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 Atomization: Water sprays for dust suppression and humidifiers, low velocity spray coating, spray drying nozzles. Cleaning and cleaning of engineering items, small electronic items and jeweler using aqueous based solvents. Cleaning and disinfection of medical instruments and food processing equipment. Processing: Dispersion of pigments and powders in liquid media and emulsification. Extraction: Essential oil, flavonoid, resin, Crystallization and Filtration (Cintas et al., 2010; Mason et al., 1996; Mason, 2000).

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 a!., 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.

RCOOH +CH3OH RCOOCH3 + H2O (1)

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.

Raw material	Catalyst	Catalyst amount (wt %)	Alcohol type	Alcohol /oil molar ratio	Reaction temp. (°C)	Reaction time	Reactor conditions	Performance (%)	Ref.
Oleic acid	H2SO4	5	Ethanol	3:1	60	2 hour	Ultrasonic cleaner 40 kHz, 1200 W	~90 (conversion)	Hanh et al., 2009a
Commercial edible oil Corn Grape seed Canola Palm	KOH	0.5	Methanol	6:1	36 ± 2	20 min	Ultrasonic cleaner 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 mehcxick	0.15	Methanol	6:1	45	1 h	21.5 kHz, 600 W	Fully transesterified	Cintas et al., 2010
Canola Soybean Corn	KOH	1	Methanol	6:1	55	30 min	450 W	95 (yield) 95 (yield)	Lee et al., 2011
Tung and Blended oil (20%Tung, 30%canola, 50%palm )	KOH	1	Methanol	6:1	20-30	30 min	25 kHz, 270 W	91.15 (yield) 94.03 (yield)	Hanh et al., 2011
Beef Tallow	KOH	0.5	Methanol	6:1	60	70s	40 kHz, 1200 W	>92 (conversion)	Teixeira et al., 2009
Triolein	KOH	1	Methanol	6:1	25	30 min	Ultrasonic cleaner 40 kHz, 1200 W	~99 (yield)	Hanh et al., 2008
Triolein	NaOH KOH	1	Methanol Ethanol	6:1	25	25 min	Ultrasonic cleaner 40 kHz, 1200 W	>95 (conversion)	Hanh et al., 2009b
Neat vegetable oil	NaOH KOH	0.5	Methanol	6:1	25	20 min	Ultrasonic cleaner 20 kHz 40 kHz 1200 W	98 (yield) 96 (yield	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
Palm	CaO SrO BaO	3	Methanol	9:1	65	60 min	30 kHz	77.3 (yield) 95 (yield) 95 (yield)	Mootabadi et al., 2010
Palm	BaO SrO	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
Soybean	Ti(Pr)4 Al(Pr)3	3	Methanol	6:1	60	2 h		64 (yield)	Ye et al., 2007
Soybean	Ncvczyn 435	6	Methanol	6:1	40	4h	40 kHz, 500 W	96 (yield)	Yu et al., 2010
Jatropha oil	Na/SiO	3	Methanol	9:1	50-70	15 min	24 kHz, 200W	98.5 (yield)	Kumar et al., 2010b
Fish oil	KOH CHONa	1 0.8	Ethanol	6:1 6:1	20-60 20-60	>30 >30	25-35 kHz 25-35 kHz	>95 (conversion ) >98 (conversion )	Armenta et al., 2007

Table 6. The studies for biodiesel production from various feedstocks at different conditions under ultrasound irradiation

Oil	Catalyst type	Catalyst amount (wt%)	Alcohol type	Alcohol: oil ratio	Reaction temperature (0C)	Reaction time	Ultrasound conditions	Performance (%)	Ref.
Waste cooking	KOH	0.7 0.3	Methano l	2.5:1 (mol) 1.5:1	20-25	10 min 20 min	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)	1 (For each step)	Methano l	4:1(mol) (For each step)	40 (For each step)	1.5 h (For each step)	210W (For each step)	95.2 (total yield )	Deng et al., 2011
Jatropha curcas L.	NaOH H2SO4	1 (For each step)	Methano l	0.4 (v/v) 6:1 (mol)	60 (For each step)	1h 30 min	210W (For each step)	96.4 (total yield)	Deng et al., 2010

Table 7. Biodiesel production with two step transterification under ultrasound irradiation

Alcohol type	Neat vegetable oil a (Stavarache et al., 2005)	Triolein b (Hanh et al., 2009b)	Soybean oil c (Colucci et al., 2005)
Performance (%) Stirring Ultrasonic	Conversion (%)	Conversion (%)
Methanol	80 (Yield) 98 (Yield) (60 min) (20 min)	98	99.3
Ethanol	79 (Yield) 88 (Yield) (20 min) (20 min)	~98	99.1
n — propanol	78 (Yield) 88 (Yield) (10 min) (10 min)	~93	—
Iso-propanol	No conversion Some conversion	3	29.2
n-butanol	83 (Yield) 92 (Yield) (>60 min) (>60 min)	~93	92.0
Iso — butanol	No conversion Some conversion	3	—
Tertiary — butanol	No conversion No conversion	—

a Reaction conditions for neat vegetable oil: 0.5% (wt/ wt) NaOH, 6:1 alcohol to oil molar ratio, 40 KHz, b Reaction conditions for triolein: 25 min, 25 °C, 0.1% (wt/ wt) KOH, 6:1 alcohol to triolein molar ratio,

40 KHz,

c Reaction conditions for soybean oil: 2h, 1.5% (wt/ wt) KOH, 6:1 alcohol to oil molar ratio, 40 KHz

Table 8. The influence of alcohol on the ultrasound assisted transesterification of different oils for biodiesel production

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.

Type of alcohol		28 kHz	40 kHz	Mechanical stirring
Methanol	Reaction time (min)	10	10	10
Yield (%)	75	68	35
Ethanol	Reaction time (min)	20	10	10
Yield (%)	75	30	47
n-propanol	Reaction time (min)	20	10	10
Yield (%)	75	78	79
n-butanol	Reaction time (min)	40	20	20
Yield (%)	87	90	89

Table 9. The yields and reaction times of FAMEs as a result of different frequencies of ultrasonic irradiation and mechanical stirring in the presence of NaOH catalyst (1.5% wt))(Stavarache et al., 2005)

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).

5. Conclusion

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.