Biodiesel production of EU in 2009 (EBB 2010)(*Data include hydrodiesel production)
The depletion of fossil fuels and their effects on environmental pollution necessitate the usage of alternative renewable energy sources in recent years. In this context, biodiesel is an important one of the alternative renewable energy sources which has been mostly used nowadays. Biodiesel is a renewable and energy-efficient fuel that is non-toxic, biodegradable in water and has lesser exhaust emissions. It can also reduce greenhouse gas effect and does not contribute to global warming due to lesser emissions. Because it does not contain carcinogens and its sulphur content is also lower than the mineral diesel (Sharma & Singh, 2009; Suppalakpanya et al., 2010). Biodiesel can be used, storaged safely and easily as a fuel besides its environmental benefits. Also it is cheaper than the fossil fuels which affect the environment in a negative way. It requires no engine conversion or fuel system modification to run biodiesel on conventional diesel engines.
Today, biodiesel is commonly produced in many countries of the world such as Malaysia, Germany, USA, France, Italy and also in Australia, Brazil, and Argentina. Biodiesel production of EU in 2009 was presented in Table 1 (European Biodiesel Board, July 2010). As can be seen from Table 1, 9 million tons biodiesel were produced in European Union countries in 2009. Germany and France are the leaders in biodiesel production. EU represents about 65% of worldwide biodiesel output. Biodiesel is also main biofuel produced and marketed in Europe. In 2009, biodiesel represented is about 75% of biofuels produced in Europe.
Firstly in 1900, Rudolph Diesel showed that diesel engines could work with peanut oil. And then, the different kinds of methods such as pyrolysis, catalytic cracking, blending and microemulsification were used to produce biodiesel from vegetable oil for diesel engines (Sharma & Singh, 2009; Varma & Madras, 2007). Finally, transesterification process was developed as the most suitable method to overcome problems due to direct use of oil in diesel engines (Varma & Madras, 2007).
Biodiesel is generally produced from different sources such as plant oils: soybean oil (Kaieda et al., 1999; Samukawa et al., 2000; Silva et al., 2010; Cao et al., 2005; Lee et al., 2009; Yu et al., 2010), cottonseed oil (Köse et al., 2002; He et al., 2007; Royon et al., 2007; Hoda, 2010; Azcan & Danisman, 2007; Rashid et al., 2009), canola oil (Dube et al., 2007; Issariyakul et al., 2008), sunflower oil (Madras et al., 2004), linseed oil (Kaieda et al., 1999), olive oil (Lee et al., 2009), peanut seed oil (Kaya et al., 2009), tobacco oil (Veljkovic et al., 2006), palm oil (Melero et al., 2009), recycled cooking oils (Issariyakul et al., 2008; Rahmanlar, 2010; Zhang et al. 2003; Demirbaş, 2009) and animal fats (Da Cunha et al., 2009; Öner & Altun, 2009; Gürü et al., 2009; Gürü et al., 2010; Tashtoush et al., 2004; Teixeira et al., 2009; Chung et al., 2009). The major economic factor to consider for input costs of biodiesel production is the feedstock. 90 % of the total cost of the biodiesel production is the resource of the feedstock. Studies to solve this economic problem especially focused on biodiesel production from cheaper raw material. Using agricultural wastes, high acid oils, soapstock, waste frying oil and alg oil as raw materials for biodiesel production are being reported in literature (Haas & Scott, 1996;Özgül & Türkay, 1993; Özgül & Türkay, 2002; Leung & Guo, 2006; Yücel et al., 2010; Özçimen & Yücel, 2010).
Transesterification process, as showed in Figure 2 (Barnard et al., 2007) is a conventional and the most common method for biodiesel production. In transesterification reaction homogeneous catalysts (alkali or acid) or heterogeneous catalysts can be used. The catalysts split the oil into glycerin and biodiesel and they could make production easier and faster.
In this method, fatty acid alkyl esters are produced by the reaction of triglycerides with an alcohol, especially ethanol or methanol, in the presence of alkali, acid or enzyme catalyst etc. The sodium hydroxide or potassium hydroxide, which is dissolved in alcohol, is generally used as catalyst in transesterification reaction (Dube et al., 2007). The products of the reaction are fatty acid methyl esters (FAMEs), which is the biodiesel, and glycerin (Vicente et al., 2004). Ethanol can be also used as alcohol instead of methanol. If ethanol is used, fatty acid ethyl ester (FAEE) is produced as product (Hanh et al., 2009b). Methyl ester rather than ethyl ester production was preferred, because methyl esters are the predominant product of commerce, and methanol is considerably cheaper than ethanol (Zhou & Boocock, 2003). However, methanol usage has an important disadvantage, it is petroleum based produced. Whereas ethanol can be produced from agricultural renewable resources, thereby attaining total independence from petroleum-based alcohols (Saifuddin & Chua, 2004; Encinar et al. 2007). Ethanol is also preferred mostly in ethanol producing countries. Propanol and butanol have been also used as alcohols in biodiesel production.
Alkali-catalyzed transesterification proceeds much time faster than that catalyzed by an acid and it is the one most used commercially (Dube et al., 2007; Freedman et al., 1984). The most commonly used alkali catalysts are NaOH, CH3ONa, and KOH (Vicente et al., 2004). Potassium hydroxide (KOH) and sodium hydroxide (NaOH) flakes are inexpensive, easy to handle in transportation and storage, and are preferred by small producers. Alkyl oxide solutions of sodium methoxide or potassium methoxide in methanol, which are now commercially available, are the preferred catalysts for large continuous-flow production processes (Singh et al., 2006).
For acid-catalyzed systems, sulfuric acid has been the most investigated catalyst, but other acids, such as HCl, BF3, H3PO4, and organic sulfonic acids, have also been used by different researchers (Lotero et al, 2005). But in alkali catalyzed method, glycerides and alcohol must be substantially anhydrous, otherwise it leads to saponification (Helwani et al., 2009). Due to saponification the catalytic efficiency decreases, the separation of glycerol becomes difficult and it also causes gel formation (Helwani et al., 2009). In homogeneous catalyzed reactions, separation of catalyst from the reaction mixture is hard and expensive. With this purpose, large amount of water is used to separate catalyst and product (Vyas et al., 2010). On the other hand, undesired by-product formation such as glycerin can be seen, the reaction lasts very long and energy consumption may be very high. Thus, researchers have focused on development of new biodiesel production methods and the optimization of the processes (Sharma et al., 2008). So, various processes such as supercritical process, microwave assisted method and ultrasound assisted method have recently developed. Alternative energy stimulants or non-classical energies have been used for many years to increase the reaction rate and to enhance the yield of particular reaction products. Novel methods or combining innovative methods and techniques are a challenge that can lead to unexpected advances in biodiesel production techniques (Nuechter et al., 2000). In this study, biodiesel production in supercritical conditions, in microwave and ultrasound techniques as novel methods through the years (2000-2011) was reviewed and presented in detail.
2. Supercritical process
Supercritical method is one of the novel methods in biodiesel production. Biodiesel production can be easily achieved by supercritical process without catalysts. A supercritical fluid is any substance at a temperature and pressure above its critical point. It can diffuse through solids like a gas, and dissolve materials like a liquid. These fluids are environment-friendly and economic. Generally, water, carbon dioxide and alcohol are used as supercritical fluids. Supercritical fluids have different application areas. One of these applications is the biodiesel production that is firstly achieved by Saka and Kusdiana in 2001. And many studies on biodiesel production in supercritical conditions were made since 2001. All studies in the literature since 2001 were reviewed and presented in Table 2. The biodiesel production have been studied by using supercritical process from different oils such as rapeseed oil (Kusdiana & Saka, 2001; Saka et al., 2010; Saka & Kusdiana, 2002; Minami & Saka, 2006; Yoo et al., 2010), algae oil (Patil et al., 2010b), chicken fat (Marulanda et al., 2010), jatropha oil (Hawash et al., 2009; Rathore & Madras, 2007; Chen et al., 2010), soybean oil (Cao et al., 2005; He et al., 2007 ; Cheng et al., 2010; Yin et al., 2008), waste cooking oil (Patil et al., 2010a; Demirbaş, 2009), sunflower oil (Demirbaş, 2007), cottonseed oil (Demirbaş, 2008), linseed oil (Demirbaş, 2009), hazelnut kernel oil (Demirbaş, 2002), coconut oil (Bunyakiat et al, 2006), palm oil (Gui et al., 2009 ; Tan et al., 2010c; Tan et al., 2009; Song et al., 2008).
In Saka’s study, rapeseed oil was converted to methyl esters with supercritical methanol (molar ratio of methanol to rapeseed oil: 42 to 1) at temperature of 350°C in 240 s. The methyl ester yield of the supercritical methanol method was higher than those obtained in the conventional method with a basic catalyst. Liquid methanol is a polar solvent and has hydrogen bonding between OH oxygen and OH hydrogen to form methanol clusters, but supercritical methanol has a hydrophobic nature with a lower dielectric constant, so non-polar triglycerides can be well solvated with supercritical methanol to form a single phase oil/methanol mixture. For this reason, the oil to methyl ester conversion rate was found to increase dramatically in the supercritical state (Saka & Kusdiana, 2001; Fukuda et al., 2001).
Main factors affecting transesteriﬁcation via supercritical process are the effect of temperature, pressure and effect of molar ratio between alcohol and oil sample.
Temperature is the most important factor in all parameters that affects the transesteriﬁcation under supercritical condition. In the study of Kusdiana & Saka, the conversion of triglyceride to methyl esters is relatively low due to the subcritical state of methanol at temperatures of 200 and 230 °C. In these conditions, methyl esters formed are most about 70 wt% for 1 h treatment. However, a high conversion of rapeseed oil to methyl esters with the yield of 95 wt% at 350 °C for 4 min reaction time (Kusdiana & Saka, 2001).
Pressure is also very important parameter, but, reaction pressure increases with the increase of temperature. Thus the effect of pressure on the transesteriﬁcation is always correlated with temperature. High pressure increases the solubility of triglyceride, thus, a contact at the molecular level between alcohol and triglyceride become closer at high pressure (Lee & Saka, 2010).
The effect of molar ratio between alcohol and oil sample is the other important parameter in supercritical condition as mentioned before. Higher molar ratio between methanol and triglyceride is favored for transesteriﬁcation reaction under supercritical condition. The reason can be that contact area between methanol and triglycerides are increased at the higher molar ratios of methanol. In Kusdiana’s study, the effect of the molar ratio of methanol to rapeseed oil was studied in the range between 3.5 and 42 on the yield of methyl esters formed for supercritical methanol treatments. For a molar ratio of 42 in methanol, almost complete conversion was achieved in a yield of 95% of methyl esters, whereas for the lower molar ratio of 6 or less, incomplete conversion was apparent with the lower yield of methyl esters (Kusdiana & Saka, 2001).
Advantages of supercritical process are the shorter reaction time, easier purification of products and more efficient reaction.Although higher temperature, pressure and molar ratio between methanol and triglyceride are favored for transesteriﬁcation reaction under supercritical condition, energy consumption, and excess amount alcohol usage are the disadvantages for the biodiesel production in supercritical conditions (Lee & Saka, 2010).
For biodiesel production, generally supercritical methanol and supercritical ethanol is used. However, supercritical carbon dioxide can be also used for this purpose since it is cheap, non-flammable and non-toxic (Varma & Madras, 2007). In recent years, two-step transesterification processes such as both subcritical and supercritical, both enzyme and supercritical fluid conditions etc. were also developed (Saka & Isayama, 2009).
Kusdiana and Saka developed a two-step biodiesel production method “Saka–Dadan process (Kusdiana & Saka, 2004). Besides the same advantages as one-step supercritical methanol process, the two-step method is found to use milder reaction condition and shorter reaction time, which may further allow the use of common stainless steel for the reactor manufacturing and lower the energy consumption (Lee & Saka, 2010). Minami & Saka (2006), Saka et al. (2010) and Cao et al. (2005) used two-step supercritical method in their studies. Therefore, two- step method has advantages that are milder reaction conditions, high reaction rate, applicable to various feedstocks, easier separation, no catalyst needed there is no high equipment cost and high alcohol oil ratio.
|Rapeseed oil||Supercritical methanol||42:1||350 °C,14 MPa||240 s||Batch-type vessel||35 (methyl ester yield)||Kusdiana & Saka, 2001|
|Wet algae||Supercritical methanol||9:1||255 °C, 1200 psi||25 min||Micro-reactor||90 (FAME yield)||Patil et al., 2010b|
|Rice bran oil|
rice bran oil
|Supercritical methanol||27:1||300 °C, 30 MPa||5 min||Stainless steel reactor||51.28|
94.84 (FAME yield)
|Kasim et al., 2009|
|Chicken fat||Supercritical methanol||6:1||400 °C, 41.1 MPa||6 min||Batch reactor||88 (FAME yield)||Marulanda et al., 2010|
|Jatropha oil||Supercritical methanol + propane||43:1||593 K, 8.4 MPa||4 min||Bench–scale reactor||100 (FAME yield)||Hawash et al., 2009|
|Soybean oil||Supercritical methanol||24:1||280 °C, 12.8 MPa||10 min||Batch-type vessel||98 (methyl ester yield)||Cao et al., 2005|
|Refined palm oil||Supercritical ethanol||33:1||349 °C, P"/>6.38 MPa||30 min||batch-type|
|79.2 (biodiesel yield)||Gui et al., 2009|
|Rapeseed oil||Supercritical methanol||42:1||350 °C, 19 MPa||4 min||Batch-type vessel||95 (methyl ester yield)||Kusdiana & Saka, 2001|
|Rapeseed oil||Supercritical methanol||42:1||350 °C, 30 MPa||240 s||Batch-type vessel||95 (conversion)||Saka & Kusdiana, 2001|
|Rapeseed oil||Supercritical methanol||42:1||350 °C, 35 MPa||240 s||Batch-type vessel||98.5 (conversion)||Saka & Kusdiana, 2002|
|Rapeseed oil||Subcritical acetic acid|
|300 °C, 20 MPa|
270 °C, 17 MPa
97 (FAME yield)
|Saka et al., 2010|
|Waste cooking oil||Supercritical methanol||10:1-50:1||300 °C, 1450 psi||10-30 min||Micro-reactor||80 (biodiesel yield)||Patil et al., 2010a|
|Waste cooking oil||Supercritical methanol||41:1||560 K||1800 s||Cylindrical autoclave||100 (biodiesel yield)||Demirbaş, 2009|
|Sunflower oil||Supercritical methanol + calcium oxide (%3 wt)||41:1||525 K, 24 Mpa||6 min||Cylindrical autoclave||100 (methyl ester yield)||Demirbaş, 2007|
|Cottonseed oil||Supercritical methanol|
70 (methyl ester yield)
|Linseed oil||Supercritical methanol|
65 (methyl ester yield)
|Hazelnut kernel oil||Supercritical methanol||41:1||350 °C||300 s||Cylindrical autoclave||95 (conversion)||Demirbaş, 2002|
|Jatropha oil||Supercritical methanol||40:1||350 °C, 200 bar||40 min||Small scale batch reactor||"/>90 (conversion)||Rathore & Madras, 2007|
|Soybean oil||Supercritical methanol||40:1||310 °C, 35 MPa||25 min||Tube reactor||96 (methyl ester yield)||He et al., 2007|
|Coconut oil and palm kernel oil||Supercritical methanol||42:1||350 °C, 19 MPa||400 s||Tubular reactor||95-96 (conversion)||Bunyakiat et al, 2006|
|Jatropha oil||Supercritical methanol||5:1||563 K, 11 MPa||15 min||Tubular reactor||100 (conversion)||Chen et al., 2010|
|R. sativus L. oil||Supercritical ethanol|
|590.5 K, 12.5 MPa|
590 K, 14.1 MPa
99.8 (ester yield)
|Valle et al., 2010|
|Purified palm oil||Supercritical methanol|
|372 °C, 29.7 MPa|
349 °C, 26.2 MPa
|Batch-type tube reactor||81.5|
79.2 (biodiesel yield)
|Tan et al., 2010c|
|Palm oil||Supercritical methanol||30:1||360 °C, 22 MPa||20 min||Batch-type tube reactor||72 (biodiesel yield)||Tan et al., 2009|
|Refined, bleached and deodorized palm oil||Supercritical methanol||45:1||350 °C, 40 MPa||5 min||Batch-type reactor||90 (FAME yield)||Song et al., 2008|
|Rapeseed oil||Subcritical water+Two-step supercritical methanol|
|270 °C, 20 MPa|
320 °C, 20 MPa
380 °C, 20 MPa
|Tubular reactor||90 (methyl ester yield)|
80 (methyl ester yield)
|Minami & Saka, 2006|
|Refined soybean oil||Supercritical methanol|
Supercritical methanol+hexane (co-solvent)
Supercritical methanol+CO2 (co-solvent)
Supercritical methanol+ KOH
|42:1||350 °C, 20 MPa|
160 °C, 10 MPa
98 (methyl ester yield)
|Yin et al., 2008|
|Waste palm cooking oil|
Refined palm oil
|Supercritical methanol||40:1||300 °C||20 min||Batch-type tube reactor||79|
80 (biodiesel yield)
|Tan et al., 2010a|
|Free fatty acids||Supercritical methanol||1.6:1||270 °C, 10 MPa||30 min||Batch reactor||97 (FAME yield)||Alenezi et al., 2010|
|Rapeseed oil||Supercritical methanol|
+metal oxide catalysts (ZnO)
% 1 (wt) ZnO
|250 °C, 105 bar||10 min||Batch- type reactor system||95.2 (FAME yield)||Yoo et al., 2010|
|Soybean oil||Supercritical methanol||40:1||375 °C, 15 MPa||1000 s||Vertical tubular reactor||92 (methyl ester yield)||Cheng et al., 2010|
Both enzyme and supercritical fluid conditions were used in recent years (Table 3). No soap formation, no pollution, easier puriﬁcation, catalyst reusable, no waste water are advantages for this mixed method. Enzymes represent an environmentally friendly alternative to chemical catalysts. Biodiesel production can further conform to environmental concerns if volatile, toxic, and ﬂammable organic solvents are avoided and replaced enzyme with supercritical carbon dioxide (Wen et al., 2009). In recent years, it has been discovered that especially lipases can be used as catalyst for transesteriﬁcation and esteriﬁcation reactions. Enzyme catalyzed transesterification, using lipase as catalyst does not produce side products and involves less energy consumption (Fjerbaek et al., 2009). However, enzyme applications have also disadvantages that they are expensive and have stricted reaction conditions and some initial activity can be lost due to volume of the oil molecule (Marchetti et al., 2007).
+Novozym 435 Candida antarctica
|350 °C, 200 bar|
350 °C, 200 bar
350 °C, 200 bar
350 °C, 200 bar
|Varma et al., 2010|
|Sunflower oil||Supercritical methanol|
+ Novozyme 435 enzyme in supercritical CO2
+Novozyme 435 enzyme in supercritical CO2
|400 °C, 200 bar|
400 °C, 200 bar
|Giridhar et al., 2004|
+ Candida antartica lipase enzyme in supercritical CO2
|45 °C, 130 bar||6 h||Batch|
|Lee et al., 2009|
|Supercritical methyl acetate||42:1||350 °C, 20 MPa||45 min||Batch-type vessel||91||Saka & Isayama, 2009|
Waste soybean oil
Jatropha curcas oil
|Supercritical methyl acetate||42:1|
|345 °C, 20 MPa|
345 °C, 20 MPa
345 °C, 20 MPa
345 °C, 20 MPa
|Campanelli et al., 2010|
|Purified palm oil||Supercritical methyl acetate||30:1||399 °C||59 min||Batch-type tube reactor||97.6 (biodiesel yield)||Tan et al., 2010b|
|Jatropha curcas oil||Sub-critical water+|
Sub-critical dimethyl carbonate
|270 °C, 27 MPa|
300 °C, 9 MPa
|Batch-type vessel||"/> 97 (methyl ester yield)||Ilham & Saka, 2010|
In supercritical processes, as solvent not only methanol but also methyl acetate and dimethyl carbonate are now good candidates. However, further researches are needed for their practical applications. Saka & Isayama (2009), Tan et al. (2010b) and Campanelli et al. (2010) studied with supercritical methyl acetate for biodiesel production (Table 4). High products recovery and no glycerol produced are advantages, however, lower reactivity than methanol is the main disadvantage for these applications of supercritical biodiesel production processes (Lee&Saka2010).
3. Microwave assisted process
Generally, heating coils are used to heat the raw material in biodiesel production process. This treatment can be also done by microwave method. An alternative heating system “microwave irradiation” has been used in transesterification reactions in recent years. Microwaves are electromagnetic radiations which represent a nonionizing radiation that influences molecular motions such as ion migration or dipole rotations, but not altering the molecular structure (Fini & Breccia, 1999; Varma, 2001; Refaat et al., 2008). The frequencies of microwave range from 300 MHz to 30 GHz, generally frequency of 2.45 GHz is preferred in laboratory applications (Taylor et al., 2005). Microwave irradiation activates the smallest degree of variance of polar molecules and ions with the continuously changing magnetic field (Azcan& Danisman, 2007). The changing electrical field, which interacts with the molecular dipoles and charged ion, causes these molecules or ions to have a rapid rotation and heat is generated due to molecular friction (Azcan& Danisman, 2007; Saifuddin & Chua, 2004). The absorption of microwaves causes a very rapid increase of the temperature of reagents, solvents and products (Fini & Breccia, 1999).
Microwave process can be explained for the biodiesel production with transesterification reaction: the oil, methanol, and base catalyst contain both polar and ionic components. Microwaves activate the smallest degree of variance of polar molecules and ions, leading to molecular friction, and therefore the initiation of chemical reactions is possible (Nuechter et al., 2000). Because the energy interacts with the sample on a molecular level, very efficient and rapid heating can be obtained in microwave heating. Since the energy is interacting with the molecules at a very fast rate, the molecules do not have time to relax and the heat generated can be for short times and much greater than the overall recorded temperature of the bulk reaction mixture. There is instantaneous localized superheating in microwave heating and the bulk temperature may not be an accurate measure of the temperature at which the actual reaction is taking place (Barnard et al., 2007; Refaat et al., 2008).
When the reaction is carried out under microwaves, transesterification is efficiently accelerated in a short reaction time. As a result, a drastic reduction in the quantity of by-products and a short separation time are obtained (Saifuddin & Chua, 2004; Hernando et al., 2007) and high yields of highly pure products are reached within a short time (Nuechter et al., 2000). So, the cost of production also decreases and less by-products occurs by this method (Öner & Altun, 2009). Therefore, microwave heating compares very favorably over conventional methods, where heating can be relatively slow and inefficient because transferring energy into a sample depends upon convection currents and the thermal conductivity of the reaction mixture (Koopmans et al., 2006; Refaat et al., 2008). Microwave assisted transesterification process schematic diagram was presented in Figure 4.
There can be also a few drawbacks of microwave assisted biodiesel production, beside the great advantages. Microwave synthesis may not be easily scalable from laboratory small-scale synthesis to industrial production. The most significant limitation of the scale up of this technology is the penetration depth of microwave radiation into the absorbing materials, which is only a few centimeters, depending on their dielectric properties. The safety aspect is another drawback of microwave reactors in industry (Yoni & Aharon, 2008; Vyas et al., 2010).
This survey of microwave assisted transformations is abstracted from the literature published from 2000 to 2011. And studies on microwave assisted method of transesterification reaction in the literature were summarized in Table 5. The biodiesel production have been studied by using microwave assisted method from different oils such as cottonseed oil (Azcan& Danisman, 2007), safflower seed oil ( Düz et al., 2011), rapeseed oil (Hernando et al., 2007; Geuens et al., 2008), soybean oil (Hernando et al., 2007; Hsiao et al., 2011; Terigar et al., 2010), corn oil (Majewski et al., 2009), macauba oil (Nogueira et al., 2010), waste frying palm oil (Lertsathapornsuk et al., 2008), micro algae oil (Patil et al., 2011), karanja oil (Venkatesh et al., 2011), jatropha oil (Shakinaz et al., 2010), yellow horn oil (Zhang et al., 2010), canola oil (Jin et al., 2011), camelina sativa oil (Patil et al., 2009), castor oil (Yuan et al., 2009), waste vegetable oils (Refaat et al., 2008), maize oil (Öztürk et al., 2010) and sunflower oil (Han et al., 2008; Kong et al., 2009).
oil molar ratio
|Cotton seed oil||KOH||1.5||Methanol||6:1||21% of 1200 W||7 min||333 K||92.4 (yield)||Azcan& Danisman, 2007|
|Safflower seed oil||NaOH||1||Methanol||10:1||300 W||6 min||333 K||98.4 (conversion)||Düz et al., 2011|
|Rapeseed oil Soybean oil||NaOH||%1.3||Methanol||18:1 1.27 ml||300 W||1 min||60 °C||97|
|Hernando et al., 2007|
DPAMs (Mesylate) DPABs(Benzenesulfonate)
|Methanol||5 g methenol / 2 g oil||-||20 min||150°C||100|
97 (methyl ester yield)
|Majewski et al., 2009|
|Waste frying oil||NaOH||1||Methanol||6:1||600 W||5 min||64°C||93.36 (methyl ester content)||Yücel et al., 2010|
|Macauba oil||Novozyme 435 Lipozyme IM||2.5|
|Ethanol Ethanol||9:1 9:1||-||15 min|
|Nogueira et al., 2010|
|Waste frying palm oil||NaOH||3||Ethanol||12:1||800 W||30 s||-||97 (conversion)||Lertsathapornsuk et al., 2008|
|Rapeseed oil||KOH NaOH||1|
|Methanol Methanol||6:1 6:1||67 % of 1200 W||5min|
|Azcan & Danisman, 2008|
|Soybean oil||nano CaO (heterogeneous catalyst)||3||Methanol||7:1||-||60 min||338 K||96.6 (conversion)||Hsiao et al., 2011|
|Soybean oil Oleic acid||sulfated zirconia||5||Methanol||20:1||-||20 min||60 °C||90 (conversion)||Kim et al., 2011|
|Dry micro algae||KOH||2||Methanol||9:1||800 W||6 min||-||80.13 (conversion)||Patil et al., 2011|
|Crude karanja oil||KOH||1.33||Methanol||%33.4 (w/w)||180 W||150 s||-||89.9 (conversion)||Venkatesh et al., 2011|
|Jatropha oil||KOH||1.50||Methanol||7.5:1||-||2 min||65°C||97.4 (conversion)||Shakinaz et al., 2010|
|Crude palm oil||KOH||1.50||Ethanol||8.5:1||70 W||5 min||70°C||85 (yield)|
|Suppalakpanya et al., 2010|
|Yellow horn oil||Heteropolyacid (HPA)||1||Methanol||12:1||500 W||10 min||60°C||96.22 (FAMEs)||Zhang et al., 2010|
|Soybean oil||NaOH||1||Methanol||6:1||900 W||1 min||303 K||97.7 (conversion)||Hsiao et al., 2011|
|Canola oil||ZnO/La2O2CO3 (heterogeneous catalyst)||< 1||Methanol||1:1 (w/w)||< 5 min||<100°C||"/> 95 (yield)||Jin et al., 2011|
|Camelina sativa oil||Heterogeneous metal oxide catalysts (BaO, SiO)||1.5|
80 (FAME yield)
|Patil et al., 2009|
|Castor bean oil||Al2O3 / 50% KOH SiO2 / 50% H2SO4 SiO2 / 30% H2SO4||1|
|Methanol Methanol Ethanol||1:6 1:6 1:6||40 W|
40 W 220 W
30 min 25 min
|Perin et al., 2008|
|Castor oil||H2SO4 / C||5||Methanol||1:12||200 W||60 min||338 K||94 (yield)||Yuan et al., 2009|
|Triolein||KOH NaOH||5||Methanol||1:6||25 W||1 min||323 K||98 (conversion)||Leadbeater & Stencel, 2006|
|Frying oil||NaOH||0.5||Ethanol||1:6||50% of 750 W||4 min||60°C||87 (conversion)||Saifuddin & Chua, 2004|
|Rapeseed oil||-||-||Supercritical 1-butanol||2.5:1||-||4 hour|
|310°C||91 (fatty acid buthyl ester|
|Geuens et al., 2008|
|Domestic waste vegetable oil|
Restaurant waste vegetable oil
Neat vegetable virgin sunflower oil
|KOH||1||Methanol||6:1||500 W||1 h||65°C||95.79|
96.15 (biodiesel yield)
|Refaat et al., 2008|
|Safflower seed oil||NaOH||1||Methanol||10:1||300 W||16 min||60°C||98.4 (methyl ester content)||Düz et al., 2011|
|Soybean oil||NaOH||1||Methanol||6:1||600 W (Ultrasonic)|
900 W (Microwave)
|333 K||97.7 (conversion)||Hsiao et al., 2010|
|Maize oil||NaOH||1.5||Methanol||10:1||-||-||-||98 (conversion)||Öztürk et al., 2010|
Rice bran oil
|NaOH||0.6||Ethanol||5:1||-||10 min 73°C||73°C||99.25|
99.34 (FAME yield)
|Terigar et al., 2010|
|Jatropha curcas||NaOH||4||Methanol||30:1||-||7 min||328 K||86.3 (conversion)||Yaakob et al., 2008|
|Sunflower oil||H2SO4||0.05||Methanol||10:1||400W||45 min-||96.2 (conversion)||Han et al., 2008|
|Sunflower oil||TiO2/SO4||0.02||Methanol||12:1||300W||-25 min||94.3 (biodiesel yield)||Kong et al., 2009|
4. Ultrasound assisted process
Ultrasonic waves are energy application of sound waves which is vibrated more than 20,000 per second. In another words, it can be defined as the sound waves beyond human hearing limit. Human hear can not hear sound waves with more high-pitched sound waves of an average of 10-12 kHz. Ultrasonic or ultrasound signals are in the order of 20 kHz- 100 kHz and above the limit of human hearing. Ultrasonic waves were used as the first for medical research and detectors in the 1930s and 1940s (Newman& Rozycki, 1998). Idea of the use of ultrasound, especially in the industry since the 1980s began to develop rapidly, and today a wide range of applications using ultrasonic waves appeared. At present, ultrasonic waves are used in areas such as
Ultrasonic irradiation has three effects according to the investigators. First one is rapid movement of fluids caused by a variation of sonic pressure. It causes solvent compression and rarefaction cycles (Mason, 1999). The second and the most important one is cavitation. If a large negative pressure gradient is applied to the liquid, the liquid will break down and cavities (cavitation bubbles) will be created. At high ultrasonic intensities, a small cavity may grow rapidly through inertial effects. So, bubbles grow and collapse violently. The formation and collapse of micro bubbles are responsible for most of the significant chemical effects (Kumar et al., 2010a). Cavitation is considered as a major factor which influences on reaction speed. Cavity collapse increases mass transfer by disrupting the interfacial boundary layers known as the liquid jet effect. The last effect of ultrasound is acoustic streaming mixing.
Ultrasound has been used to accelerate the rates of numerous chemical reactions, and the rate enhancements, mediated by cavitations, are believed to be originated from the build-up of high local pressures (up to 1000 atm) and temperatures (up to 5000 K), as well as increased catalytic surface areas and improve mass transfer (Yu et al., 2010). Low frequency ultrasonic irradiation is widely used for biodiesel production in recent years. In transesterification reaction, mixing is important factor for increasing biodiesel yield. Oil and methanol are not miscible completely in biodiesel processing. Ultrasonic mixing is an effective mixing method to achieve a better mixing and enchancing liquid–liquid mass transfer (Ji et al., 2006). Vigorous mixing increases the contact area between oil and alcohol phases with producing smaller droplets than conventional stirring (Mikkola & Salmi, 2001; Stavarache et al., 2006). Cavitation effects increase mass and heat transfer in the medium and hence increase the reaction rate and yields (Adewuyi, 2001). Ultrasonic cavitation also provides the necessary activation energy for initiating transesterification reaction.
Ultrasonic waves are produced with the power converter (transducer) which is piezoelectric material. Sound waves are converted to ultrasonic waves vibrating at high frequency with quartz crystal oscillator. If ultrasound waves are used in chemical reactions and processes it is called as sonochemistry. Industrial sonochemial reactors were designed more than 40 years ago by Sarocco and Arzono (Cintas et al., 2010). They showed that reactor geometry affected enormously the reaction kinetics. Later many rectors have been developed by researchers for different chemical reactions. For conventional biodiesel production, batch and continuous reactors have been developed in industry. Ultrasonic cleaning bath, ultrasonic probe which are usually operated at a fixed frequency are mainly used as ultrasonic apparatus. Frequency is dependent on particular type of transducer which is 20 kHz for probes and 40 kHz for bath. Figure 5 shows schematic diagram of biodiesel production via ultrasound assisted method.
Ultrasonic processing of biodiesel involves the following steps: 1. Mixing vegetable oil is with the alcohol (methanol or ethanol) and catalyst, 2. Heating the mixture, 3. The heated mixture is being sonicated inline, 4. Glycerin separation by using centrifuge. Alternative reactors have also been developed to lower energy consumption. Cintas et al., (2010) designed a flow reactor constituted by three transducers and showed that considerable energy saving could be achieved by large-scale multiple transducer sonochemical reactors operating in a continuous mode.
The factors affecting ultrasound assisted biodiesel production are: -Effect of catalyst type on ultrasound assisted biodiesel production, -Effect of alcohol type on ultrasound assisted biodiesel production, -Effect of ultrasonic power on biodiesel processing, -Frequency effect on ultrasonic assisted biodiesel production.
Effect of catalyst type on ultrasound assisted biodiesel production: In ultrasonic assisted biodiesel studies homogen (alkaline, acid), heterogen and enzyme catalyst were studied with many edible and nonedible oils under ultrasonic irradiation. Transesterification reactions have been studied with KOH catalyst for corn oil (Stavarache et al., 2007a; Lee et al., 2011), grape (Stavarache et al., 2007a), canola (Stavarache et al., 2007a; Thanh et al., 2010a; Lee et al., 2011), palm (Stavarache et al., 2007a), tung (Hanh et al., 2011), beef tallow (Teixeira et al.,2009), coconut (Kumar et al., 2010), soybean (Ji et al., 2006; Mahamuni & Adewuyi, 2009;Thanh et al., 2010a; Lee et al., 2011), triolein (Hanh et al., 2008; Hanh et al., 2009b), fish oil (Armenta et al.,2007),neat vegetable oil (Stavarache et al., 2005), waste cooking oil (Thanh et al., 2010b; Hingu et al.,2010).These studies were presented in Table 6 (one step transesterification), and Table 7 (two-step esterification). Generally KOH was preferred for transesterification reactions instead of NaOH. Soybean (Ji et al., 2006), neat vegetable oil (Stavarache et al., 2005), jatropha curcas L. (Deng et al., 2010) (in the second transesterification step) and triolein (Hanh et al., 2009b) were transesterified with NaOH. KOH and NaOH were used for ultrasound assisted transesterification of neat vegetable oil. They used 0.5%, 1% and 1.5 % alkali catalyst amount, 6:1 molar ratio methanol to oil and room temperature. The researchers reported that there were no great differences in the time to complete conversion between two types of catalyst (Stavarache et al., 2005).. 98% and 96% yields were achieved with 0.5 % NaOH and KOH catalyst, respectively. They also reported that when KOH was used, high yields were obtained even for 1.5% catalyst concentration. Potassium soap is softer, more soluble in water and does not make as much foam as sodium soap. The washing of esters when using potassium hydroxide is easier and the yields of isolated product are higher. In alkali catalyzed ultrasonic transesterification for biodiesel production (Tables 6 and 7), 0.3-1.5 % alkali catalyzed amounts were used. Apart from that, Cintas et al., (2010) developed a new ultrasonic flow reactor to scale up biodiesel from soybean oil in presence of (Na or K methoxide). Na and K methoxide, are alkaline metal alkoxides (as CH3ONa for the methanolysis) are the most active catalysts because of stronger hydroxide group. In their reacton mixture of oil (1.6 L), methanol and sodium methoxide 30% in methanol (wt/wt ratio 80:19.5:0.5, respectively) was fully transesterified at about 45°C in 1 h (21.5 kHz, 600 W, flow rate 55 mL/min).
Heterogen catalysts were tried by researchers in a few studies (Ye et al., 2007; Salamatinia, 2010; Mootabadi et al., 2010;Kumar et al., 2010b). As it is known, ultrasound increase mixing of oil and alcohol with catalyst phases, as well as increase catalytic surface area. Catalyst can be broken into smaller particles by ultrasonic irradiation to create new sites of the subsequent reaction. Thus, solid catalyst is expected to last longer in the ultrasonic-assisted process (Mootabadi et al., 2010). Single component alkaline earth metal oxides (BaO, SrO, CaO) having lower solubility in alcohol catalyzed palm transesterification processes with methanol (Mootabadi et al., 2010). The catalytic activities of the three catalysts were correlated well with their basic strengths and found as the sequence of CaO < SrO < BaO. BaO catalyst achieved 95.2% of biodiesel yield within 60 min in the ultrasonic-assisted process while SrO catalyst generally demonstrated slightly lower result. CaO showed the lowest yield with 77.3%yield under optimum conditions. Although high activity of BaO as catalyst, this activity dropped severely in the BaO reusability test, especially under ultrasonic condition (compared to mechanical stirring). In another study, aluminum isopropoxide or titanium isopropoxide as heterogeneous transesterification catalysis are employed to produce nanoemulsions with large interfacial area for easy catalyst separation and enhanced reaction rate (Ye et al., 2007). These catalysts are produced by partial polymerization and metal alkoxides are connected by metal-oxygen bonds. Alkoxide parts in the polymer matrix catalyst gives the catalyst amphiphilic properties that help form and stabilize alcohol/ triglycerides nanoemulsion (Ye et al., 2007). The study showed that titanium isopropoxide also showed good catalytic activity and considerable amphiphilic properties in forming nanoemulsions. With aluminum isopropoxide or titanium isopropoxide, transparent alcohol/oil emulsions can be formed in less than four minutes and can significantly enhance the transesterification reaction rate. The micelle size was observed to be as low as 5.1 nm.
High acidity oils (Jatropha curcas L, waste frying oil) can be transesterified by two-step processes. In the first step, free fatty acids are converted to esters by direct esterification with acid catalyst. Eq. 1 shows esterification of fatty acids. In the second step, basic catalyst was used to esterify triglycerides as it was shown in Figure 2.
In production of biodiesel from Jatropha curcas L. oil (non edible oil) Deng et al., (2011) used a two-step process. The first step pretreatment (acid-esterification) of Jatropha oil was performed at 318 K an ultrasonic reactor for 1.5 h in their first study (Deng, et al., 2010). After reaction, the acid value of Jatropha oil was reduced to 0.7 mg KOH/g and 93.3% esterification rate was achieved. The second step, a base-catalyzed transesterification was performed with nano sized Mg/Al oxides under different conditions. At the optimized condition, (Table 6) 95.2% biodiesel yield was achieved, and the Jatropha oil biodiesel properties were found to be close to those of the German standard. It was reported that the catalyst could be reused for 8 times.
Although it is known that ultrasonic mixing has a significant effect on enzymatic transesterification there are a little study about using of lipases as enzyme catalyst. It has been reported that enzyme activity of Novozym 435 enhanced by ultrasound irradiation (Sinisterra, 1992; Lin & Liu, 1995). Novozym 435 (Candida antarctica lipase B immobilized on polyacrylic resin) was used in biodiesel production from soybean oil and methanol with a low frequency ultrasonic (40 kHz) waves to see enzyme activity and compare their overall effects under two different conditions—ultrasonic irradiation and vibration (Yu et al., 2010). They investigated effects of reaction conditions, such as ultrasonic power, water content, organic solvents, ratio of solvent/oil, and ratio of methanol/oil, enzyme dosage and temperature on the activity of Novozym 435. Novozym 435 activity significantly increased by ultrasonic irradiation compared with vibration and reaction rate was further increased under the condition of ultrasonic irradiation with vibration (UIV). Yu et al (2010) indicated that 96% yield of fatty acid methyl ester (FAME) could be achieved in 4 h under the optimum conditions: 50% of ultrasonic power, 50 rpm vibration, water content of 0.5%, tert-amyl alcohol/oil volume ratio of 1:1, methanol/oil molar ratio of 6:1, 6% Novozym 435 and 40 °C. Since the lipase enzyme is expensive catalyst it is important to reuse the catalyst in biodiesel industrial productions. The researchers also pointed out that Novozym 435 was not deactivated under UIV, only 4 % enzyme activity slightly decreased after five cycles.
Effect of alcohol type on ultrasound assisted biodiesel production: Methanol was mostly used in transesterification reaction under ultrasonic irradiation with oils shown in Tables 6 and 7. High conversion and yields were obtained with methanol and ethanol using. Stavarache et al., (2007a) used methanol in transesterification of commercial edible oil, corn, grapeseed, canola and palm oil. Excellent yields (99%) were obtained for all type oils in 20 minutes with 6:1 methanol to oil molar ratio at 36 °C. As it is shown in Figure 6, triglycerides are converted to di and monoglycerides to produce biodiesel to produce biodiesel and glycerin. They also examined the transesterification reaction mechanism under low frequency (40 kHz) ultrasonically driven esterification.
They have reported that the major part of the transesterification took place in the first 3-10 minutes of reaction if not faster and the rate- determining reaction switches from diglyceride (DG) → monoglyceride (MG) (classical mechanic agitation) to MG + ROH→Gly + ME (ultrasonically driven transesterification). In another study, the conversion of FAME greater than 99.4 % was achieved after about 15 minutes at 40 °C with ultrasonic agitation for 6:1 methanol: oil molar ratio (Calucci et al., 2005). They have also concluded that hydrolysis rate constants of DG and TG are three to five times higher than those of mechanical agitation. Ji et al., (2006) used ultrasonic transesterification process for soybean oil transesterification with methanol and reported 99% yield at 10 min reaction time with 6:1 methanol to oil molar ratio at 45°C. Oleic acid, triolein, coconut were esterified with ethanol and 90% conversion, about 99% yield and >92% yields were achieved respectively (Hanh et al., 2009a; Hanh et al., 2009b; Kumar et al., 2010a). Table 8 shows the some biodiesel yield and conversion with various monoalcohols and comparing of the alcohols.
Stravarache et al., (2005) studied effects of alcohol type on transesterification of neat vegetable oil under ultrasonic and mechanical stirring. The results of transesterfication with primary, secondary and tertiary alcohols after 60 min of reaction were presented in Table 8.
|Oleic acid||5||Ethanol||3:1||60||2 hour||Ultrasonic|
40 kHz, 1200 W
|~90 (conversion)||Hanh et al., 2009a|
|Commercial edible oil|
Grape seed Canola
|KOH||0.5||Methanol||6:1||36 ± 2||20 min||Ultrasonic|
40 kHz,1200 W
|~ 99 (conversion)||Stavarache et al., 2007a|
|Refined soybean oil||KOH||1.5||Methanol||6:1||40||15 min||20 kHz, 14.49 W||"/>99.4 (conversion)||Colucci et al., 2005|
|Soybean||NaOH||1||Methanol||6:1||45||10 min||197 kHz, 150W||99 (yield)||Ji et al., 2006|
|Soybean||KOH||0.5||Methanol||6:1||26 - 45||30 min<||611 kHz, 139 W||"/>90 (conversion)||Mahamuni & Adewuyi, 2009|
|Soybean||Na or K methoxide||0.15||Methanol||6:1||45||1 h||21.5 kHz, 600 W||Fully transesterified||Cintas et al., 2010|
|KOH||1||Methanol||6:1||55||30 min||450 W||95 (yield)|
|Lee et al., 2011|
Blended oil (20%Tung, 30%canola, 50%palm )
|KOH||1||Methanol||6:1||20-30||30 min||25 kHz, 270 W||91.15 (yield)|
|Hanh et al., 2011|
|Beef Tallow||KOH||0.5||Methanol||6:1||60||70 s||40 kHz, 1200 W||"/>92 (conversion)||Teixeira et al., 2009|
40 kHz, 1200 W
|~99 (yield)||Hanh et al., 2008|
40 kHz, 1200 W
|"/>95 (conversion)||Hanh et al., 2009b|
|Neat vegetable oil||NaOH KOH||0.5||Methanol||6:1||25||20 min||Ultrasonic|
40 kHz 1200 W
|Stavarache et al., 2005|
|Coconut||KOH||0.75||Ethanol||6:1||-||7 min||24kHz, 200 W||"/>92 (yield)||Kumar et al., 2010a|
|Waste cooking oil||KOH||1||Methanol||6:1||45||40 min||20 kHz, 200 W||89 (conversion)||Hingu et al., 2010|
|Palm||KOH||-||Methanol||6:1||38–40||20 min||45 kHz,600 W||95 (yield)||Stavarache et al., 2007b|
|3||Methanol||9:1||65||60 min||30 kHz||77.3 (yield)|
|Mootabadi et al., 2010|
|2.8||Methanol||9:1||65||50 min <||20 kHz, 200 W||"/>92 (yield)||Salamatinia et al., 2010|
|Canola||KOH||0.7||Methanol||5:1||25||50 min||20 kHz, 1000 W||"/>95 (conversion)||Thanh et al., 2010a|
|3||Methanol||6:1||60||2 h||-||64 (yield)||Ye et al., 2007|
|Soybean||Novozym 435||6||Methanol||6:1||40||4h||40 kHz, 500 W||96 (yield)||Yu et al., 2010|
|Jatropha oil||Na/SiO2||3||Methanol||9:1||50-70||15 min||24 kHz, 200W||98.5 (yield)||Kumar et al., 2010b|
|"/>95 (conversion )|
"/>98 (conversion )
|Armenta et al., 2007|
|20 kHz, 1000W|
(For each step)
|81 (yield )|
99 (yield )
|Thanh et al., 2010b|
|Jatropha curcas L.||H2SO4 (1. step)|
Mg/Al oxides (2. step)
(For each step)
(For each step)
(For each step)
(For each step)
(For each step)
|95.2 (total yield )||Deng et al., 2011|
|Jatropha curcas L.||NaOH H2SO4||1|
(For each step)
(For each step)
(For each step)
|96.4 (total yield)||Deng et al., 2010|
|Alcohol type||Neat vegetable oil a|
(Stavarache et al., 2005)
(Hanh et al., 2009b)
|Soybean oil c|
(Colucci et al., 2005)
|Methanol||80 (Yield) 98 (Yield)|
(60 min) (20 min)
|Ethanol||79 (Yield) 88 (Yield)|
(20 min) (20 min)
|n- propanol||78 (Yield) 88 (Yield)|
(10 min) (10 min)
|Iso-propanol||No conversion Some conversion||3||29.2|
|n-butanol||83 (Yield) 92 (Yield)|
("/>60 min) ("/>60 min)
|Iso- butanol||No conversion Some conversion||3||-|
|Tertiary- butanol||No conversion No conversion||-|
N- chain alcohols (methanol, ethanol, n- propanol, and n-butanol) showed the high yields between 88-98% in 10-20 min reaction time. The yields of biodiesel in ultrasound activation were higher than mechanical stirring since ultrasound produce less soap. By using ultrasound the reaction time was found much shorter than mechanical stirring. The secondary alcohols showed some conversion while transesterification reaction took place under stirring. Tertiary-butanol had no conversion with both type of procedure. Hanh et al., (2009b) produced biodiesel with triolein and various alcohols (methanol, ethanol, propanol, butanol, hexanol, octanol and decanol). The productions were performed at molar ratio 6:1 (alcohol: triolein) and 25°C in the presence of base catalysts (NaOH and KOH) under ultrasonic irradiation (40 kHz) and mechanical stirring (1800 rot/min) conditions. The rate of ester formation depended on alcohol types; as the alcohol carbon number increased, reaction rate decreased. The secondary alcohols such as 2-propanol, 2-butanol, 2-hexanol, and 2-octanol showed 3% conversion, suggesting that the steric hindrance strongly affected the transesterification of triolein. N-propanol showed approximately 93% conversion under ultrasonic irradiation, while 75% conversion was obtained under mechanical stirring. Soybean oil was transesterifed with methanol, ethanol, n-butanol, and iso-propanol over 2 h reaction period with 1.5 % KOH as the catalyst and a 6:1 molar ratio of alcohol/oil at 60°C (Colucci et al., 2005). The similar results obtained with methanol, ethanol and n-butanol compared to other studies.
Effect of ultrasonic power on biodiesel processing: The effect of ultrasonic power on the biodiesel formation has been reported (Mahamuni& Adewuyi, 2009; Hingu et al., 2010; Lee et al.,2011). Biodiesel yield increased with increasing ultrasonic power in all the studies. Nahamuni& Adewuyi (2009) studied this effect for three different frequencies and various powers (181, 90, 181 W at 1300 kHz, 104, 139,68 W at 611 kHz, 181, 117, 81, 49 W at 581 kHz). The reactions were carried out for 60-180 minutes. The reaction rate increased with increasing ultrasound power at any given frequency and biodiesel yield was obtained above 90%. At start of the reaction, reaction rate is very low because of low interfacial area available for the reaction. As time increased the reaction rate increased. This increase is due to the amount and size of the emulsion formation varies because of ultrasonic cavitation. Ultrasonic cavitation produces finer and stable emulsion and following this higher mass transfer and hence, higher biodiesel formation. When the ultrasonic power increases acoustic amplitude increases. So, cavitation bubble will collapse each other violently resulting in high velocity and micromixing at the phase boundary between two immiscible phases. Ultrasonication can result in mean droplet sizes much lower than those generated by conventional agitation, and can be a more powerful tool in breaking methanol into small droplets (Wu et al., 2007). The emulsion droplet size of methanol/soybean oil dispersions for ultrasonic and mechanical stirring was investigated and was shown that emulsion droplet size in ultrasonic mixing 2.4 times lower than that of conventional agitation. The mean droplet sizes were 148 and 146 nm with ultrasonic energy at 50 and 70 W, respectively. However, the droplet size was about 340 nm with impeller at 1000 rpm.
Higher power levels usually gives lower conversions because of cushioning effect and hence lower cavitational activity (Ji et al., 2006; Hingu et al., 2010; Lee et al., 2011). Hingu et al. (2010) observed that while the biodiesel conversion was obtained around 66% at 150 W power 89% of conversion was obtained when the power dissipation was increased to 200 W. But further increase in power from 200 W to 250 W resulted in lower FAME conversion. FAME conversion rate also depends on the emulsification degree of reaction system (Ji et al., 2006). These authors also noted that the order of affecting factors on FAME yield was substrate molar ratio > temperature > pulse frequency > ultrasonic power.
Ultrasound pulse (few seconds on followed by second off) effects the biodiesel conversion (Hingu et al., 2010; Ji et al., 2006). Higher conversion can be obtained when higher pulse is applied to system. For example, while biodiesel conversion was obtained for the pulse 2 s ON and 2 s OFF, the conversion were 65.5% for 5 s ON and 1 s OF (Hingu et al., 2010). For a pulse duration as 1 min ON and 5 s OFF, conversion of 89.5% was obtained because of better emulsification of the methanol and oil layers. The effect of horn position on biodiesel production was investigated by same researchers. They kept the reaction parameters constant such as 6:1, methanol to waste cooking oil molar ratio, 1% catalyst concentration, 45°C temperature, 200W power ad 1 min ON and 5 s OFF pulse. Cavitation intensity depends on some parameters physicochemical properties namely viscosity, surface tension and density. Cavitation is generated due to the presence of horn in oil or methanol. According to the horn position various results can be observed. Hingu et al. (2010) applied there different positions: in the oil phase, at the interface and in methanol. While maximum conversion was achieved as 89.5% when the horn was dipped in methanol rich layer, the lowest conversion was obtained as 8.5% when the horn is dipped in the oil phase. 58.5% conversion was observed when the horn is located at the interface of two phases. Maximum ester conversion was obtained since methanol contributed cavitating conditions significantly.
Frequency effect on ultrasonic assisted biodiesel production: The effect of ultrasonic frequency was studied on the yield of transesterification reaction of vegetable oils and shortchain alcohols (Stavarache et al., 2005). NaOH or KOH were used as base catalysts. It was observed that the reaction time gets shorter (the reaction fastens) as the ultrasonicirradiation increases but the yield slightly decreases. At 40 KHz, the reaction time was shorter than 28 KHz, but the yield was obtained higher when studied at 28 kHz.. This is because of the higher formation of soap at 40 KHz and higher quantity of soap makes the purification process harder. The more soap is formed, more esters gets trapped in the soap micelles and the yield of the reaction decreases at 40 KHz as a result.
General comparison of ultrasound irradiation with conventional stirring: Ultrasonic assisted transesterification of oil presents some advantages compared to conventional stirring methods such as; reducing reaction time, increase the chemical reaction speed and decrease molar ratio and methanol, increase yield and conversion. Ultrasound irradiation reduce the reaction time compared to conventional stirring operation (Stavarache et al., 2005; Ji et al., 2006; Hanh, et al., 2008; Mootabadi, et al., 2010; Hingu et al., 2010; Lee et al., 2011). Stavarache et al. (2005) studied transesterification of vegetable oil with short-chain alcohols, in the presence of NaOH, by means of low frequency ultrasound (28 and 40 kHz). By using ultrasounds the reaction time was found much shorter (10–40 min) than for mechanical stirring. The optimal conditions for triolein methanolysis was methanol/triolein molar ratio of 6/1, KOH concentration of 1 wt% and irradiation time of 30 min. But the optimal conditions for the conventional stirring method were found to be as were methanol/triolein molar ratio of 6/1, KOH concentration of 1 wt% and 4 h (Hanh et al.., 2008). In transesterification of waste cooking oil with methanol 89.5% conversion was obtained in 40 minutes whereas conventional stirring resulted in 57.5% conversion (Hingu et al., 2010). Palm oil was esterified with 95% yield in 60 minutes compared to 2–4 h with conventional magnetic stirring under optimal conditions. Ultrasonic irradiation method enabled to reduce the reaction time by 30 min or more comparing to conventional heating method in production of biodiesel from various vegetable oils. Also this method improved conversion rate (Hanh et al., 2007; Lee et al., 2011). In transesterification reaction, mixing is important factor for increasing biodiesel yield. Ultrasonic effect induces an effective emulsification and mass transfer compared to conventional stirring thus reaction rate increase (Hanh et al., 2009; Hingu et al., 2010). Comparison of yield and conversion of vegetable oilwith various alcohols was presented in Table 8 and also was explained in the effect of alcohol type on ultrasound assisted biodiesel production section.
Ultrasound assisted method has a similar effect as microwave assisted method that both of them reduce the separation time from 5 to 10 hours to less than 60 minutes compared to conventional transesterification method (Kumar et al., 2010). Also, during production of biodiesel via acid or base catalyst, ultrasound irradiation provides a fast and easy route (Yu et al., 2010) and the purity of glycerin increases.
The production of biodiesel from non-edible vegetable oil and waste cooking oil using ultrasonication allows under ambient operating conditions (Kumar et al., 2010a; Hingu et al., 2010). Also, biodiesel production works from vegetable oils given in Table 6 illustrates the applicability of ultrasonic irradiation under atmospheric and ambient conditions. The transesterification reaction with methanol is usually performed at 60°C with classical stirring. Roomtemperature is hardly competitive in terms of energy consumption. Room temperature is hardly competitive in terms of energy consumption. The production of biodiesel with ultrasound is effective and time and energy saving and economically functional method (Ji et al., 2006; Kumar et al., 2010a; Hanh et al., 2011). Power ultrasonic method required approximately a half of the energy that was consumed by the mechanical stirring method (Ji et al., 2006). Special mixing devices can be used to increase mass transfer. It was reported that sonochemical reactors consume only about one third the energy required for a specialty mixer for same conversion (Lifka & Ondruschka, 2004). All these results clearly indicate that ultrasonic method inexpensive, simple and efficient and would be promising to the conventional stirring method.
As seen from the Table 9, the length of the alcohol chain affects the yield of the reaction, as the frequency of the ultrasonic irradiation affects the reaction time. In longer alcohol chains, the yield of the reaction is higher. The longer alcohol chains increases the solubility (miscibility) of alcohol into the oil. 40 kHz of ultrasonic irradiation is preferable if faster reaction is needed but it has to be taken into account that the yield decreases as the reaction fastens because of the higher formation of soap in faster reactions. In conclusion, miscibility of oil and alcohol is better under the control of ultrasonic waves. This effect increases the surface area and higher yields of isolated methyl esters can be achieved. The mass transfer is better so that the soap formation is lower resulting as better and easier isolation of methyl esters. Power of the ultrasonic irradiation makes the reaction faster, as the yield slightly decreases under higher frequencies (40 kHz).
Due to the growing energy necessity and environmental problems the studies focused on renewable alternative energy sources. Biodiesel is one of the important renewable energy sources used in many countries in the world as an alternative diesel fuel. Biodiesel is generally produced transesterfication reaction of vegetable and animal oils with catalyst under conventional stirring with batch and continuous processes. Because of the economical causes, choosing efficient transesterification method for biodiesel production has become important in recent years. In this context, the researchers have been investigating different new processes such as supercritical, microwave assisted and ultrasound assisted process to avoid inefficient processes. It is found that these methods have several distinctions compared to conventional methods. Homogenous catalyst (sulfuric acid, sodium hydroxide, potassium hydroxide, sodium and potassium metoxide etc.), heterogeneous catalyst (ZnO, SiO, MgO, BaO, SrO etc.) and enzymatic catalyst (lipase) are also easily being used in microwave and ultrasonic assisted processes. However, supercrital transesterfication reaction of vegetable oils is a noncatalytic reaction and higher yields can be obtained with compared to conventional methods. New methods for biodiesel production offer more advantages but these methods have also some negative effects. For example, energy consumption, excess amount alcohol usage are the disadvantages of supercritical process.
Microwave synthesis is still in lab-scale synthesis and it is not viable in large scale for industrial production due to penetration depth of microwave radiation into the absorbing materials. The safety aspect is another drawback of microwave reactors for industry. Ultrasonic biodiesel production could be advantageous for small producers, but in large scale processing maybe challenging because of necessity of many ultrasound probes. Although there are some disadvantages of novel methods in biodiesel production, these methods give several important advantages for the transesterification of oils such as: reducing reaction time and reaction temperature, unwanted by-products; and increasing ester yields, conversion easier compared to conventional method. In conclusion, these methods with their important advantages can be more preferred than conventional method anymore.