Ind. Eng. Chem. Res. 2006, 45, 5411-5417
5411
Single-Phase and Two-Phase Base-Catalyzed Transesterification of Canola Oil to Fatty Acid Methyl Esters at Ambient Conditions Fadi Ataya,† Marc A. Dube´ ,*,† and Marten Ternan†,‡ Department of Chemical Engineering, Centre for Catalysis Research and InnoVation, UniVersity of Ottawa, 161 Louis Pasteur Street, Ottawa, Ontario, K1N 6N5, Canada, and EnPross Inc., 147 Banning Road, Ottawa, Ontario, K2L 1C5, Canada
Experiments were performed at ambient temperature to investigate mass transfer during the transesterification reaction of canola oil with methanol (MeOH) to form fatty acid methyl esters by use of a sodium hydroxide (NaOH) base catalyst. Reactions were conducted at two NaOH catalyst concentrations under quiescent conditions, agitated two-phase conditions, and agitated single-phase conditions that were achieved by the addition of a solvent, tetrahydrofuran. A MeOH to oil molar ratio of 6:1 was employed. Small conversions, at ambient conditions, accentuated the effects of mass transfer on the transesterification reaction. For twophase reactions, it was postulated that the reaction occurred at the interface between the two phases. The influence of mass transfer was indicated by the increased reaction rate resulting from (a) stirring a two-phase reaction mixture and (b) changing a two-phase reaction to a single-phase reaction through the addition of a solvent. In some experiments an induction period was observed. Factors influencing the induction period were identified. CH3OH + Na+ -OH ) Na+ -OCH3 + H2O
Introduction Biodiesel, as defined by the American Society for Testing and Materials (ASTM), is a fuel comprised of monoalkyl esters of long-chain fatty acids derived from vegetable oils or animal fats. The molecular similarities between biodiesel and petroleum diesel indicate that biodiesel may satisfy the requirements of a diesel engine.1 Biodiesel is a domestic, clean-burning, renewable, liquid fuel that can be used in compression-ignition engines instead of petroleum diesel with little or no modifications to the engine components. The dominant biodiesel production process, namely, transesterification, typically involves the reaction of an alkyl alcohol with a long-chain ester linkage in the presence of a catalyst to yield monoalkyl esters (biodiesel) and glycerol. An extensive review of the biodiesel production process was published by Ma et al.2 The transesterification reaction consists of a number of consecutive reversible steps. When methanol (MeOH) is the alcohol, 1 mol of triglyceride (TG) is converted sequentially to diglyceride (DG), monoglyceride (MG), and finally fatty acid methyl ester (FAME) and glycerol (GLY). The overall reaction sequence for the transesterification reaction is shown below:
Overall Reaction: 1 TG + 3 CH3OH T GLY + 3 FAME
(1)
Stepwise Reactions: 1 TG + 1 CH3OH T DG + 1 FAME
(2)
1 DG + 1 CH3OH T MG + 1 FAME
(3)
1 MG + 1 CH3OH T GLY + 1 FAME
(4)
The reaction mechanism involves methoxy ions formed from MeOH: * To whom correspondence should be addressed. Tel.: (613) 5625800, ext.6108. Fax: (613) 562-5172. E-mail:
[email protected]. † University of Ottawa. ‡ EnPross Inc.
(5)
It is the methoxy ions that react with the glyceride molecules. The reaction system is essentially heterogeneous, as the nonpolar (mostly triglyceride) phase and polar (mostly alcohol) phase are immiscible. The formation of a saponified reaction product with a basic catalyst promotes emulsification, allowing the reaction to proceed rapidly. In contrast, the acid-catalyzed reaction requires longer reaction times to reach the necessary conversion to biodiesel.2 Base-catalyzed biodiesel production has been the focus of many laboratory-scale investigations. The variables affecting the yields of fatty esters from transesterified vegetable oils were studied by Freedman et al.3 They found that the presence of moisture and free fatty acids (FFA) significantly decreased the yield of the biodiesel product. Methanolysis and butanolysis of soybean oil were examined by Freedman et al.4 at molar ratios of 6:1 and 30:1 for different temperatures in the range of 20-60 °C. The results for butanolysis indicated that, at a molar ratio of 30:1, the forward reaction followed pseudo-first-order kinetics, while second-order kinetics were observed at a molar ratio of 6:1. For methanolysis, the reaction deviated from second-order kinetics and was significantly slower. The appearance of methyl esters without the corresponding increase in diglyceride (DG) concentration and decrease in monoglyceride (MG) concentration prompted the proposal of a “shunt” reaction wherein three molecules of MeOH reacted simultaneously with one TG molecule. The presence of an induction period prior to the initiation of reaction for the two-phase medium was explained by Boocock et al.5 on the basis of the work by Freedman et al.4 They reported that the butanolysis reaction of soybean oil is faster than the methanolysis reaction at similar reaction conditions and that both reactions are slow at higher conversions. The catalyst concentration utilized was considered to be insufficient for fast completion of the reaction. The catalyst is located only in the MeOH phase, and the minuscule concentration of oil in that phase limits the reaction. The methanolysis mixture consists of two phases while the butanolysis mixture mainly consists of a single phase. The reason is that the nonpolar methyl group of MeOH is too small
10.1021/ie060152o CCC: $33.50 © 2006 American Chemical Society Published on Web 06/20/2006
5412
Ind. Eng. Chem. Res., Vol. 45, No. 15, 2006
to interact with the oil, resulting in a limited solubility of the two phases. On the other hand, the butyl group of butanol is sufficiently large to cause miscibility even at ambient temperatures, thereby improving the rate of reaction. The deviation from second-order kinetics for the methanolysis reaction was rationalized without reverting to the “shunt” scheme. Instead they noted that the DG and MG have two and one hydroxyl groups, respectively, and that should give them a greater solubility in the polar phase than TG, because hydroxyl groups are soluble in a polar phase. They expected that some presence of DG and MG in the polar phase containing the catalyst would give them the opportunity to react further. They suggested that the additional conversion of DG and MG in the polar phase would prevent them from attaining the large concentrations predicted by second-order kinetics. The solubilities of MeOH and ethanol in beef tallow at various temperatures were analyzed by Ma et al.6 They reported that MeOH is less soluble than ethanol in beef tallow. The solubility of MeOH was found to be 8 wt % at 45 °C, whereas the solubility of ethanol was approximately 24 wt % at 45 °C, reaching 100 wt % at 68 °C. However, the solubility of MeOH only reached 19 wt % at 100 °C, increasing at a rate of 2-3 wt % per 10 °C. Because the methyl group of MeOH is smaller than the corresponding ethyl group of ethanol, the solubility of MeOH in beef tallow is limited, even at the higher temperatures studied. The influence of mixing on the transesterification reaction has been reported in several studies.7-10 The effect of mixing on the transesterification of beef tallow with MeOH at an alcohol to oil molar ratio of 6:1 with NaOH as the catalyst at a reaction temperature of 80 °C was investigated by Ma et al.7 It was concluded that mixing increased the reaction area between the immiscible reactants, thereby improving the initiation of the reaction. Moreover, without mixing, the reaction was regarded as impractical because it occurred only at the interface of the phases and was extremely slow. Finally, upon initiation of the reaction, the formation of an emulsion occurred and stirring was no longer required, as the effect of reaction time rather than the effect of mixing dominated. The combined effects of mixing and kinetics on the transesterification of soybean oil and MeOH were examined by Noureddini and Zhu8 under basic conditions. The methyl ester concentration profile indicated the presence of a lag at the beginning of the reaction that was postulated to be representative of a mass transfer-controlled region followed by a fast, kinetically controlled region and finally, a slow region toward the completion of reaction. It was stated that the methyl esters produced acted as a mutual solvent, resulting in the formation of a single-phase medium. Mixing decreased the lag time in the mass transfer-controlled region. In contrast, the impact of mixing in the single-phase region was minimal and the effect of temperature dominated the rate of reaction. Increased temperatures had a similar impact in the mass transfer-controlled region. The elevated temperatures improved solubility between the phases and resulted in a greater frequency of collision between the molecules. However, at the higher levels of mixing, the impact of temperature on the mass transfer-controlled region was less significant. The effects of mixing and temperature on the base-catalyzed glycerolysis of soybean oil were examined by Noureddini et al.10 The results showed that both temperature and mixing were key factors affecting the solubility of glycerol in the oil phase, thereby limiting the conversion of TG to MG. Increased temperatures improved the production of MG. However, the
effects became less significant as mixing increased. Increasing the temperature improved the solubility between the phases, and the effect of mixing became insignificant at the elevated temperatures. Mass transfer limitations were evident when little mixing occurred, whereas with increased mixing the diffusion limitations that represent a barrier to interfacial transport were minimized. The base-catalysts NaOH and KOH were used by Muniyappa et al.9 for the glycerolysis of TG at concentrations ranging from 0.05 to 0.20 wt % and at a maximum reaction temperature of 250 °C. It was determined that the key factor limiting the conversion of TG is the degree of solubility between the phases. Saponification promoted the reaction through emulsification of the medium, and temperature increases resulted in an increased mutual solubility of the oil and glycerol phases, leading to faster reaction rates. It was concluded that reductions in temperature and reaction time were feasible through effective contact between the immiscible nonpolar and polar phases. Other methods of overcoming mass transfer include ultrasonic irradiation,11 the use of supercritical conditions,1 and the creation of a homogeneous medium with the addition of a cosolvent.5 A single-phase reaction system for methanolysis was investigated by Boocock et al.5 at ambient temperature. At a 6:1 molar ratio of MeOH to oil, the addition of tetrahydrofuran (THF) cosolvent at a THF:MeOH volumetric ratio of 1.25:1 resulted in the formation of a one-phase system in which the reaction rate of methanolysis at 20 °C was almost as fast as that of butanolysis at 30 °C. A pseudo-single-phase methanolysis of soybean oil with THF under basic conditions was studied by Mao et al.12 with a MeOH to oil molar ratio of 6:1 at 23 °C. The results showed that the reaction starts as a single phase wherein fast reactions rates were observed, followed by a sudden shift in the reaction brought about by a slower reaction rate as a two-phase medium was formed. This was attributed to the formation and subsequent separation of a glycerol phase wherein the catalyst was located. The catalyst removal was seen as the major contributor to the deceleration in the reaction rate; however, the decrease in MeOH concentration was also seen as a source of the reaction rate deceleration. Furthermore, the concentration profiles for the homogeneous medium indicated that, toward the end of the reaction, the concentration of TG is lower than concentrations of DG and MG. In this paper, a series of base-catalyzed transesterifications was performed at ambient conditions under varying conditions of homogenization. The goal was to investigate the effects of catalyst concentration, agitation, and removal of the two-phase interface on the conversion of TG and yield of FAME. Ambient temperature conditions were used to decrease the reaction kinetics sufficiently to enable mass transfer effects to become more apparent. Experimental Section Experiments were conducted in order to measure the conversion of TG and DG in canola oil and the yields of DG, MG, and FAME. All experiments were performed at ambient temperature (20 °C) and pressure (1 atm) over a time period of 2 h. The reaction mixture had a MeOH to canola oil ratio of 6:1 and the catalyst was NaOH (either 1 or 3 wt % by weight of the canola oil). NaOH anhydrous pellets (40 g; Fisher Scientific Company) were dissolved in 1 L of MeOH (Commercial Alcohols Inc.), resulting in a 1M stock solution. The feedstock canola oil was the “no-name” brand marketed by Loeb grocery stores. The reaction mixture was placed in 120 mL vials.
Ind. Eng. Chem. Res., Vol. 45, No. 15, 2006 5413
Our analysis showed that the reaction mixture initially contained TG, DG, and FFA. The mass ratios of DG:TG and FFA:TG in the feedstock were determined by HPLC to be 0.0135 and 0.0152, respectively. Three experimental conditions were tested at each of two catalyst concentrations (1 and 3 wt % NaOH by weight of canola oil) for a total of six experiments: quiescent two-phase reaction, agitated two-phase reaction, and agitated single-phase reaction. For the single-phase reactions, the homogeneous medium was obtained by adding 20 mL of HPLC-grade THF (SigmaAldrich) to the reaction vials at a volumetric ratio of THF to total sample of 1.5:1. The quiescent two-phase reaction vials were left standing for specified times, while the mixing during the agitated two-phase and single-phase reactions was performed on a Labline multi-wrist shaker at its maximum frequency for specified times. The analysis of the reaction products included several steps. After reaction, the samples were immediately water-washed, resulting in a lower polar phase, which was discarded, and an upper nonpolar phase. This upper phase was then filtered through a 0.2 µm poly(tetrafluoroethylene) (PTFE) syringe filter and analyzed by high-performance liquid chromatography (HPLC) according to the methods reported by Dube´ et al.13 Samples (0.04 g portions) were weighed into the HPLC vials and diluted with THF to make up 20 mg/mL sample solutions for HPLC analysis. The HPLC system (Waters Corp.) 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 by use of 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 sample injection loop was 200 µL and the injected sample volume was 20 µL. The running time required for product characterization was approximately 60 min. Calibration curves13 were generated for the following standards (Sigma-Aldrich): 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 amounts of product constituents were subjected to further computations to provide the conversions and yields of the components used in the analysis. 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 ratio and the FFA/TG ratio, respectively. The number of moles of MeOH in the feed was equal to 6 multiplied by (number of moles of TG + number of moles of DG) in the feed. The moles of “MeOH in the product” and “glycerol in the product” were calculated from the moles of the product constituents plus 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/moles of feed) and yield (moles of feed converted to a product constituent/moles of feed). Results and Discussion The conversions of TG in the presence of 1 and 3 wt % NaOH catalyst were measured as a function of time for the transesterification of canola oil. Results are shown in Figures 1 (1 wt % NaOH) and 2 (3 wt % NaOH) for three different experimental
Figure 1. Fractional TG conversion to all products versus time, with 1 wt % NaOH (percent by weight of the canola oil). For all figures, unless otherwise indicated, symbols are as follows: ([) quiescent two-phase reaction; (0) agitated two-phase reaction; (2) agitated single-phase reaction.
Figure 2. Fractional TG conversion to all products versus time, with 3 wt % NaOH. See caption for Figure 1.
regimes: a quiescent two-phase reaction, an agitated two-phase reaction, and an agitated single-phase reaction. As expected, the conversions increased with time. The effect of agitation can be seen by comparing the TG conversion profiles for the quiescent two-phase reactions with those for the agitated two-phase reactions in the presence of 1 wt % NaOH catalyst (Figure 1) and in the presence of 3 wt % NaOH catalyst (Figure 2). For both catalyst concentrations, almost no TG conversion occurred with the quiescent two-phase reaction, whereas substantial TG conversions occurred for the agitated two-phase reactions. Except for shaking, the reaction conditions were identical. This indicated that improved mixing of the two phases had a beneficial effect. The change in conversion obtained by adding THF that caused the two-phase reaction to become a single-phase reaction can also be seen in Figures 1 and 2. Conversions were much greater in the experiments in the agitated single-phase system than with the agitated two-phase system. In a single phase, there is no interface between phases and therefore there can be no limitations due to interphase mass transfer. These observations are consistent with the notion that mass transfer influences the rate of reaction when an interface is present.5 The effect of the NaOH catalyst concentration on TG conversion can be seen in a comparison of Figures 1 and 2. In general, TG conversion increased when the catalyst concentration increased from 1 wt % (Figure 1) to 3 wt % (Figure 2). This was true for both the agitated two-phase and agitated singlephase reactions.
5414
Ind. Eng. Chem. Res., Vol. 45, No. 15, 2006
Figure 3. Fractional DG conversion to all products versus time, with 1 wt % NaOH. See caption for Figure 1.
Figure 4. Fractional DG conversion to all products versus time, with 3 wt % NaOH. See caption for Figure 1.
One hypothesis that explains the above observations is that, in a two-phase reaction, the transesterification reaction occurred at the interface. Specifically, the TG conversion reactions occurred at the interface between droplets of the polar phase (i.e., MeOH and NaOH catalyst) that were dispersed in a continuous nonpolar phase (i.e., TG, DG, FFA, and FAME). In addition, mass transfer of reactants (TG and MeOH) to the interface and/or mass transport of products (FAME and glycerol) from the interface influences the reaction rate. Some components such as DG, MG, and FAME are expected to be soluble in both phases, at least to some extent. Agitation might increase the number of droplets and might also decrease their dimensions, thereby increasing the interfacial surface area available for reaction. The increased conversion observed in Figures 1 and 2 with agitation compared to that with no agitation is consistent with this “reaction at the interface” hypothesis. The addition of THF to form a single phase in which all species are mutually soluble would eliminate the interface and eliminate mass transfer barriers. The fact that the maximum conversion was obtained when the two-phase interface was eliminated is also consistent with the “reaction at the interface” hypothesis. Increasing catalyst concentration in the dispersed polar phase would also increase the catalyst concentration at the two-phase interface. That too would be consistent with the “reaction at the interface” hypothesis. The conversion of DG during the transesterification of canola oil is shown as a function of time, with 1 wt % NaOH catalyst (Figure 3) and 3 wt % NaOH catalyst (Figure 4). The negative conversions indicate that DG formation (caused by the conversion of TG according to eq 2) exceeded DG conversion (to
FAME according to eq 3). The values of the fractional DG negative conversions are much lesser than -1 because there is such a small amount of DG present in the feedstock compared to the amount of TG. The DG conversion results are consistent with the “reaction at the interface” hypothesis. For both catalyst concentrations, the fractional DG formation at early reaction times in the singlephase reaction exceeded that in the two-phase reaction (see Figures 3 and 4). This is a second indication that the elimination of an interface, and thereby the elimination of mass transfer limitations, enhanced the reaction. Also, a comparison of the quiescent two-phase reaction and the agitated two-phase reaction, with 3 wt % NaOH catalyst, in Figure 4, showed that DG conversion with agitation exceeded that of the standing quiescent condition where almost zero DG conversion took place, thereby demonstrating the beneficial effect of mixing on improved mass transfer between the two phases. Visual observations were made to further characterize the nature of the two phases. When reaction mixtures were allowed to settle in vials, the top phase was observed to be a continuous polar MeOH phase. The bottom phase was an emulsion containing the nonpolar material. We hypothesize that the emulsion consisted of a continuous nonpolar phase plus dispersed micelles composed of water, MeOH, and NaOH that were stabilized by soap at their interfaces. FFA present as impurities in the canola oil feedstock are the origin of the soap molecules. At the interface, the FFA molecules (RCOOH) will react predominantly with the OH- ions, via eq 6, and to a much lesser extent with the CH3O- ions, via eq 7:
RCOOH + Na+ -OH ) RCOO- Na+ + H2O
(6)
RCOOH + -OCH3 ) RCOOCH3 + -OH
(7)
The reaction with the OH- ions will result in the formation of soap (ROO- Na+), a polar molecule. In addition, the water within the micelle interiors could originate as an impurity in MeOH and as a product from the saponification reaction (see eq 6). Water is a more polar molecule than MeOH. It is likely that micelles composed of soap molecules on the exterior and water molecules inside stabilized the emulsion in the nonpolar phase that was seen in our visual observations. The water could extract other species, such as NaOH and MeOH. At the micelle interface, the carboxylic acid end of the soap molecules will reside in the polar phase, while the hydrocarbon end of the soap molecules will reside in the continuous nonpolar phase. An induction period was observed prior to the initiation of TG conversion in the agitated two-phase reaction containing 1 wt % NaOH catalyst (Figure 1). The induction period is also seen in the DG conversion data (Figure 3). The existence of induction periods has been reported previously by Freedman et al.,4 Noureddini and Zhu,8 and Darnoko and Cheryan.14 Boocock et al.5 suggested that the induction period is caused by the twophase nature of the medium. A comparison of the two-phase agitated reactions in Figures 1 and 2 shows that an induction period was observed when 1 wt % NaOH was used but not when 3 wt % NaOH was used. This observation is consistent with the “reaction at the interface” hypothesis. The amount of micellar material formed was proportional to the amount of FFA in the feedstock. If there was insufficient water in the micelles to extract all the NaOH from the polar phase, then surplus NaOH would be present. Thus, during agitation, reaction at the interface between the dispersed MeOH-rich polar phase and the continuous nonpolar phase would proceed with the surplus NaOH catalyst that is not contained within the micelles.
Ind. Eng. Chem. Res., Vol. 45, No. 15, 2006 5415
Figure 5. Fractional conversion versus time for a quiescent reaction with 3 wt % NaOH: (], ---) TG conversion; (0, s) DG conversion.
The end of the induction period seems to occur simultaneously with the transition from DG formation to DG conversion and back to DG formation. This can be seen for the agitated two-phase reaction containing 1 wt % NaOH catalyst in Figure 3. The end of the TG induction period is seen in Figure 1 at ∼20 min, and the formation-conversion-formation transition in DG is shown in Figure 3 at the same reaction time. The source of DG for the initial DG formation in the two-phase reaction in Figure 3, at ∼10 min, is the conversion of TG that occurs during that time interval. Although the initial conversion of TG in the two-phase reaction, at ∼10 min, in Figure 1 appears to be negligible, it definitely has a finite positive value that is reproducible when shown on an expanded scale. The transition from DG formation to DG conversion and back to DG formation was also observed in the quiescent two-phase reaction containing 3 wt % NaOH catalyst (see Figure 5). For the quiescent reaction, the DG fractional conversion was less than 0.02 and the TG fractional conversion was an order of magnitude smaller. Nevertheless, the same trends shown in Figure 3 are seen in Figure 5. In particular, the TG induction period appears to be completed after 80 min. There were three factors that influenced the TG induction period. The first was the catalyst concentration. For agitated reaction mixtures, when the NaOH catalyst concentration was increased from 1 to 3 wt %, the induction period was eliminated, as can be seen by comparing the agitated two-phase reactions in Figures 1 and 2. A second factor was agitation. The induction period in a quiescent reaction mixture containing 3 wt % NaOH was described above, in Figure 5. When the same reaction mixture was agitated, there was no induction period, as seen in Figures 2 and 4. This is one indication that, at some reaction conditions, mixing or enhanced mass transfer can affect the induction period. A third factor was the interface between the two phases. The induction period in an agitated reaction mixture containing 1 wt % NaOH was mentioned above (see Figure 1). In contrast, no induction period was observed in an agitated single-phase reaction containing 1 wt % NaOH (see Figure 1). For agitated single-phase reactions, TG conversions after reaction times of 10 min were greater with 3 than with 1 wt % NaOH (compare Figures 1 and 2). The slopes of the lines in Figures 1 and 2 correspond to the rates of TG conversion. For a single-phase reaction, TG conversion decreased abruptly after the reaction time exceeded 10 min. Mao et al.12 reported a similar slowing of the reaction. This decrease has been attributed to the transition from a single-phase to a two-phase reaction. The second phase likely occurs after sufficient glycerol has been formed and its solubility in the single-phase reaction mixture
Figure 6. 1/(1 - XTG)) versus time, with 1 wt % NaOH, where XTG is the TG fractional conversion. See caption for Figure 1.
Figure 7. 1/(1 - XTG)) versus time, with 3 wt % NaOH, where XTG is the TG fractional conversion. See caption for Figure 1.
is exceeded. Because glycerol is a polar species, the other polar molecules, NaOH catalyst and MeOH, will tend to be associated with it. The decrease in the rate of TG conversion thus would be due to the removal of both catalyst and MeOH from the nonpolar phase containing TG. Finally, it is apparent that after 2 h the TG conversion is essentially the same with both 1 wt % NaOH (Figure 1) and 3 wt % NaOH (Figure 2). This indicates that catalyst concentration no longer has a dominant effect on TG conversion. 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. Plots of this type are shown in Figures6 and 7 for 1 and 3 wt % NaOH concentrations, respectively. For the agitated two-phase reaction with the 1 wt % NaOH catalyst concentration (Figure 6), the induction period was represented by a line of zero slope. It was followed by a line of positive slope (k ) 0.0017 ( 0.0003 min-1; note that the variations here and in all subsequently reported k values represent 95% confidence intervals) at longer reaction times. In contrast, with 3 wt % NaOH catalyst concentration, a single line (k ) 0.0076 ( 0.0016 min-1) was drawn through all the data points for the agitated two-phase reaction (Figure 7). The steeper slope obtained with 3 wt % compared to 1 wt % NaOH catalyst concentration is consistent with an increase in a kinetic rate constant and/or a mass transfer coefficient. For the agitated single-phase reaction, two lines of markedly different slopes have been drawn in both Figures 6 and 7. The ones at shorter reaction times have a much steeper slope (for 1 wt % NaOH, k ) 0.2044 ( 0.0310 min-1, and for 3 wt %
5416
Ind. Eng. Chem. Res., Vol. 45, No. 15, 2006
Figure 8. Agitated single-phase reaction: Conversion of TG (XTG) and yields of DG (YDG), MG (YMG), and FAME (YFAME) versus time at 3 wt % NaOH.
will apply and both DG and MG will react in accordance with their respective concentrations and rate constants. The data shown in Figure 8 at reaction times less than 10 min may represent a kinetically controlled regime, and therefore suggest that MG reacts more slowly than DG in a single-phase reaction. In contrast, in Figure 9 the conversion of DG at the interface of a two-phase reaction medium will involve the disappearance of DG and the instantaneous appearance of the MG product at its location at the interface. The MG molecule is ideally located for subsequent conversion to FAME. Thus, even though the MG conversion reaction may be kinetically slower than the DG conversion reaction, it will be sterically promoted because of its position at the interface. In contrast, additional DG molecules will have to displace a MG molecule or some other species to obtain a location at the interface, before it can react. The steric advantage that MG has over DG in a two-phase reaction is consistent with the ratio of MG yield to DG yield being less than unity, as shown in Figure 9. The data in Figures 8 and 9 are consistent with other reports in the literature. Results for a single-phase reaction medium reported by Mao et al.12 show the same trend indicated in Figure 8 even though their experiments were for shorter reaction times. Results obtained in two-phase systems reported by Freedman et al.,4 Noureddini and Zhu,8 and Darnoko and Cheryan14 showed the same trends indicated in Figure 9. Conclusions
Figure 9. Agitated two-phase reaction: Conversion of TG (XTG) and yields of DG (YDG), MG (YMG), and FAME (YFAME) versus time at 3 wt % NaOH.
NaOH, k ) 0.3182 ( 0.0619 min-1) than the ones at longer reaction times (for 1 wt % NaOH, k ) 0.0096 ( 0.0082 min-1, and for 3 wt % NaOH, k ) 0.0088 min-1; note that not enough data points were available to calculate a confidence interval for the latter value). We attribute this to a transition from a kinetically controlled single-phase reaction to a two-phase reaction where mass transfer makes a significant contribution. The slope at longer reaction times for the single-phase reaction is not significantly different from the slope obtained during the two-phase agitated reaction with 3 wt % NaOH catalyst. Again, this strongly suggests that mass transfer may influence the rates of the two-phase agitated reactions. The compositions of the transesterification products are shown in Figures8 and 9 as a function of time. The fraction of unreacted TG in the product is the difference between 1.0 and the fractional TG conversion shown in the figures. The fractional yields of DG and MG are expressed as amount of product formed per amount of feedstock fed. The fractional yield of FAME was calculated on the basis of FAME produced from the complete conversion of TG and does not include the FAME produced from the partial conversion of TG to either DG or MG. On this basis, the sum of the yields of DG, MG, and FAME will equal the TG conversion. The conversions and yields from the single-phase reaction mixtures were greater than those from the two-phase agitated reaction mixtures. For the agitated single-phase reactions (Figure 8), the ratio of MG yield to DG yield was greater than unity. In contrast, for the agitated two-phase reactions (Figure 9), the ratio of MG yield to DG yield was less than unity. These observations are consistent with the “reaction at the interface” hypothesis. In a single-phase reaction, homogeneous kinetics
Canola oil transesterification experiments have been performed as both two-phase reactions and single-phase reactions, with varying NaOH catalyst concentrations. For the two-phase reaction medium it was hypothesized that the reaction occurred at the interface between the two phases. The data were found to be consistent with this hypothesis. An induction period was observed for some but not all of the two-phase reactions. The variables that were found to influence the induction period were agitation, the presence of the interface between the two phases, and NaOH catalyst concentration. For the single-phase medium, two distinct regimes have been identified, one at the short reaction times that may be related to reaction kinetics and the other at longer reaction times that may be related to mass transport. The rates of conversion in the two-phase medium were similar to the slower conversion rates observed with the singlephase reaction medium. The “reaction at the interface” hypothesis was used to provide an explanation for the ratio of MG yield to DG yield being different in two-phase reactions from those in single-phase reactions. With these findings in mind, one method to improve performance in two-phase biodiesel production systems will be obtained by maximizing the interfacial surface area. Acknowledgment We acknowledge helpful discussions with Dr. Andre´ Tremblay. The BIOCAP Canada Foundation and the Natural Sciences and Engineering Research Council (NSERC) of Canada are gratefully acknowledged for financial support for the project. Nomenclature FAME ) fatty acid methyl ester FFA ) free fatty acid MG ) monoglyceride DG ) diglyceride TG ) triglyceride THF ) tetrahydrofuran
Ind. Eng. Chem. Res., Vol. 45, No. 15, 2006 5417
GLY ) glycerol MeOH ) methanol NaOH ) sodium hydroxide KOH ) potassium hydroxide X ) conversion Y ) yield k ) reaction rate constant Literature Cited (1) Saka, S.; Kusdiana, D. Biodiesel Fuel from Rapeseed Oil as Prepared in Supercritical Methanol. Fuel 2001, 80, 225. (2) Ma, F.; Clements, L. D.; Hanna, M. A. Biodiesel Production: A Review. Bioresour. Technol. 1999, 70, 1. (3) 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. (4) Freedman, B.; Butterfield, R. O.; Pryde, E. H. Transesterification Kinetics of Soybean Oil. J. Am. Oil Chem. Soc. 1986, 63, 1375. (5) Boocock, D. G. B.; Konar, S. K.; Mao, V.; Sidi, H. Fast One-Phase Oil-Rich Processes for the Preparation of Vegetable Oil Methyl Esters. Biomass Bioenergy. 1996, 11, 43. (6) 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. (7) Ma, F.; Clements, L. D.; Hanna, M. A. The Effect of Mixing on Transesterification of Beef Tallow. Bioresour. Technol. 1999, 69, 289.
(8) Noureddini, H.; Zhu, D. Kinetics of Transesterification of Soybean Oil. J. Am. Oil Chem. Soc. 1997, 74, 1457. (9) Muniyappa, P. R.; Brammer, S. C.; Noureddini, H. Improved Converison of Plant Oils and Animal Fats into Biodiesel and Co-Product. Bioresour. Technol. 1996, 56, 19. (10) 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. (11) Stavarache, C.; Vinatoru, M.; Nishimura, R.; Maeda, Y. Conversion of Vegetable Oil to Biodiesel Using Ultrasonic Irradiation. Chem. Lett. 2003, 32, 716. (12) Mao, V.; Konar, S. K.; Boocock, D. G. B. The Pseudo-SinglePhase, Base-Catalyzed Transmethylation of Soybean Oil. J. Am. Oil Chem. Soc. 2004, 81, 803. (13) Dube´, 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. (14) Darnoko, D.; Cheryan, M. Kinetics of Palm Oil Transesterification in a Batch Reactor. J. Am. Oil Chem. Soc. 2000, 77, 1269. (15) McMurry, J. Organic Chemistry, 4th ed.; Nelson Canada: Scarborough, Ontario, Canada, 1996.
ReceiVed for reView February 6, 2006 ReVised manuscript receiVed May 16, 2006 Accepted May 20, 2006 IE060152O
