Waste Management 34 (2014) 2611–2620

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Transesterification of waste vegetable oil under pulse sonication using ethanol, methanol and ethanol–methanol mixtures Edith Martinez-Guerra, Veera Gnaneswar Gude ⇑ Civil and Environmental Engineering Department, Mississippi State University, Mississippi State, MS 39762, United States

a r t i c l e

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Article history: Received 24 April 2014 Accepted 26 July 2014 Available online 29 August 2014 Keywords: Pulse sonication Alcohol mixture Process parameters Waste vegetable oil Ultrasound intensity Power density

a b s t r a c t This study reports on the effects of direct pulse sonication and the type of alcohol (methanol and ethanol) on the transesterification reaction of waste vegetable oil without any external heating or mechanical mixing. Biodiesel yields and optimum process conditions for the transesterification reaction involving ethanol, methanol, and ethanol–methanol mixtures were evaluated. The effects of ultrasonic power densities (by varying sample volumes), power output rates (in W), and ultrasonic intensities (by varying the reactor size) were studied for transesterification reaction with ethanol, methanol and ethanol–methanol (50%-50%) mixtures. The optimum process conditions for ethanol or methanol based transesterification reaction of waste vegetable oil were determined as: 9:1 alcohol to oil ratio, 1% wt. catalyst amount, 1–2 min reaction time at a power output rate between 75 and 150 W. It was shown that the transesterification reactions using ethanol–methanol mixtures resulted in biodiesel yields as high as >99% at lower power density and ultrasound intensity when compared to ethanol or methanol based transesterification reactions. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction The United States Energy Information Administration has estimated that about 138 million tons of waste cooking oil is produced per year in the USA with a per capita production of 9 lb per year (Radich, 2006). In Canada, approximately 135,000 tons per year of waste cooking oil produced (Canada, 2006). In the European Union (EU) countries, the total waste cooking oil production is approximately 700,000–1,000,000 tons per year (Kulkarni and Dalai, 2006; Patil et al., 2009) while for the UK, the production is about 200,000 tons of waste cooking oil per year (Carter et al., 2005; Chhetri et al., 2008). These waste cooking oils may pose an environmental threat if they are not reused or disposed properly (Charpe and Rathod, 2011). However, waste cooking oils may serve as local feedstock for biodiesel production (Ramos et al., 2013; Gude and Grant, 2013; Grant and Gude, 2013; Gude et al., 2013a,b). This use will reduce the biodiesel production costs by at least 2–3 times since feedstock costs can count for up to 80% of the total biodiesel costs. The waste cooking oils are sold at a cost 2–3 times lower than fresh or virgin feedstock and are often available free of cost (Ramos et al., 2013). As a comparison, the diesel

⇑ Corresponding author. E-mail address: [email protected] (V.G. Gude). http://dx.doi.org/10.1016/j.wasman.2014.07.023 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.

production from petroleum costs about 1.00 $/L in 2008 which is a very competitive price for biodiesel production from other feedstock as well. Additionally, utilizing waste oil for biodiesel production will mitigate the environmental impacts related to feedstock production (Sivasamy et al., 2009). Biodiesel from waste cooking oils can be produced by a wellknown transesterification reaction. Addition of an alcohol group is required to complete this reaction (Felizardo et al., 2006). The short chain alcohols such as methanol, ethanol, and butanol are frequently used, with methanol being the most common. All of these alcohols possess different physical and chemical properties and therefore exhibit different patterns of transesterification reaction kinetics and biodiesel yield rates. Methanol and ethanol produce superior results in transesterification reactions. Currently, methanol is produced from the fossil sources and mineral oils (Andrade Torres et al., 2013) which impact the availability and cost of the oil resources. For long term sustainability of biodiesel production, use of ethanol provides a more environmentally friendly perspective since it can be derived from natural and renewable sources like plants and crops (Encinar et al., 2002; Andrade Torres et al., 2013). Biodiesel containing ethyl esters are more beneficial compared to methyl esters because ethanol can be derived from renewable sources, and the extra carbon atom in the ethanol molecule slightly increases the heat content and the cetane number. In addition, another important advantage is that the ethyl

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esters generally have lower cloud and pour points than methyl esters (Encinar et al., 2007). Mass transfer issues are often not discussed in the transesterification reactions. The Transesterification reaction is mainly dominated by the mass transfer limitations, then followed by a kinetics controlled region, with oils and methanol (Likozar and Levec, 2014a; Narvaez et al., 2009). Mass transfer between two organic phases (methanol and oil) plays a critical role during the transesterification (methanolysis) and controls the reaction kinetics. In transesterification reaction kinetics, three regimes are wellrecognized, that is an initial mass transfer-controlled regime (slow), followed by a chemically-controlled regime (fast), and a final regime, close to equilibrium (slow) (Stamenkovic et al., 2008). Therefore, methanol is not effectively used for the reactions due to the interfacial mass transfer resistance (Kai et al., 2010). The mass transfer limitations on biodiesel production may be overcome with efficient mixing mechanism such as ultrasonic mixing. Ultrasound can enhance the mass transfer between two immiscible liquids (Leung et al., 2010). Cavitation mainly affects the mass transfer rates and ensures a uniform distribution of the reactants, as one concludes from the fact that a significant effect on both the reaction rate and the equilibrium conversion is only observed in the later stages of the reactions when heterogeneity is removed (Likozar and Levec, 2014b). The rationale for this study can be explained as follows: transesterification reaction using methanol as reactant is a mass transfer limited operation due to poor solubility of oil in methanol. But methanol has a higher equilibrium conversion due to the higher reactive intermediate methyl group. On the other hand, ethanol has better solvent properties and can be obtained from renewable resources (Issariyakul et al., 2007). However, the formation of emulsion after the transesterification of oil with ethanol makes the separation of ester very difficult. When methanol/ethanol mixtures are employed for the transesterification reaction, this combination may benefit from ethanol’s better solvent properties and methanol’s faster equilibrium conversion kinetics. In addition, esters obtained from the mixture of alcohols possess better lubricating abilities and render as lubricating additive. Finally, replacing methanol with ethanol will also make the biodiesel production sustainable by reducing dependence on non-renewable methanol production (Kulkarni et al., 2007). Moreover, the ultrasound mixing provides superior mixing compared to conventional heating in transesterification reaction promoting mass transfer between the oils and alcohols. In this paper, we report the transesterification of waste vegetable oils using methanol and ethanol under a pulse sonication effect.

Since the two different alcohols have different physical and chemical characteristics, their effect on the transesterification reactions under ultrasound irradiation could be very distinguishable. The molar ratio of alcohol to oil, catalyst amount, and the reaction time were evaluated under direct pulse sonication without any external heating or mixing. Additionally, the effects of power density, ultrasound intensity and power output rates were also evaluated. Finally, the effect of ethanol/methanol mixtures (50%-50%) on the transesterification reaction along with power density, ultrasonic intensity and power output rate are presented. 2. Materials and methods This section provides details on the materials used and experimental procedures followed in this study. 2.1. Materials Waste vegetable oil (Canola) was obtained from a local restaurant near the Mississippi State University (MSU) campus. The acid value of the oil was found to be 3.5 mg KOH/g, corresponding to a free fatty acid (FFA) level of 1.7% and base catalyzed transesterification is suitable for feedstock with FFA content less than 4% (Patil et al., 2010, 2012). Ethanol, methanol and the catalyst (sodium hydroxide, NaOH) were purchased from Fisher Scientific. Methanol used in this study was of ACS (American Chemical Society) certified grade and ethanol was of reagent grade. 2.2. Experimental procedures The pulse sonication of transesterification reaction was performed using a NO-MS100 ultrasonicator manufactured by Columbia International Technologies with a maximum of 1000 W power output capacity. The ultrasonic frequency was 25 kHz. The horn is made of titanium alloy with variable power output rates and a 2.54 cm diameter tapered to 0.254 cm at the tip. The transesterification process involves direct sonication of the sample mixture (known amount of waste vegetable oil mixed with pre-prepared sodium methoxide solution) to be converted into biodiesel followed by separation/washing/drying cycles. The transesterification reaction was carried out in a 50 ml glass batch reactor (beaker) equipped with an ultrasonic horn and a digital temperature probe (Fig. 1). The reaction sample temperature was recorded using a digital thermometer at every 10 s of reaction time. The molar ratio of methanol and/or ethanol to oil was varied between 4.5:1, 6:1, 9:1, and 13.5:1, while three catalyst loadings

Fig. 1. Biodiesel production from waste vegetable oil under pulse-sonication.

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were at 0.5, 1.25 and 2.0% (wt./wt.). The molar ratios were calculated based on the molecular weight of the waste vegetable oil (obtained from GC-FID. The molecular weight of waste vegetable oil was 856.94 g/mol. The sample volume of the oil was fixed at 10 ml for all experiments. The reaction times were varied between 0.5 min and 2.5 min at 0.5 min increments. After the ultrasonic processing, the reaction mixture was allowed to settle and to separate into two distinct layers (biodiesel and glycerol). The glycerol was drained and discarded, and the remaining reaction mixture was washed and dried in the oven at 60 °C for eight hours. This was to ensure complete removal of the unreacted methanol and byproducts. The biodiesel sampler was then collected for gas chromatographic (GC–MS) analysis. The analysis was carried out on a Varian Gas Chromatography (GC) with cool-on column injection and FID detection as required by ASTM 6584 method for B100. The operating scheme for the biodiesel analysis is presented elsewhere (Gude and Grant, 2013). Fig. 1 shows the process scheme for the experimental studies. 2.3. Ultrasonic transesterification 2.3.1. Reaction scheme The reaction considered for this study is a typical transesterification reaction of oil and alcohol in the presence of a strong Lewis base catalyst producing alkyl esters and glycerol. Ultrasound irradiation was utilized as a non-conventional energy source to promote mixing and heating in the transesterification reaction. The temperature rise in the reaction mixture comes from two effects: (1) continuous expansion and rarefaction cycles caused by cavitation bubbles, and (2) the exothermic nature of the transesterification reaction. 2.3.2. Ultrasonic mechanism Novel process-intensification techniques such as microwaves and ultrasound provide convenient and efficient heating and mixing for transesterification (Martinez-Guerra et al., 2014a,b; Martinez-Guerra and Gude, 2014). Ultrasonic excitation of reactions increases the interfacial area of the reaction mixtures through emulsification, which is important for the formation of vapor and

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cavitation bubbles in viscous liquids such as oils, especially in biodiesel production. Vapor bubbles generated by ultrasound effect in oils and methanol mixtures oscillate and move with the steady currents in the bulk liquid caused by the acoustic streaming. This acoustic streaming is caused by differences in the ultrasound velocity, which is a property of the materials. For example, the traveling velocity of acoustic waves in ethanol is 1182 m/s (Darrigo and Paparelli, 1988) and in vegetable oils is about 1430 m/s (Wong et al., 2008). Simultaneously, the cavitation bubbling action pushes the liquid toward the interfacial surfaces of the vapor bubbles, where the reaction mixture interacts. This phenomenon enhances the mass transfer across the interfaces of the bubbles and, thus, accelerates the chemical reaction rates under diffusion-limited conditions such as the early stage of transesterification of oils in biodiesel production. Meanwhile, ultrasonic cavitation bubbles are much smaller in size and the total number per volume is much more stable than those generated mechanically. The very large number of fine cavitation bubbles greatly increases the interfacial area available for chemical reactions. Ultrasonic cavitation also reportedly produces a microenvironment of very high temperature and pressure, which may create highly active reaction intermediates and lead to faster transesterification reaction rates (He and Van Gerpen, 2012). Energy transformation in ultrasonic processing of chemical reactions typically follows three steps: (1) the transformation of electrical input to mechanical energy through a piezoelectric or piezomagnetic transducer; (2) the delivery of vibrational energy (acoustic energy) from the emission tip of the transducer to the liquid medium; and (3) the conversion of the energy of ultrasonic streaming to the energy that activates reactants by acoustic cavitation. Energy losses in any step will influence the total energy efficiency (Luo et al., 2014).

3. Results and discussion 3.1. Effect of alcohol to oil ratio The type and amount of alcohol used in the transesterification reaction plays a very important role. Fig. 2 shows the temperature

Fig. 2. Effect of alcohol to oil ratio on biodiesel yield.

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profiles and the biodiesel yields for transesterification reactions using methanol, ethanol and ethanol–methanol alcohol mixtures (50%/50% molar ratios). It should be noted that the reaction temperatures for reactions involving ethanol are higher compared to the methanol based transesterification reactions. Methanol has a lower boiling temperature (64.5 °C) compared to ethanol (78.6 °C). The temperature of methanol prior to mixing with the waste vegetable oil was 25 °C while the temperature of ethanol was measured at 30 °C; the alcohol mixtures had an intermediate temperature. The maximum reaction temperatures recorded at two minutes of reaction time were 37.5 °C, 46.6 °C, and 53.2 °C respectively for methanol, ethanol and alcohol mixture mediated transesterification reactions. The biodiesel yields for different molar ratios of methanol, ethanol and alcohol mixtures are shown in Fig. 2. At 4.5:1 methanol to oil molar ratio, the biodiesel yields were 89.5%, 10% and 89.5% respectively for methanol, ethanol, and alcohol mixtures. The low yield of ethanol based reaction at lower molar ratio can be attributed to the higher solubility of ethanol in the oil phase to form an emulsion which proves to be difficult for washing and purification (Meher et al., 2006). In general, the higher the molecular chain of the alcohol group, the higher will be the miscibility between the oil and alcohol (Stavarache et al., 2005). This is a desired phenomenon since it reduces the reaction time. But it should be carefully monitored to avoid suspension (solidification) of biodiesel and glycerol mixtures after the reaction. It was reported that the formation of methyl esters was 15 times slower than the butyl esters from soybean oil (Boocock et al., 1996). Although the biodiesel yield from methanol reactions is higher than other alcohol combinations, it is not acceptable for practical purposes. Further increases in the molar ratios of different alcohols have shown increase in biodiesel yields. It is obvious that at molar ratios of 6:1, 9:1, and 13.5, the biodiesel yield was higher for all alcohol combinations, but at molar ratios of 9:1 and 13.5, the ethanol– methanol alcohol mixture resulted in higher biodiesel yields of 98.7% and 98% respectively. The transesterification reaction by the ethanol–methanol alcohol mixture can possibly be enhanced by: (1) the higher solubility of ethanol in oil; and (2) the active methoxide intermediate group forwarding the reaction to completion. Not only does the use of ethanol–methanol mixtures improve the biodiesel yields, they also improve the lubricating properties of the biodiesel produced, which is a desired feature for diesel engines (Issariyakul et al., 2007).

3.2. Effect of catalyst By using catalysts the conversion of oils by transesterification into biodiesel results in marked economic and environmental benefits due to reduction in alcohol requirements and the reaction time (Sabudak and Yildiz, 2010). Sodium hydroxide (NaOH) is the most widely used catalyst in biodiesel production (Brito et al., 2012). The effect of the sodium hydroxide catalyst on the biodiesel yield was investigated using two different alcohols. Three different weight percentages of catalyst were tested: 0.5, 1, and 2% wt. NaOH. The ethanol/methanol to oil ratio, sample volume, reaction time and the power output rate were maintained at 9:1, 10 mL, 2 min and 150 W respectively. Fig. 3 shows the temperature profiles for methanol and ethanol based transesterification reactions. It can be noted that the reaction temperature profiles generally were slightly higher at higher catalyst concentrations. The biodiesel yields for methanol transesterification reactions were consistently higher compared to the ethanol mediated reactions. The highest yield obtained yield 96.8% for 1% wt. catalyst using methanol, while using ethanol the highest biodiesel yield was 92.5%. It

Fig. 3. Effect of catalyst amount on biodiesel yield using (A) methanol; (B) ethanol and (C) comparison.

was observed that saponification was higher when ethanol was used. This indicates that lower NaOH amounts should be used for transesterification reactions with ethanol. This was proven when a 2% wt. catalyst was added to the mixture of ethanol and oil, producing a biodiesel yield of 47% while it was 86% using the same amount of catalyst (2% wt.) and methanol. In general, increasing the catalyst concentration will increase the biodiesel yield and reaction kinetics. However, higher catalyst concentrations tend to form emulsions, which increases the viscosity and formation of a gel that traps the FAMES and glycerols making product separation difficult. Higher catalyst concentrations (>1%) have generally resulted in a strong soap formation whether catalyzed by sodium hydroxide or potassium hydroxide as in (Veljkovic et al., 2012). Since higher catalyst concentrations speed up the transesterification reaction, the reaction should be stopped in a short reaction time to avoid soap formation.

3.3. Effect of reaction time The temperature profiles and the biodiesel yields for different reaction times are shown in Fig. 4. It is very interesting to note that

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the boiling temperatures cause vapors to form which interfere with the cavitation effects of acoustic streaming (Veljkovic et al., 2012). The formation of bubbles and their collapse will be drastically affected by the vapors forming and the significant variations in the viscosity of the reaction mixture. In view of the above discussion, the reaction temperatures observed in our studies were favorable for ultrasound mediated transesterification reaction even in the absence of external heating. Also, other studies have shown that the ultrasound based transesterification reaction dramatically reduced the reaction time from hours to minutes with lower alcohol and catalyst consumption (Rokhina et al., 2009; Mahamuni and Adewuyi, 2010).

3.4. Effect of ultrasound power output rate

Fig. 4. Effect of ultrasound exposure (reaction time) on biodiesel yield: temperature profiles for reaction time using (A) methanol, (B) ethanol, and (C) biodiesel yield for methanol and ethanol.

shorter reaction times are favored for ethanol based transesterification reactions due to its superior miscibility with oil molecules. At higher reaction times, stable emulsions were formed causing separation and purification issues and lower biodiesel yields. However, for methanol, longer reaction times favored the biodiesel yields. This can be explained based on the heterogeneity and the poor solubility of methanol in the oil phase which requires some mixing to form a homogeneous suspension before the methoxide intermediate group could take part in the transesterification reaction. At one minute reaction time, the biodiesel yields were 99% and 95% for ethanol and methanol respectively. The ethanol/methanol to oil ratio, catalyst amount, sample volume, and the power output rate were maintained at 9:1, 1% wt., 10 mL, and 150 W respectively. The temperature profiles (Fig. 4A and B) for methanol and ethanol show that the reaction temperatures in general increased with reaction times. But the reaction temperatures for ethanol were consistently higher than methanol reaching as high as 56 °C, while the highest temperature recorded for methanol was 36.7 °C. Transesterification reactions are usually conducted at temperatures lower than 60 °C since the reaction temperatures up to the boiling point of alcohols increase the biodiesel yields. For ultrasound mediated reactions, reaction temperatures at or beyond

Reducing energy consumption should be one of the goals in the production of biodiesel. It is critical to take into account the amount of energy consumed during the biodiesel production. In this study, different power outputs were tested (75 W, 150 W, 300 W, and 450 W) on the transesterification reaction to observe the biodiesel yields. For these tests, the sample volume was maintained at 10 mL, while the other parameters were kept at: ethanol/ methanol to oil ratio of 9:1, catalyst amount of 1% wt., and reaction time of 2 min. The temperature profiles and biodiesel yields are shown in Fig. 5. It was noted that the biodiesel yield increased with power output rate in the case of methanol, but the opposite trend was observed for ethanol. The yield using methanol increased as the power increased and a yield of 98% was obtained at 300 W and 450 W. It is interesting to note that a 75 W power output rate resulted in a comparable yield of 94% using ethanol and 95% using methanol indicating the beneficial use of low power output rates. The tests were repeated to verify the accuracy of the results, and similar yields were obtained. During the washing/ separation process of the reaction products, a significant amount of soap was removed for ethanol derived biodiesel samples. However, after drying the samples, the physical quality of the biodiesel produced using ethanol was higher than that using methanol. The highest reaction temperatures were 44, 62.6 and 60.5 °C for methanol, ethanol and ethanol–methanol mixtures respectively observed at 450 W power output rates. To summarize the experimental studies, the optimum process conditions for ethanol or methanol based transesterification reactions of waste vegetable oil were determined as: 9:1 alcohol to oil ratio, 1% wt. catalyst amount, 1–2 min reaction time at a power output rate of 75–150 W.

3.5. Effect of power density Frequency of ultrasonic irradiation may impact the physical and chemical effects of the transesterification reaction significantly. Low frequency ultrasound irradiation between 20 and 50 kHz have been recommended for transesterification reactions due to better physical cavitation effects (Zhang et al., 2011; Dehghani et al., 2007). In this study, an ultrasound frequency of 25 kHz was applied. Other parameters that could affect the ultrasound efficiency are the power dissipation, reactor diameter (discussed earlier), depth of the sample volume and material of construction (Gole and Gogate, 2012). Accordingly, we have evaluated the effect of power density on the transesterification reaction using different sample volumes and different alcohol combinations. Fig. 6 shows the temperature and biodiesel profiles for different sample volumes of 10, 20, and 30 mL of oil and corresponding alcohol amounts at a fixed power output rate

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Fig. 5. Effect of ultrasound power output rate on biodiesel yield.

Fig. 6. Effect of reaction mixture volume on biodiesel yield.

of 150 W. The longer the alcohol chain, the higher the alcohol amount required for transesterification reaction will be and hence the total amount of reaction mixture will change accordingly. As expected the reaction temperatures were higher for reactions involving ethanol as a reactant. But it can be noted that the biodiesel yield was higher for methanol (97.5%) and ethanol (97%) reactions at 20 mL oil volume while the biodiesel yield was the highest for a ethanol–methanol mixture at 10 mL oil volume (98.7%). But the ethanol–methanol alcohol mixture resulted in

the highest yield (97%) at 30 mL oil volume for the given power dissipation rate. Fig. 7 shows the power density effect on the transesterification reaction mediated by different alcohols. It is very clear from the profiles that the ethanol–methanol alcohol mixture has better yields at even lower power densities. This improvement could be attributed to the many distinct advantages of using ethanol–methanol mixtures discussed previously. The biodiesel yields for alcohol mixtures were quite stable across the range indicating that mixing

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Fig. 7. Effect of power density on biodiesel yield.

the longer chain alcohols with methanol may reduce the power dissipation and power density requirements for transesterification due to other physical and chemical effects induced by the ultrasound irradiation. 3.6. Effect of ultrasound intensity The propagation of ultrasound waves affects biodiesel production. In order to evaluate that effect, we tested 50 mL, 100 mL, 150 mL, and 250 mL size reactors with diameters of 1.5, 1.75, 2.0,

and 2.55 in. respectively. The ultrasonic intensity applied on the surface of reaction mixtures were 13.6, 10.0, 7.6, and 5.1 W/cm2 respectively. It appears that the ultrasonic intensity has some effect on the transesterification reaction and thus biodiesel yields. Higher ultrasonic intensities were favorable for methanol, but lower ultrasonic intensity favored ethanol based transesterification reactions probably due to the higher solubility of ethanol and its eventual product separation issues as discussed previously (Fig. 8). Within the evaluated range of intensity, the biodiesel yields did not change significantly. For the ethanol–methanol mix-

Fig. 8. Effect of ultrasound intensity on biodiesel yield.

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ture, lower ultrasound intensity appeared to be adequate resulting in a higher biodiesel yield (99%) which was consistent through the range of intensities tested. When methanol was used as the reactant, the lowest biodiesel yield obtained was 95% for 250 mL reactor with 2.5 in diameter which was not much lower than the highest yield of 96.8% for 50 mL reactor. The results using ethanol were slightly different from those using methanol. However, the methanol and ethanol based transesterification reaction had similar biodiesel yields (96%) at an ultrasonic intensity of 10 W/cm2. It can be noted that the reaction temperatures were higher for the reactions involving ethanol. In the transesterification reaction with ethanol, the reaction proceeds quickly increasing the temperature of the reaction mixture since the transesterification is mostly exothermic. In the case of methanol, the mass transfer and diffusive limitations slow the reaction, the reaction kinetics are heavily dominated by the intermediate functional group (methoxide) effect. Also, the activation energy for the transesterification reaction using methanol (methyl esters) is reportedly higher than that with ethanol (ethyl esters; Dantas et al., 2007). It was also noticed that for the majority of the samples, the yield decreased with the increase in the reactor size; it can be assumed that this was caused by the surface area of the reactor which allows alcohols to escape faster from the reaction. The fact that the yields are higher using methanol can be attributed to the difference in evaporation rates between ethanol and methanol, and also that saponification is higher when using ethanol.

3.7. Optimum conditions and the biodiesel composition Optimum conditions for the process parameters based on biodiesel yields were summarized in Table 1. Fatty acid methyl ester (FAME), fatty acid ethyl ester (FAEE), and the composition of the methanol–ethanol mixture analysis (FAAE) for biodiesel from waste vegetable oil have shown the following major and minor compounds (Table 2): Octanoic (C8:0), Lauric (C12:0); Myristic (C14:0); Palmitic (C16:0); Palmitoleic (C16:1); Stearic (C18:0); Oleic (C18:1); Linoleic (C18:2); Linolenic (C18:3); Archidic (C20:0); Behenic (C22:0); Erucic (C22:1); and Lignoceric (C24:0). Fig. 9 shows a comparison of the fatty acid alkyl ester compositions for methanol, ethanol and methanol–ethanol mixtures. Previous studies report that the biodiesel properties have met the ASTM standards for acid value, viscosity, and cetane number of all the esters prepared from waste vegetable oils and they have shown good performance as a lubricity additive in the engines (Issariyakul et al., 2007).

4. Conclusions This study reported on the effect of pulsed sonication on the transesterification reaction of waste vegetable oil. The optimum process conditions for ethanol or methanol or alcohol mixture based transesterification reaction of waste vegetable oil were

Table 1 Summary of optimized conditions based on biodiesel yield. Reactant

Process parameters

Optimum conditions based on biodiesel yield

Ethanol

Molar ratio: 4.5:1; 6:1; 9:1; 13.5:1 Catalyst: 0.5%, 1%, and 2 wt.% Reaction time: 0.5–2.5 min, increments of 0.5 min Oil volume: 10, 20, and 30 mL Reactor volume: 50, 100, 150, 250 mL Power: 80, 150, 300, and 450 W

9:1 – 92.5% 1 wt.% – 92.5% 1 min – 99% 20 mL – 97.5% 100 mL – 96% 75 W – 94%

Methanol

Molar ratio: 4.5:1; 6:1; 9:1; 13.5:1 Catalyst: 0.5%, 1%, 2% Reaction time: 0.5–2.5 min, increments of 0.5 min Oil volume: 10, 20, and 30 mL Reactor volume: 50, 100, 150, and 250 mL Power: 80, 150, 300, and 450 W

9:1 – 96.8% 1 wt.% – 96.8% 2.5 min – 98% 20 mL – 98% 50 mL – 96.8% 300 and 400 W – 98%

Ethanol–methanol mixture

Molar ratio: 4.5:1; 6:1; 9:1; 13.5:1 Catalyst: 1% Reaction time: 2 min Oil volume: 10, 20, and 30 mL Reactor volume: 50, 100, 150, and 250 mL Power: 80, 150, 300, and 450 W

9:1–98.7% 1% 1-2 min 10 mL – 98.7% 100, 150, and 250 mL – 99% 300 W – 99%

Table 2 FAAE compositions for biodiesel obtained from different reactants. Group name

Methanol

Ethanol

Ethanol–methanol mixture

Octanoic acid (C8:0) Lauric (C12:0) Myristic acid (C14:0) Palmitic acid (C16:0) Palmitoleic acid (C16:1) Linoleic acid (C18:2) Linolenic acid (C18:3) Arachidic (C20:0) Behenic (C22:0) Erucic (C22:1) Lignoceric (C24:0) Other acids

0.01 0.03 0.04 33.12 0.00 64.25 0.55 0.25 0.14 0.88 0.72 0.02

0.02 0.02 0.05 0.00 16.25 0.00 78.27 2.06 0.45 0.63 2.25 0.01

0.01 0.01 0.02 8.13 22.17 65.80 0.71 0.43 0.22 0.23 1.06 1.19

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Fig. 9. GC-Chromatograms for methyl, ethyl, and methyl-ethyl esters.

determined as: 9:1 alcohol to oil ratio, 1% wt. catalyst amount, and 1–2 min reaction time at a power output rate of 75–150 W. Methanol and ethanol based transesterification reactions had similar biodiesel yields (96%) at an ultrasonic intensity of 10 W/cm2 while ethanol–methanol (50%-50%) mixtures resulted in higher biodiesel yields compared to ethanol or methanol due to the combination of ultrasonic effect and the better solubility of ethanol at lower power density and ultrasound intensity. These factors may result in better process operational flexibility and yield improvements in large scale design and applications for biodiesel production. Since the biodiesel properties are reportedly better for the ethanol–methanol mixtures, further research in this area could have profound impact on current processes for biodiesel production. Finally, ethanol can be produced from renewable materials, which reduces the stress on the existing methanol sources and associated gasoline price fluctuations enhancing the sustainability of biodiesel production.

Acknowledgements This research was supported by the Office of Research and Economic Development (ORED), the Bagley College of Engineering (BCoE), and the Department of Civil and Environmental Engineering (CEE) at Mississippi State University.

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