Energy Conversion and Management 88 (2014) 633–640
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Transesterification of used vegetable oil catalyzed by barium oxide under simultaneous microwave and ultrasound irradiations 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
i n f o
Article history: Received 13 June 2014 Accepted 26 August 2014
Keywords: Biodiesel Ultrasound Microwave Process parameters Barium oxide Power density
a b s t r a c t This study presents a novel application of simultaneous microwave and ultrasound (MW/US) irradiations on transesterification of used vegetable oil catalyzed by barium oxide, heterogeneous catalyst. Experiments were conducted to study the optimum process conditions, synergistic effect of microwave and ultrasound irradiations and the effect of power density. From the process parametric optimization study, the following conditions were determined as optimum: 6:1 methanol to oil ratio, 0.75% barium oxide catalyst by wt.%, and 2 min of reaction time at a combined power output rate of 200 W (100/100 MW/US). The biodiesel yields were higher for the simultaneous MW/US mediated reactions (93.5%) when compared to MW (91%) and US (83.5%) irradiations individually. Additionally, the effect of power density and a discussion on the synergistic effect of the microwave and ultrasound mediated reactions were presented. A power density of 7.6 W/mL appears to be effective for MW, and MW/US irradiated reactions (94.4% and 94.7% biodiesel yields respectively), while a power density of 5.1 W/mL was appropriate for ultrasound irradiation (93.5%). This study concludes that the combined microwave and ultrasound irradiations result in a synergistic effect that reduces the heterogeneity of the transesterification reaction catalyzed by heterogeneous catalysts to enhance the biodiesel yields significantly. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The persisting global energy crisis and the escalating greenhouse gas emissions at global levels from fossil fuel consumption have provided impetus for research and development in the areas of renewable energy and fuels production. Several renewable energy sources such as solar, nuclear, geothermal, wind, and biomass have been explored and appropriate technologies have been developed in the past few decades to serve as carbon–neutral energy sources. Among these renewable energy sources, energy production from biomass and its derived feedstock (i.e., oil) appears to be a very attractive option since the energy/fuel derivatives from these sources possess high energy content/density with minimum environmental emissions. Biodiesel produced from biomass derived oils is also an excellent example of carbon–neutral transportation fuel. As such, it is critical to develop energy-efficient technologies for biodiesel production to enhance the environmental benefits, as well as the net energy benefits of the overall process.
⇑ Corresponding author. E-mail address:
[email protected] (V.G. Gude). http://dx.doi.org/10.1016/j.enconman.2014.08.060 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.
For long-term sustainability of biodiesel production, the following must be considered: (1) developing the best scientific, engineering and technological solutions for converting the feedstock into energy/fuel sources; (2) minimizing the environmental impact by reducing the water and energy consumption; and (3) addressing long-term availability of the feedstock supplies, preferably from local production/sources [1,2]. Accordingly, in this research a novel application of simultaneous microwave and ultrasound irradiations was evaluated for biodiesel production with the goal of understanding their synergistic effect on the transesterification reaction. Microwaves and ultrasound have been used as constructive, rapid and safe methods for biodiesel production [3,4]. Used vegetable oil was utilized as feedstock since it is often available at a very low cost or even free locally for biodiesel production [2,5–10]. Feedstock costs usually account for up to 80% of the total biodiesel costs [11]. Biodiesel from virgin or used oil feedstock is usually produced through a well-known method called ‘‘transesterification reaction’’. The transesterification reaction, also known as alcoholysis, is the displacement of alcohol from an ester by another alcohol in a process similar to hydrolysis, except that an alcohol is used instead of water. The result is that triglyceride molecules (90– 98% of the oil), which are long and branched, are transformed into smaller esters whose size and properties are similar to those of diesel oils [12,13]. The transesterification reaction can be conducted
634
E. Martinez-Guerra, V.G. Gude / Energy Conversion and Management 88 (2014) 633–640
by using various catalysts categorized as homogeneous or heterogeneous catalysts [14]. Homogeneous catalysts such as ‘‘sodium hydroxide’’ or ‘‘potassium hydroxide’’ dissolve in the biodiesel, and their separation process produces large quantities of wastewater impacting the environment, adding to the treatment processes and finally increasing product costs. These issues can be circumvented by employing heterogeneous catalysts that do not dissolve or sparingly dissolve in oil or methanol phases and eliminate the extensive cleaning, separation, and drying processes. Heterogeneous catalysis has many advantages such as non-corrosiveness, being an environmentally benign process, fewer disposal problems, and better biodiesel production economics [15]. They are also much easier to separate from liquid products and can be designed for higher activity, selectivity, and long catalyst lifetimes [16,17]. Many types of heterogeneous catalysts, such as alkaline earth metal oxides, and various alkaline metal compounds supported on alumina or zeolite can catalyze transesterification reactions [18–20]. This paper presents the simultaneous effect of microwave and ultrasound irradiations on transesterification of used vegetable oils catalyzed by heterogeneous catalyst, barium oxide (BaO). The effect of various process parameters such as methanol to oil ratio, catalyst amount, and the reaction time were evaluated. The synergistic effects [21] of the two novel technologies on the heterogeneously catalyzed transesterification reaction were investigated. The following sections present the experimental studies and the results with a discussion in detail. 2. Materials and methods 2.1. Materials The used vegetable oil (UVO) was obtained from a local restaurant in Starkville, MS. The acid value of UVO was found to be 3.5 mg KOH/g, corresponding to a free fatty acid (FFA) level of 1.7%. Although the solid acid catalysts are preferred for simultaneous esterification and transesterification of feedstock possessing high acid value (free fatty acids, FFA%), we have used solid base catalyst (BaO) which has higher catalytic performance for transesterification than solid acid catalyst. Also, it should be noted that the base catalyzed transesterification is suitable for feedstock with FFA content less than 4% [22]. The molecular weight of the used vegetable oil was calculated to be 837 g/mol from GC–FID analysis. Methanol and catalyst (barium oxide, BaO) were purchased from Fisher Scientific and are of analytical grade. The transesterification experiments were conducted in a microwave/ultrasound reactor with temperature and power control functions manufactured by Columbia International TechnologiesÒ, USA. Microwave irradiation at 2.45 GHz frequency was applied while the ultrasound frequency was at 25 kHz. This ultrasound frequency was chosen because most of the previous biodiesel research studies have reported superior results at this frequency [4]. Microwave-transparent, three-neck custom-fabricated reaction vessels made of borosilicate glass (provided by Columbia International Technologies) were used as reaction vessels. 2.2. Methods The experimental setup consists of a microwave unit combined with ultrasound horn and a thermocouple and a reflux condenser. To identify the combined effect of the microwaves and ultrasound (MW/US), a matrix of conditions was evaluated by fixing a sample volume and energy output rate and by changing the process conditions. For the first set of tests, 20 mL of UVO was added to the mixture of methanol and BaO (catalyst) and was then subjected
to MW/US (100/100 W) irradiations. A three-neck reactor equipped with a reflux condenser and a temperature probe was used. The reflux condenser allowed for cooling of the evaporating solvent (methanol) and returning to the reaction mixture. Temperature and power readings were recorded every 10 s for all the experiments. The molar ratio of methanol to oil tested were 4.5:1, 6:1, 9:1, and 12:1, while the catalyst loads ranged from 0.25% to 1.5% (wt./wt.) by increments of 0.25%. Different reaction times and different power output rates were also evaluated. For 100/100 W (MW/US) power output rates, the reaction times of 1–5 min at one minute intervals were tested. After the exposure of the reaction mixture to MW/US irradiation and before measuring the yield, the samples were allowed to settle for 12 h and then washed with warm deionized water in a pear-shaped separatory funnel to eliminate any soap and unreacted methanol in the biodiesel. The Glycerol layer was separated from the biodiesel (fatty acid methyl esters – FAMEs) layer prior to washing. The samples were then placed in an oven at 60 °C before measuring the yield. The biodiesel quality was confirmed by the GC–FID analysis by standard ASTM B100 method. 1, 3-DBC was used as an internal standard and 193 BHT was used as antioxidant [8]. For the microwave and ultrasound individual and synergistic effect tests, the power output rate of 100/100 W, 2 min reaction time, 1% catalyst, and 6:1 methanol to oil molar ratio were fixed during the experiment. For the power density tests, different sample volumes (20, 40, 60, and 80 mL) were evaluated using a 6:1 methanol to oil ratio, reaction time of 2 min, and catalyst of 0.75 wt.% for 100/100, 200/0, and 0/200 Watts of MW/US power output rates. 2.3. Heterogeneous catalysis mechanism The main mechanism of heterogeneous catalysis follows the principle similar to homogeneous catalysis of either acid or base systems [23,24]. The important factor in homogeneous base catalyzed reaction is to create nucleophilic alkoxide from the alcohol to attack the electrophilic part of the carbonyl group of the triglycerides [25], while in acid catalysis the carbonyl group in triglycerides is protonated, and the alcohol attacks the protonated carbon to create a tetrahedral intermediate. The catalyst efficiency depends on several factors such as specific surface area, pore size, pore volume and active site concentration [26]. The order of activity among alkaline earth oxide catalysts is BaO > SrO > CaO > MgO [27–29]. The structure of metal oxides is made up of positive metal ions (cations) which possess Lewis acidity, i.e. they behave as electron acceptors, and negative oxygen ions (anions) which behave as proton acceptors and are thus Brønsted bases. This has consequences for adsorption. In methanolysis of oils, it provides sufficient adsorptive sites for methanol, in which the (O–H) bonds readily break into methoxide anions and hydrogen cations (Fig. 1). The methoxide anions then react with triglyceride molecules to yield methyl esters [30,31]. Barium oxide catalyzes the transesterification reaction by forming barium methoxide with methanol. Due to its very low methanol-solubility, barium methoxide acts mainly as a heterogeneous catalyst. Pure barium methoxide, which is strongly basic, shows high catalytic activity. It is not soluble in methanol but it forms a suspensoid, whereby its active surface is very well developed. Metal alkoxides can be homogeneous catalysts if they are well soluble in methanol or if they can constitute active centers on the surface of heterogeneous catalysts. The alkalinity of a given compound is a key factor, which determines its catalytic activity in alcoholysis. Alkaline-earth metal compounds are heterogeneous catalysts and the degree of their dispersion in the reaction system has a considerable influence on the level of their catalytic activity, which is determined by diffusion. The classic mechanism of base-catalyzed
E. Martinez-Guerra, V.G. Gude / Energy Conversion and Management 88 (2014) 633–640
Fig. 1. Surface structure of metal oxides and the possible mechanism for transesterification reaction catalyzed by heterogeneous catalyst, barium oxide (BaO).
alcoholysis assumes an attack of the alkoxide anion on the carboxylate carbon of the ester group converting it to a tetrahedral intermediate carrying a negative charge. Methoxide anions can be introduced as alcoholates directly into the reaction if they are soluble in methanol and dissociate easily. The well-known catalytic activity of sodium methoxide in alcoholysis is a good example [32]. Methoxides can also result from reversible reactions of some metal oxides or hydroxides with methyl alcohol [33]; e.g. the reaction of sodium hydroxide with methanol. The mechanism of alcoholysis on heterogeneous alkoxide catalysts can be proposed as shown in Fig. 1. [34] with an example for barium oxide. 3. Results and discussions 3.1. Effect of methanol to oil molar ratio The influence of different methanol to oil molar ratios on the transesterification reaction between 4.5:1 and 12:1 ratios was
635
investigated. The other reaction conditions were set at: catalyst amount of 1 wt.% of oil; reaction time of 2 min; and a power output rate of 200 W at 100/100 W of microwave and ultrasound irradiations. Fig. 2a shows the biodiesel yields obtained for different methanol to oil molar ratios. The biodiesel yield was higher at lower methanol to oil ratios, and yields decreased with increasing ratios. The biodiesel yield was 93.5% at both 4.5:1 and 6:1 molar ratios when compared to the 91% and 86% obtained at 9:1 and 12:1 molar ratios respectively. This could be explained as barium oxide has a strong catalytic activity and its concentration in methanol is higher at low molar ratios, which enhances the mass transfer effect and availability of the catalyst decreases with higher methanol ratios. Although, a wide range of methanol to oil ratios (10:1 to 25:1) were reported [23] for heterogeneous catalysts, higher molar ratios are not preferable, even if they improve the yields slightly, simply due to additional cost and energy requirements. In microwave mediated reactions, energy supplied would be utilized to heat the solvent (since methanol is microwave absorbing material due to higher dipole momentum compared to oil) to result in higher solvent and thus energy losses [35]. Similarly, the ultrasound effect will diminish with increased amounts of reaction compounds or mixture. Further, higher methanol ratios result in unreacted methanol which poses additional concerns for biodiesel and product separation. In this study, we notice that about 4.5:1 or 6:1 molar ratios were adequate to drive the transesterification reaction forward with an acceptable yield of 93.5%. Considering the process economics, it may be desirable to conduct the reaction at 4.5:1 molar ratios, although slightly higher molar ratios ensure reaction completion [36]. The temperature and power profiles for these tests are shown in Fig. 3a and b respectively. It can be noted that at higher molar ratios the reaction temperature was lower which could be due to evaporation of the solvent which provides the cooling effect on the surface. Also, higher reaction mixtures absorb more energy to increase the temperature but not necessarily driving the reaction forward. Additionally, in ultrasound mediated transesterification reactions, an oil-alcohol emulsion may form due to intense physical mixing
Fig. 2. Effect of the process parameters: (a) methanol to oil ratios; (b) catalyst (wt.%); (c) reaction time on microwave and ultrasound irradiated transesterification reaction at 100/100 W power output rates; (d) comparison of MW/US irradiation effects; combined and individual.
636
E. Martinez-Guerra, V.G. Gude / Energy Conversion and Management 88 (2014) 633–640
Fig. 3. Temperature and power profiles for process parameters at 100/100 W power output rates (methanol to oil molar ratios, catalyst wt.%, and reaction time).
(higher impact than mechanical mixing) which reduces the final biodiesel yield. 3.2. Effect of catalyst amount The catalyst plays a vital role in biodiesel preparation and thus the type of catalyst to be employed in transesterification reaction needs some serious considerations. A catalyst with strong catalytic activity will improve the process performance by increasing the yield and mitigating other process related issues. The effect of catalyst amount on transesterification reaction is shown in Fig. 2b. A small amount of catalyst, about 0.25% (wt./wt.), was able to provide a FAME yield of 89.75%. This shows the strong catalytic activity of barium oxide catalyst. The biodiesel yield increased at higher catalyst amounts with yields 91% and 96% at 0.50% and 0.75% catalyst amounts. However, the yield did not increase considerably at higher catalyst percentages, but rather decreased. The lowest biodiesel yield obtained was 88.5% for 1.5% wt. catalyst due to saponification effect. The other process conditions for these tests were: methanol to oil ratio of 6:1; reaction time of 2 min; and a power output rate of 200 W at 100/100 W of microwave and ultrasound irradiations. A maximum biodiesel yield of 96% was obtained at
0.75% catalyst amount. This is much lower than the usual catalyst amounts applied in heterogeneous catalytic transesterification reactions which are usually between 3 and 10 wt.% [23]. The temperature and power profiles are shown in Fig. 3c and d respectively. The reaction temperatures were higher at higher catalyst amounts (73.3 °C at 1.25% and 66.1 °C at 0.25%) indicating the catalyst activity. Since transesterification is exothermic reaction (releasing heat) and the reaction rate increases the heat released during the reaction enhanced by higher catalyst amount. However, for transesterification reaction, higher reaction temperatures are not required. Reaction temperatures well beyond the methanol boiling point may result in solvent losses and favor the reactions with water moisture (if present) via saponification reaction leading to lower biodiesel yields and byproduct formation. 3.3. Effect of reaction time The effect of reaction time on the transesterification reaction is shown in Fig. 2c. Five different reaction times were considered, starting with 1 min reaction time and at one minute intervals up to 5 min. The other process conditions for these tests were: methanol to oil ratio of 6:1; catalyst amount of 0.75% wt.; and a power
E. Martinez-Guerra, V.G. Gude / Energy Conversion and Management 88 (2014) 633–640
output rate of 200 W at 100/100 W of microwave and ultrasound irradiations. It can be seen (from Fig. 2c) that about two minutes of reaction time is sufficient to complete the transesterification reaction. Transesterification reaction by conventional heating methods whether catalyzed by homogeneous or heterogeneous catalysts requires much longer reaction times often extending to several hours and at high reaction temperatures in the range of 100–250 °C (for heterogeneous catalysts) although a few studies have reported between 60 and 70 °C [37,38]. The short reaction times can be completely attributed to the ability of microwave irradiations to enhance the chemistry of the transesterification reaction. Microwave mediated reactions are reportedly very fast, up to 1000 times faster than conventional methods due to various factors such as uniform heating with inverse temperature gradient, ionic conduction, and excitation [35,39]. The temperature and power profiles are shown in Fig. 3e and f respectively. The reaction temperatures were higher at longer reaction times; reaching as high as 92 °C at five minute reaction times and 84.1, 81.8, 73.6 and 45.6 °C respectively at reaction times 4, 3, 2 and 1 min. However, for transesterification reaction, higher reaction temperatures are not required due to the aforementioned reasons. Longer reaction times may result in emulsions leading to lower biodiesel yields and byproduct formation (saponification effect).
637
3.4. Evaluation of combined effect of microwave and ultrasound irradiations Following the evaluation of process parametric effects, the synergistic effect of microwave and ultrasound was investigated with the optimum process conditions (methanol to oil ratio of 6:1, catalyst amount of 0.75% wt. and 2 min reaction time). Microwaves and ultrasound applied whether simultaneously or individually, the power output rate was kept at 200 W with a sample volume of 20 mL. Different reaction times of 1, 2, and 3 min were considered to understand the synergistic effect of both irradiations. The biodiesel yields obtained from these tests are shown in Fig. 2d. For a reaction time of 1 min, the microwave irradiation worked superior to others probably due to the temperature effect caused by the dielectric momentum of the methanol. The yields at one minute reaction time were 86%, 79% and 71% for microwave, ultrasound and microwave/ultrasound irradiations respectively. None of these yields are in the acceptable range for practical biodiesel production. The lower biodiesel yield for ultrasound can be attributed to the lack of ability to raise the reaction temperature. For microwave/ultrasound combination, the yield was significantly lower since the microwave output rate was quite low during the initial stages for 100 W output rate. However, the biodiesel yields
Fig. 4. Temperature and power profiles for microwave, ultrasound and microwave/ultrasound mediated transesterification reactions; combined and individual.
E. Martinez-Guerra, V.G. Gude / Energy Conversion and Management 88 (2014) 633–640
increased significantly for all the energy combinations at two minutes of reaction time. The yields at two minutes of reaction time were 91%, 83.5% and 93.5% for microwave (MW), ultrasound (US) and microwave/ultrasound (MW/US) irradiations respectively. The reasons for yield improvement in the microwave/ultrasound irradiated reactions can be explained by exposure time and the reaction temperature, which can significantly affect the conversion process. The temperature profiles in Fig. 4 show that ultrasound irradiation did not increase the reaction temperature significantly since it was applied in a pulse mode (5 s ON and 1 s OFF) to provide for relaxation effect, which also affects the combined microwave and ultrasound irradiation in a similar manner in terms of reaction temperature. In our previous study, where we applied continuous sonication the reaction temperatures reached over 80 °C due to intense mixing provided by ultrasound [29]. Microwave based reactions resulted in higher temperatures due to the superheating effect. It could be noticed that the temperature profiles were uniform indicating reliability of the experimental unit in generating the temperature effects, although the temperatures increased with increasing reaction time. The same applies to the power profiles. The power profiles were uniform with minimum deviation. Further exposure (beyond 2 min) to microwave and/or ultrasound did not favorably affect the yield, except for ultrasound irradiation which was about 91%. This result is similar to the results reported in the literature because prolonged exposure to ultrasound usually provides a better yield, but at additional energy expenses [4].
100
Biodiesel Yield (%)
638
95
90
MW/US
85
MW US
80 0
5
10
15
20
Power Density (W/mL) Fig. 5a. Effect of power density for microwave, ultrasound and microwave/ ultrasound mediated transesterification reaction.
3.5. Other benefits of synergistic microwave and ultrasound effects The most commonly used homogeneous catalysts are alkaline metal hydroxides such as sodium hydroxide and potassium hydroxide (NaOH and KOH result in soap formation, which makes the process inefficient by reducing alkyl ester yield and interrupting glycerol recovery [40]. Although, there are many advantages with heterogeneously catalyzed methanolysis (transesterification) reaction, it is very complex because it occurs in a three-phase system consisting of a solid (heterogeneous catalyst) and two immiscible liquid phases (oil and methanol). But, by applying microwaves and ultrasound simultaneously, the heterogeneity of the reaction mixture (or diffusion limitations) can be greatly reduced. Ultrasound provides for intense mixing that is more effective than conventional mechanical mixing. The Microwave effect will be greatly enhanced when the mass transfer limitation is eliminated by simultaneous ultrasound mixing providing uniform and rapid heating throughout the reaction mixture. In the past, to overcome this mass transfer problem in heterogeneous catalysts, co-solvents were introduced to promote miscibility of oil and methanol [41]. Tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), n-hexane, and ethanol were the more commonly used co-solvents in transesterification of vegetable oils with methanol and solid catalysts [34]. Another way to mitigate mass transfer problems associated with heterogeneous catalysts is using structure promoters or catalyst supports which provide more specific surface area and pores for active species where they can anchor and react with large triglyceride molecules [42]. However, in simultaneous microwave and ultrasound mediated reactions, such addition of co-solvents or surface modifications are not required. 3.6. Effect of power density The effect of power density is shown in Fig. 5a. The power density is defined as the ratio of the power supplied to the amount (mL) of sample processed. The amount of sample includes the methanol volume (fixed at 6:1) and catalyst amount of 0.75% (wt./wt.). A power output rate of 400 W (200 W for 2 min reaction
Fig. 5b. Effect of sample volume for microwave, ultrasound and microwave/ ultrasound mediated transesterification reaction.
time) was applied through microwave and ultrasound irradiations. The power densities were calculated as 3.8 W/mL; 5.1 W/mL; 7.6 W/mL; and 15.3 W/mL for sample volumes 80, 60, 40, and 20 mL respectively. Power densities tests help optimize the approximate power output for a desired reaction. Higher power output rate does not necessarily result in higher yields in a reaction but rather increase the energy costs and offset the net energy benefits, especially in biodiesel production [43]. It can be noticed that higher power densities did not necessarily result in higher biodiesel yields due to excess energy and solvent losses. In other words, the power output should be properly adjusted to improve the energy efficiency of the reaction. A power density of 7.6 W/mL appears to be effective for microwave irradiated reactions (94.4% and 94.7%) but a power density of 5.1 W/mL was appropriate for ultrasound irradiation (93.5%). The biodiesel yield for ultrasound reaction (93.5%) is slightly lower than those mediated by microwave/ultrasound mediated reaction (93.8%) at a power density of 5.1 W/mL. Fig. 5b shows the relation between the biodiesel yield and the sample volume at a fixed power output rate (200 W for 2 min) to the sample. It is obvious that the biodiesel yields increased with the sample volumes. This could be explained as efficient utilization of the MW and US irradiations applied for the reaction. The synergistic MW/US effect produced superior results when compared to individual MW and US irradiations, MW being slightly superior to the US. The temperature and power profiles for the power density tests are shown in Fig. 6. It can be noted that microwave mediated transesterification reactions result in higher reaction temperatures.
E. Martinez-Guerra, V.G. Gude / Energy Conversion and Management 88 (2014) 633–640
639
Fig. 6. Temperature and power profiles for power density effect of microwave, ultrasound and microwave/ultrasound mediated transesterification reaction. Table 1 FAME composition of biodiesel obtained from used vegetable oil. Group name
Oil
MW/US
MW
US
Palmitic (C16:0) Palmitoleic (C16:1) Linoleic (C18:2) Linolenic (C18:3) Arachidic (C20:0) Behenic (C22:0) Erucic (C22:1) Lignoceric (C24:0)
15.23 35.39 47.65 1.12 0.10 0.05 0.2 0.2
27.59 27.41 43.16 1.34 0.12 0.03 0.17 0.18
6.44 11.39 80.91 0.62 0.08 0.07 0.21 0.22
4.45 0.00 93.92 0.98 0.08 0.06 0.08 0.34
3.7. Biodiesel composition FAME analysis for biodiesel from used vegetable oil has shown the following major compounds (Table 1): Palmitic (C16:0); Palmitoleic (C16:1); Linoleic (C18:2); Linolenic (C18:3); Archidic (C20:0); Behenic (C22:0); Erucic (C22:1); and Lignoceric (C24:0). The calculated molecular weight for the waste vegetable oil used is 836.97 g/mol and it was used to calculate the reactant to oil molar ratio. Some differences in FAME compositions can be noticed
due to unique heating and mixing effects caused by microwave and ultrasound irradiations whether combined or alone [21]. The fatty acid composition as well as the FAME composition of the biodiesel will depend on the origin of the oil and the specific conditions at which it has been stored or exposed. The type of heating and the exposure time and the temperature have pronounced effect on the fatty acid composition of different oils [44,45]. In literature, significant differences in fatty acid compositions were reported for different oils before and after heating. For example, the influence of frying with four different oils (sunflower oil, soybean oil, olive oil, and corn oil) on the fatty acid composition of silver carp was evaluated by Naseri et al. [44]. They have found that frying led to the exchange of fatty acids between the silver carp lipid and frying fats. As a result of interactions, MUFA (monounsaturated fatty acids), PUFA (polyunsaturated fatty acids), and PUFA/ SFA (saturated fatty acids) ratio of samples fried in sunflower, soybean, and corn oil significantly increased while the amounts of SFA decreased. Kowalski [45] reported similar changes in fatty acid profiles when sunflower and olive oils were heated with caffeic acid and protocatechuic acids. Moreover, in our experiments, an alkaline metal oxide catalyst (BaO) was used, which has a different
640
E. Martinez-Guerra, V.G. Gude / Energy Conversion and Management 88 (2014) 633–640
transesterification chemistry since the barium oxide molecule is able to interact with two carbonyl or methanol groups at the same time [46,47]. Additionally, the type of reactants (methanol, ethanol, and methanol–ethanol mixtures) might have a significant effect on the transesterification chemistry and therefore on the FAME composition, compounded by the microwave and ultrasound process intensification effects [48,49]. 4. Conclusions This study demonstrated the simultaneous application of microwaves and ultrasound on the transesterification reaction of used vegetable oils using heterogeneous catalyst, barium oxide. It was shown that the combined irradiation provides superior results due to elimination of mass – heat transfer related limitations and the improvement of homogeneity of the reaction mixture. The power density evaluation proves that the energy requirements for the transesterification reaction can be optimized by varying the power output rates of the non-conventional heating and mixing techniques providing energy-efficient biodiesel production. Further optimization studies and process improvements employing combined microwave and ultrasound mediated reactions would be beneficial for biodiesel production with low energy and chemical consumption. 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. References [1] Chung JN. Grand challenges in bioenergy and biofuel research: engineering and technology development, environmental impact, and sustainability. Front Energy Res 2013;1:1–4. [2] Gude VG, Grant GE, Patil PD, Deng S. Biodiesel production from low cost and renewable feedstock. Cent Eur J Eng 2013;3:595–605. [3] Wahidin S, Idris A, Shaleh SRM. Rapid biodiesel production using wet microalgae via microwave irradiation. Energy Convers Manage 2014;84:227–33. [4] Luo J, Fang Z, Smith RL. Ultrasound-enhanced conversion of biomass to biofuels. Prog Energy Combust Sci 2013;41:56–93. [5] Gude VG, Grant GE. Biodiesel from waste cooking oils via direct sonication. Appl Energy 2013;109:135–44. [6] Al-Hamamre Z, Yamin J. Parametric study of the alkali catalyzed transesterification of waste frying oil for biodiesel production. Energy Convers Manage 2014;79:246–54. [7] Martinez-Guerra E, Gude VG, Mondala A, Holmes W, Hernandez R. Microwave and ultrasound enhanced extractive-transesterification of algal lipids. Appl Energy 2014;129(354–363):2014. [8] Martinez-Guerra E, Gude VG, Mondala A, Holmes W, Hernandez R. Extractive transesterification of algal lipids under microwave irradiation using hexane as solvent. Bioresour Technol 2014;156:240–7. [9] Patil PD, Gude VG, Deng S, Reddy H, Muppaneni T. Biodiesel production from waste cooking oil using sulfuric acid and microwave irradiation processes. J Environ Prot 2012;3:107–13. [10] Grant GE, Gude VG. Kinetics of ultrasonic transesterification of waste cooking oils. Environ Prog Sustain Energy 2014;33:1051–8. [11] Lam MK, Lee KT, Mohamed AR. Homogeneous, heterogeneous and enzymatic catalysis for transesterification of high free fatty acid oil (waste cooking oil) to biodiesel: a review. Biotechnol Adv 2010;28:500–18. [12] Encinar JM, Gonzalez JF, Rodriguez JJ, Tejedor A. Biodiesel fuels from vegetable oils: transesterification of Cynara cardunculus L. oils with ethanol. Energy Fuels 2002;16:443–50. [13] Quik GR, Woodmore PJ, Wilson BT. Engine evaluation of linseed oil and derivatives, in vegetable oils diesel fuel: seminar III, ARM-NC-28. Bagby MO, Pryde EH, editors. Peoria, IL: U.S. Department of Agriculture; 1983. p. 138. [14] Sivasamy A, Cheah KY, Fornasiero P, Kemausuor F, Zinoviev S, Miertus S. Catalytic applications in the production of biodiesel from vegetable oils. ChemSusChem 2009;2:278–300. [15] Di Serio M, Cozzolino M, Giordano M, Tesser R, Patrono P, Santacesaria E. From homogeneous to heterogeneous catalysts in biodiesel production. Ind Eng Chem Res 2007;46:6379–84.
[16] Liu X, He H, Wang Y, Zhu S, Piao X. Transesterification of soybean oil to biodiesel using CaO as a solid base catalyst. Fuel 2008;87:216–21. [17] Xie WL, Peng H, Chen LG. Transesterification of soybean oil catalyzed by potassium loaded on alumina as a solid-base catalyst. Appl Catal A 2006;300:67–74. [18] Suppes GJ, Dasari MA, Doskocil EJ. Transesterification of soybean oil with zeolite and metal catalysts. Appl Catal A 2004;257:213–23. [19] Tsuji H, Yagi F, Hattori H, Kita H. Self-condensation of n-butyraldehyde over solid base catalysts. J Catal 1994;148:759–70. [20] Cantrell DG, Gillie LJ, Lee AF, Wilson K. Structure-reactivity correlations in MgAl hydrotalcite catalysts for biodiesel synthesis. Appl Catal A 2005;287:183–90. [21] Martinez-Guerra E, Gude VG. Synergistic effect of simultaneous microwave and ultrasound irradiations on transesterification of waste vegetable oil. Fuel 2014;137:100–8. [22] Sharma YC, Singh B. Advancements in solid acid catalysts for ecofriendly and economically viable synthesis of biodiesel. Biofuels Bioprod Bioref 2011;5:69–92. [23] Lee DW, Park YM, Lee KY. Heterogeneous base catalysts for transesterification in biodiesel synthesis. Catal Surv Asia 2009;13:63–77. [24] Di Serio M, Tesser R, Pengmei L, Santacesaria E. Heterogeneous catalysts for biodiesel production. Energy Fuels 2008;22:207–17. [25] Schuchardt U, Sercheli R, Varga RM. Transesterification of vegetable oils: a review. J Braz Chem Soc 1998;9:199–210. [26] Smith VG, Notheisz F. Heterogeneous catalysis in organic chemistry. New York, NY, USA: Academic Press Inc.; 2006. [27] Bender ML. Mechanisms of catalysis of nucleophilic reactions of carboxylic acid derivatives. Chem Rev 1960;60:53–113. [28] Demirbas A. Comparison of transesterification methods for production of biodiesel from vegetable oils and fats. Energy Convers Manage 2008;49:125–30. [29] Yan S, Di Maggio C, Mohan S, Kim M, Salley SO, Simon KY. Advancements in heterogeneous catalysis for biodiesel synthesis. Top Catal 2010;53:721–36. [30] Chorkendorff I, Niemantsverdriet JW. Concepts of modern catalysis and kinetics. Weinheim, Germany: Wiley-VCH; 2003. [31] Refaat AA. Biodiesel production using solid metal oxide catalysts. Int J Environ Sci Technol 2011;8:203–21. [32] Freedman B, Pyrde EH, Mounts TL. Variables affecting the yields of fatty esters from transesterified vegetable oils. J Am Oil Chem Soc 1984;61:1638–43. [33] Bradley DC, Mehrotra RC, Gaur DP. Metal alkoxides. New York: Academic Press; 1978. [34] Gryglewicz S. Rapeseed oil methyl esters preparation using heterogeneous catalysts. Bioresour Technol 1999;70:249–53. [35] Gude VG, Patil PD, Deng S, Nirmalakhandan N. Microwave enhanced methods for biodiesel production and other environmental applications. In: Green chemistry for environmental remediation. New York: Wiley Interscience; 2011. p. 209–49. [36] Tasic MB, Stamenkovic OS, Veljkovic VB. Cost analysis of simulated basecatalyzed biodiesel production processes. Energy Convers Manage 2014;84:405–13. [37] Endalew AK, Kiros Y, Zanzi R. Inorganic heterogeneous catalysts for biodiesel production from vegetable oils. Biomass Bioenergy 2011;35(9):3787–809. [38] Yunus khan TM, Atabani AE, Badruddin IA, Badarudin A, Khayoon MS, Triwahyono S. Recent scenario and technologies to utilize non edible oils for biodiesel production. Renew Sustain Energy Rev 2014;37:840–51. [39] Gude VG, Patil PD, Martinez-Guerra E, Deng S, Nirmalakhandan N. Microwave energy potential for biodiesel production. Sustain Chem Process 2013:1–5. [40] Freedman B, Butterfield RO, Pryde EH. Transesterification kinetics of soybean oil. J Am Oil Chem Soc 1986;63:1375–80. [41] Alhassan Y, Kumar N, Bugaje IM, Pali HS, Kathkar P. Co-solvents transesterification of cotton seed oil into biodiesel: effects of reaction conditions on quality of fatty acids methyl esters. Energy Convers Manage 2014;84:640–8. [42] Zabeti M, Daud WMAW, Aroua MK. Activity of solid catalysts for biodiesel production: a review. Fuel Process Technol 2009;90:770–7. [43] Patil PD, Gude VG, Deng S. Microwave-assisted catalytic transesterification of camelina sativa oil. Energy Fuels 2010;24:1298–304. [44] Naseri M, Abedi E, Mohammadzadeh B, Afsharnaderi A. Effect of frying in different culinary fats on the fatty acid composition of silver carp. Food Sci Nutr 2013;1(4):292–7. [45] Kowalski R. GC analysis of changes in the fatty acid composition of sunflower and olive oils heated with quercetin, caffeic acid, protocatechuic acid, and butylated hydroxyanisole. Acta Chromatogr 2007;18:15–23. [46] Boey P-L, Maniam GP, Abd Hamid S. Performance of calcium oxide as a heterogeneous catalyst in biodiesel production: a review. Chem Eng J 2011;168:15–22. [47] Islam A, Taufiq-Yap YH, Chua C-M, Chanc E-S, Ravindra P. Studies on design of heterogeneous catalysts for biodiesel production. Process Safe Environ Prot 2013;91:131–44. [48] Martinez-Guerra E, Gude VG. Transesterification of waste vegetable oil under pulse sonication using ethanol, methanol, and ethanol–methanol mixtures. Waste Manage 2014. http://dx.doi.org/10.1016/j.wasman.2014.07.023. [49] Patil P, Gude VG, Pinappu S, Deng S. Transesterification kinetics of Camelina sativa oil on metal oxide catalysts under conventional and microwave heating conditions. Chem Eng J 2011;168(3):1296–300.
