemical reaction of the species involved.15 In this paper, acid-catalyzed transesterification reactions were performed as agitated two-phase and single-phase reactions at ambient temperature. The effects of water and FFA content, the molar ratio of alcohol to oil, and the alcohol type on the TG induction period were studied in an effort to understand the mass transfer limitations present during the initial period of the two-phase reaction. The temperature, the mode of contacting, and the degree of agitation imposed on the system were maintained constant for all experiments. Variations in the chemical composition of the reaction mixture were made, and the resulting variations in TG conversion were measured. Experimental Section Transesterification experiments were performed in order to measure the conversion of TG from canola oil and the corresponding yields of DG, MG, and FAME. All experiments were performed at ambient temperature (20 °C) and pressure (1 atm) over a time period of 24 h. The alcohol in the reaction mixture was one of the following: dehydrated MeOH (Sigma-Aldrich), MeOH (Commercial Alcohols Incorporated), EtOH (Sigma-Aldrich), or BuOH (SigmaAldrich). The feedstock canola oil was the “no-name” brand marketed by Loeb grocery stores. The mass of canola oil used in each batch was approximately 5 g. FFA, in the form of oleic acid (Sigma-Aldrich), was used for the excess FFA experiments. Sulfuric acid catalyst, H2SO4 (BDH Inc.), at 3 wt % by weight of the canola oil was used for all reactions. The reaction mixtures were placed in 40 mL cylindrical vials with an inside diameter of ∼24 mm and an inside length of ∼90 mm. HPLC analysis showed that the feedstock canola oil initially contained TG, DG, and FFA. The mass ratios of DG/TG and FFA/TG in the feedstock were determined to be 0.0135 and 0.0152, respectively. The experimental conditions consisted of two-phase reactions (via methanolysis or ethanolysis) or single-phase reactions (via butanolysis). The reaction vials were agitated using a Labline multiwrist shaker at its maximum frequency of 500 shakes/min for
682 Energy & Fuels, Vol. 22, No. 1, 2008 the times specified. After reaction, the samples were immediately washed with 20 mL of water, resulting in a lower polar phase, which was discarded, and an upper nonpolar phase. This upper phase was filtered through a 0.2 µm polytetrafluoroethylene syringe filter and analyzed using high performance liquid chromatography (HPLC) according to the method reported by Dubé et al.19 HPLC vials containing 0.04 g of sample were weighed and diluted with THF to make up 20 mg/mL sample solutions for HPLC analysis. The HPLC (Waters Corporation) consisted of a pump, a flow and temperature controller, a differential refractive index detector, and two 300 × 7.5 mm Phenogel columns of 3 µm and 100 Å pore size (Phenomenex) connected in series. The system was operated using Waters Millennium 32 software. HPLC grade THF was used as the mobile phase at a flow rate of 0.05 mL/min at 38 °C. The injected sample volume was 20 µL. The running time required for product characterization was approximately 60 min. Calibration curves (Dubé et al.19) were generated for the standards (SigmaAldrich): triolein (TG), diolein (DG), monoolein (MG), methyl oleate (FAME), oleic acid (FFA), and glycerol (GLY). The areas under the peaks in the chromatograms for the product samples were used together with the calibration curves to determine the moles of the constituents (TG, DG, MG, FFA, and FAME) present in the nonpolar phase. The fatty acid ethyl ester (FAEE) and fatty acid butyl ester (FABE) yield calculations were based on the calibration curves generated for the methyl oleate/FAME standard. The moles of TG in the feedstock were calculated from the measured number of moles of the product constituents plus the stoichiometry of the reactions. The moles of DG and FFA in the feed were calculated from the moles of TG in the feed and the DG/TG feed and FFA/TG ratios, respectively. The moles of MeOH in the feed were 6 × (moles of TG + moles of DG) in the feed. The moles of “MeOH in the product” and “GLY in the product” were calculated using the moles of the product constituents plus the stoichiometry of the reactions. With the above information on feed and product constituents, the conversions and yields were calculated. Standard IUPAC definitions were used for conversion (moles of feed converted/mole of feed) and yield (moles of feed converted to a product constituent/mole of feed).
Results and Discussion The compositions of the transesterification products as a function of time are shown in Figures 2-8. The yields of DG, MG, and FAME are shown along with the conversion of TG to all products over a time period of 24 h using 3 wt % H2SO4 (% by weight of the oil). The effects of water and FFA content, the molar ratio of alcohol to oil, and the alcohol type on the TG induction period were studied in an effort to understand the phenomena occurring during the initial period of the reaction. The shortest induction periods are seen in Figure 3 (dehydrated methanol) and Figure 4 (ethanol). The shapes of the other plots of TG conversion versus time are similar to those in Figures 3 and 4. Figure 2 shows the conversion of TG and yields of DG, MG, and FAME for a MeOH-to-oil molar ratio of 6 to 1. After the induction period, from 0 to 12 h, the TG conversion continued to increase. The TG conversion also increased during the first portion of the induction period from 0 to 6 h. This was followed by the second portion of the induction period where a decrease in TG conversion occurred, from 6 to 12 h. The TG conversion then increased from 12 to 24 h, which accounted for the majority of the TG conversion. The maximum TG conversion attained for the reaction period was approximately 4.0%. The DG yield results indicate DG formation from 0 to 6 h occurring during the first portion of the TG induction period, followed by a net DG conversion during the second portion from 6 to 12 h, and (23) McMurry, J. Organic Chemistry, 4th ed.; Brooks/Cole Pub. Co.: New York, 1996.
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Figure 3. Agitated two-phase reaction. Conversion of TG (XTG) and yields of DG (YDG), MG (YMG), and FAME (YFAME), versus time at 3 wt % H2SO4 using a 1-to-6 molar ratio of oil to dehydrated MeOH.
Figure 4. Agitated two-phase reaction. Conversion of TG (XTG) and yields of DG (YDG), MG (YMG), and FAEE (YFAEE), versus time at 3 wt % H2SO4 using a 1-to-6 molar ratio of oil to EtOH.
finally an increase in DG yield from 12 to 24 h. The initial FAME yield was zero for the time period 0 to 6 h followed by an increase from 6 to 24 h. The MG yield was zero throughout the course of the reaction. The experimental results observed were found to be consistent with those presented in a previous study15 of the acid-catalyzed transesterification to biodiesel. It is likely that the reaction between TG and H2O (eq 5) occurred in the first portion of the induction period, since water in the FFA-H3O+ micelles was in direct contact with TG. The results indicate no FAME production for the time period 0 to 6 h, that is, in the first portion of the TG induction period. The TG-H2O reaction is consistent with the first portion results in Figure 2, as the TG conversion equals the DG yield and the FAME yield equals zero. The TG-H2O reaction results in a decrease in the water content of the reaction medium as TG conversion proceeds during the first portion of the TG induction period. The TG, H2O, DG, and FFA in the canola oil were arguably at equilibrium prior to entering the reactor. The conversion of TG, via eq 5, was the re-establishment of that equilibrium, in the nonpolar reaction mixture, to account for the additional water that was added with the methanol and the sulfuric acid. The results in Figure 2 from 6 to 12 h, during the second portion of the induction period, also indicate the beginning of a FAME yield, a decrease in the conversion of TG, and a decrease in the yield of DG (i.e., an increase in the conversion of DG) without the formation of MG. The combination of the
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reactions described in eqs 7 and 8 explain the production of FAME and the observed zero yield of MG. DG + MeOH T MG + FAME
(7)
MG + MeOH T GLY + FAME (8) The conversion of DG in the second portion of the induction period can be explained both by the combination of eqs 7 and 8 and by the reaction in eq 9: DG + FAME T TG + MeOH (9) Moreover, once FAME was formed, the reaction in eq 10 (the addition of the reactions in eqs 5 and 9) resulted in the hydrolysis reaction of FAME. FAME + H2O T FFA + MeOH
(10)
The FAME-H2O reaction (eq 10) resulted in a decrease in the water content of the reaction medium. The combination of eqs 7-10 is consistent with the observed increase in the FAME yield from the reactions in eqs 7 and 8. The FFA formed via eq 10 provided a driving force for the reverse of the reaction in eq 5. The re-establishment of the equilibrium of the reaction in eq 5 was required again. In this context, the water content indirectly adjusted to the presence of FAME in the reaction mixture. Both eqs 5 and 10 are consistent with water being removed during both portions of the induction period. They provide an explanation for the observations in the previous literature,6 that water inhibits the transesterification reaction. This suggests that water removal is an important phenomenon during the TG induction period. From the viewpoint of biodiesel plant operations, eq 10 suggests that adding FAME to the initial reaction mixture to increase the FAME/H2O molar ratio might be a suitable strategy for water removal and could result in the production of a shorter induction period. Figure 3 shows the conversion of TG and yields of DG, MG, and FAME for the transesterification at a MeOH-to-oil molar ratio of 6 to 1 using dehydrated MeOH. The results show a similar trend to that seen in Figure 2. However, under dehydrated conditions, the TG induction period was shorter. The TG induction period, from 0 to 6 h, consisted of a first portion showing TG conversion from 0 to 4 h and a second portion showing TG formation from 4 to 6 h. The TG conversion then increased more rapidly from 6 to 12 h. The maximum TG conversion for the 24 h reaction period was 5.8%. The DG yield indicates an initial DG formation from 0 to 4 h during the first portion of the TG induction period, followed by a net DG conversion from 4 to 6 h, prior to the more rapid increase in DG yield from 6 to 12 h. The initial FAME yield was zero from 0 to 4 h followed by an increase in the FAME yield from 4 to 24 h. For XTG, YDG, and YMG, the use of dehydrated MeOH resulted in the same trends as those shown in Figure 2. However, the observed phenomena occurred sooner. The MG yield was zero throughout the course of the reaction. Dehydrated conditions resulted in a decrease in the water content of the reaction medium. The same reactions that occurred during the induction period in Figure 2 also appear to have occurred in Figure 3. Because dehydrated methanol was used to obtain the results in Figure 3, (a) less water was available for reaction with TG, via eq 5, during the first portion of the induction period, and (b) less water was available for reaction with FAME, via eq 10, during the second portion of the induction period. Therefore, the explanation for the data in Figure 3 is exactly the same as that for Figure 2. Furthermore, it suggests that the duration of the induction period appears to be related to the amount of water in the reaction system and
that the water content must be decreased prior to the end of the induction period. Other researchers have also reported the deleterious effect of water. Liu et al.21,22 studied the acid-catalyzed esterification of acetic acid with MeOH and reported that the reaction was inhibited as a result of the preferential solvation of the acidic species by water over MeOH. Lotero et al.24 reviewed the synthesis of biodiesel via acid catalysis and stated that the water molecules affect the catalyst accessibility to the TG molecules and inhibit the reaction. The dehydrated conditions served to decrease the water content of the reaction medium and, thus, decreased the concentration of the FFA-H3O+ micelles in the system, and increased the concentration of catalytic protons that were available to participate in the reaction. Therefore, the rates of all reactions increased as a result of the increased concentration of accessible catalyst species. This resulted in the increased rate of TG conversion and the decreased TG induction period. Figure 4 shows the conversion of TG and yields of DG, MG, and FAME for the transesterification at an ethanol-to-oil molar ratio of 6 to 1. TG conversion occurred during the first portion of the induction period from 0 to 2 h. This was followed by a slight decrease in the TG conversion during the second portion of the induction period from 2 to 4 h. The TG conversion then increased rapidly from 4 to 6 h and eventually reached 10%. An initial DG formation from 0 to 2 h occurred during the induction period followed by a net DG conversion from 2 to 4 h prior to the rapid increase in DG yield from 4 to 6 h. The initial FAEE yield was zero for the time period from 0 to 4 h followed by an increase from 4 to 24 h. The MG yield was zero from 0 to18 h and increased from 18 to 24 h. The formation of FAEE from EtOH (analogous to FAME from MeOH) will remove water via the reaction in eq 11. This is consistent with the suggestion that adding FAME (in this case FAEE) is expected to shorten the duration of the TG induction period. The solubility of the alcohol in the nonpolar-TG phase influenced the duration of the TG induction period, as the solubility of EtOH in the nonpolar-TG phase was slightly greater than that of MeOH. Ma et al.16 analyzed the solubilities of MeOH and EtOH in beef tallow and reported that MeOH is less soluble than EtOH in beef tallow. The solubility of MeOH in beef tallow is limited because the methyl group of MeOH is smaller than the corresponding ethyl group of ethanol. The 24 h TG conversions with ethanol (Figure 4) being greater than those with methanol (Figure 2) might be explained by the solubility in TG of ethanol being greater than that of methanol and therefore by the better contact between the alcohol and TG. (11) FAEE + H2O T FFA + MeOH Figure 5 shows the conversion of TG and yields of DG, MG, and FABE for the single-phase transesterification at a butanolto-oil molar ratio of 6 to 1. The reaction medium was homogeneous, and all species present were mutually soluble. The TG conversion was positive and increased steadily from 0 to 24 h to approximately 70%. DG formation occurred throughout the experiment. The DG yield, however, did not increase beyond the 4 h point and started to decrease by the end of the reaction. The FABE yield was positive for the entire experiment. The MG yield was zero from 0 to 2 h and increased from 2 to 24 h. The butanolysis reaction of soybean oil is faster than the methanolysis reaction under similar reaction conditions.11 The (24) Lotero, E.; Liu, Y.; Lopez, D. E.; Suwannakarn, K.; Bruce, D. A.; Goodwin, J. G. Synthesis of Biodiesel via Acid Catalysis. Ind. Eng. Chem. Res. 2005, 44, 5353.
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Figure 5. Agitated single-phase reaction. Conversion of TG (XTG) and yields of DG (YDG), MG (YMG), and FABE (YFABE), versus time at 3 wt % H2SO4 using a 1-to-6 molar ratio of oil to BuOH.
Figure 6. Agitated single-phase reaction. ln(1/(1 - XTG)) versus time at 3 wt % H2SO4 using a 1-to-6 molar ratio of oil to BuOH.
methanolysis mixture consists of two phases, while the butanolysis mixture consists of a single phase. This is because the nonpolar methyl group of MeOH is too small to interact with the oil, resulting in a limited solubility of the two phases, whereas the butyl group of BuOH is sufficiently large to cause miscibility, even under ambient conditions. The infinite solubility of BuOH in the nonpolar-TG phase created a single-phase reaction mixture having no interfacial barriers to limit mass transfer, thus resulting in the elimination of the TG induction period. These results are comparable to those presented previously15 for the single-phase experiments carried out with MeOH using tetrahydrofuran (THF) as a cosolvent; however, the maximum TG conversion achieved here is approximately 2 times greater. An explanation for the 24 h TG conversion obtained with a single-phase butanol mixture being greater than that with a single-phase methanol/THF mixture is that dilution from the addition of THF made both the TG and alcohol concentrations smaller in the methanol/THF mixture. Figure 6 shows ln(1/(1 - X)) as a function of time for the acid-catalyzed biodiesel reaction under single-phase conditions at a butanol-to-oil molar ratio of 6 to 1 using 3 wt % H2SO4 catalyst. For a first-order reaction, a plot of ln(1/(1 - X)) as a function of time will be linear with a slope equal to the reaction rate constant, k. The slope of the line for the agitated singlephase TG-DG reaction was used to determine a rate constant value of 1.35 × 10-5 s-1. In contrast, a rate constant of 5.55 × 10-6 s-1 was obtained15 with methanol and THF as a cosolvent. This is explained in terms of the dilution effect resulting from the volume of THF required to achieve homogeneous conditions.
Ataya et al.
Figure 7. Agitated two-phase reaction. Conversion of TG (XTG) and yields of DG (YDG), MG (YMG), and FAME (YFAME), versus time at 3 wt % H2SO4 using a 1-to-6 molar ratio of oil to MeOH and 5 wt % excess FFA.
Figure 8. Agitated two-phase reaction. Conversion of TG (XTG) and yields of DG (YDG), MG (YMG), and FAME (YFAME), versus time at 3 wt % H2SO4 using a 1-to-30 molar ratio of oil to MeOH.
The equilibrium concentration of butoxy anions will therefore be greater than that of methoxy anions, causing the concentration of one of the reactants, the alkoxy ions, to be greater with butanol than with methanol. Figure 7 shows the conversion of TG and yields of DG, MG, and FAME for the transesterification at a MeOH-to-oil molar ratio of 6 to 1, with excess FFA, that is, oleic acid, added to the reaction medium at 5 wt % by weight of the canola oil. The TG induction period was increased compared to the case described in Figure 2. TG conversion occurred during the induction period from 0 to 24 h. The maximum TG conversion attained for the reaction period was 0.9%. The DG yield results indicate an initial DG formation from 0 to 12 h, during the induction period, followed by a net DG conversion from 12 to 24 h. The initial FAME yield was instantaneous and increased steadily from 0 to 4 h before stabilizing for the remainder of the experiment. The MG yield was zero throughout the course of the reaction. The excess amount of FFA in the reaction medium provided a driving force for the reverse of the reaction in eq 10, in which FFA and MeOH reacted to form FAME and H2O. The amount of FAME is indicated in Figure 7. A comparable amount of water was also formed, since the stoichiometry of the reverse of the reaction in eq 10 indicates H2O and FAME were formed in equal amounts. On the basis of the previous discussion, the increase in the amount of water increased the duration of the TG induction period. The increase in the water content of the reaction medium also increased the concentration of the FFA-H3O+ micelles in
Acid-Catalyzed Transesterification
Energy & Fuels, Vol. 22, No. 1, 2008 685 Table 1. Experimental Features
figure
experimental condition
time at end of first portion of induction period (h)
maximum TG conversion at end of first induction period (XTG)MAX
time at end of second portion of induction period (h)
minimum TG conversion at end of second induction period (XTG)MIN
2 3 4 5 7 not shown 8 not shown
MeOH/oil ) 6/1 dehydrated MeOH EtOH BuOH 5 wt % FFA MeOH/oil ) 15/1 MeOH/oil ) 30/1 MeOH/oil ) 60/1
6 4 4 0 15 6 6 2
0.007 0.004 0.004 N/A 0.016 0.01 0.004 0.002
12 8 4 0 >24 12 6 4
0 0 0 N/A N/A 0.005 0.004 0.001
the system and decreased the concentration of catalytic protons that were available to participate in the reaction. Figure 8 shows the conversion of TG and yields of DG, MG, and FAME for the transesterification at a MeOH-to-oil molar ratio of 30 to 1. The results show a variation in the reaction profiles when compared to the conditions described in Figure 2. TG conversion occurred during the induction period from 0 to 6 h. The TG conversion increased more rapidly from 6 to 24 h. The maximum TG conversion was 2.2%. DG formation occurred for the entire duration of the experiment. The initial FAME yield was zero from 0 to 6 h followed by an increase from 6 to 24 h. The MG yield was zero throughout the course of the reaction. Other experiments were performed at MeOH/oil molar ratios of 15 to 1 and 60 to 1. The results are not shown, since they were similar in shape to those at 30 to 1. Some of the features from the various experiments, including the variations in MeOH/ oil molar ratio are listed in Table 1. Although the results for the induction period duration are not linear in MeOH/oil molar ratio, in general, the duration of the total induction period (first portion plus second portion) became shorter as the MeOH/oil molar ratio increased. Although our measurements identified the role of water during the induction period, there may be a more fundamental underlying factor. As we noted previously, Lotero et al.24 described the relationship between water and the accessibility of the catalyst to the TG molecules. Since the first step in the reaction mechanism is protonation, it seems possible that accessibility to the catalyst might be an important feature during the induction period. Conclusions Agitated two-phase and single-phase transesterification experiments were performed to study the induction period in the transesterification of canola oil to FAME or biodiesel. Results
were explained in terms of the physical model (see Figure 1) that was developed using a “reaction at the interface” hypothesis. The overall duration of the TG induction period was linked to water content in the reaction system. Experiments in which the initial water content changed, the FFA content changed, the specific alcohol (MeOH, EtOH, BuOH) changed, and the MeOH/oil molar ratio changed were all found to influence the water content either directly or through ester (FAME, FAEE) production. The addition of FAME to the initial reaction mixture was proposed as a strategy to decrease the duration of the TG induction period. Acknowledgment. The authors wish to acknowledge the BIOCAP Canada Foundation and the Natural Sciences and Engineering Research Council (NSERC) of Canada for financial support of this research.
Nomenclature FAAE ) fatty acid alkyl ester FAME ) fatty acid methyl ester FAEE ) fatty acid ethyl ester FABE ) fatty acid butyl ester FFA ) free fatty acid MG ) monoglyceride DG ) diglyceride TG ) triglyceride THF ) tetrahydrofuran GLY ) glycerol MeOH ) methanol EtOH ) ethanol BuOH ) butanol H2SO4 ) sulfuric acid NaOH ) sodium hydroxide Xi ) conversion of species i (mol/mol) Yi ) yield of species i (mol/mol) k ) reaction rate constant EF7005386
