**2. Fatty acid alkylesters production assisted by microwaves**

Electromagnetic radiation (EMR) is a form of energy exhibiting wave like behaviour as it travels through space. EMR has both electric and magnetic field components, which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation. Electromagnetic radiation is classified according to the frequency of its wave. In order of increasing frequency and decreasing wavelength, these are radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays (Serway and Jewett, 2004).

Microwaves belong to the portion of the electromagnetic spectrum with wavelengths from 1 mm to 1 m with corresponding frequencies between 300 MHz and 300 GHz.

Within this portion of the electromagnetic spectrum there are frequencies that are used for cellular phones, radar, and television satellite communications. For microwave heating, two frequencies, reserved by the Federal Communications Commission (FCC) for industrial, scientific, and medical (ISM) purposes are commonly used for microwave heating. The two most commonly used frequencies are 0.915 and 2.45 GHz. Recently, microwave furnaces that allow processing at variable frequencies from 0.9 to 18 GHz have been developed for material processing (Thostenson and Chou, 1999). Microwave radiation was discovered as a heating method in 1946, with the first commercial domestic microwaves being introduced in the 1950s. The first commercial microwave for laboratory utilization was recognized in 1978 (Gedye et al., 1986; Giguere et al., 1986).

Over the last decade, microwave dielectric heating as an environmentally benign process has developed into a highly valuable technique, offering an efficient alternative energy source for numerous chemical reactions and processes. It has many advantages compared to conventional oil-bath heating, such as non-contact heating, energy transfer instead of heat

fractions that crystallize and can block the filters of the engines. One of the alternatives to reduce the flow properties at low temperatures (FPLT) of methyl esters specially the obtained from oil palm is use alkyl esters, obtained through of trans-esterification with branched alcohols, that prevent the agglomeration and formation of crystals of these methyl

Alkyl esters can be produced through trans-esterification of triglycerides, which are separated by immiscibility and higher density. (Marchetti et al., 2007; Ma and Hanna, 1999;

Very few studies have been made with the aim to obtain alkyl esters and all are obtained by homogeneous catalysis (Lee et al., 1995). Yields of these reactions are very low by the high steric hindering that presenting the branched alcohols. To increase the conversion, in this

The preparation of fatty acid alkylester using alternative methods, such as: electromagnetic radiation (microwave, radio frequency) and ultrasound, offers a fast, easy route to this valuable biofuel with advantages of a short reaction time, a low reactive ratio, an ease of operation a drastic reduction in the quantity of by-products, and all with reduced energy

In this work the revision of the relevant aspects of the production optimization, intrinsic effects and parameters more relevant in the synthesis and characterization of fatty acid alkylesters (biodiesel) using as alternative methods: Microwaves, Radio Frequency and

Electromagnetic radiation (EMR) is a form of energy exhibiting wave like behaviour as it travels through space. EMR has both electric and magnetic field components, which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation. Electromagnetic radiation is classified according to the frequency of its wave. In order of increasing frequency and decreasing wavelength, these are radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays

Microwaves belong to the portion of the electromagnetic spectrum with wavelengths from 1

Within this portion of the electromagnetic spectrum there are frequencies that are used for cellular phones, radar, and television satellite communications. For microwave heating, two frequencies, reserved by the Federal Communications Commission (FCC) for industrial, scientific, and medical (ISM) purposes are commonly used for microwave heating. The two most commonly used frequencies are 0.915 and 2.45 GHz. Recently, microwave furnaces that allow processing at variable frequencies from 0.9 to 18 GHz have been developed for material processing (Thostenson and Chou, 1999). Microwave radiation was discovered as a heating method in 1946, with the first commercial domestic microwaves being introduced in the 1950s. The first commercial microwave for laboratory utilization was recognized in 1978

Over the last decade, microwave dielectric heating as an environmentally benign process has developed into a highly valuable technique, offering an efficient alternative energy source for numerous chemical reactions and processes. It has many advantages compared to conventional oil-bath heating, such as non-contact heating, energy transfer instead of heat

work, we propose use assisted reactions by alternative methods.

**2. Fatty acid alkylesters production assisted by microwaves** 

mm to 1 m with corresponding frequencies between 300 MHz and 300 GHz.

esters.

Vicente et al., 2004)

consumption.

Ultrasound is proposed.

(Serway and Jewett, 2004).

(Gedye et al., 1986; Giguere et al., 1986).

transfer, higher heating rate, rapid start-up and stopping of the heating, uniform heating with minimal thermal gradients, selective heating properties, reverse thermal effects (heating starting from the interior of the material body), energy savings and higher yields in shorter reaction time (Tierney and Lidstrom, 2005). Microwave heating is based dielectric heating, the ability of some polar liquids and solids to absorb and convert microwave energy into heat. In this context, a significant property is the mobility of the dipoles by either ionic conduction or dipolar polarization and the ability to orient them according to the direction of the electric field. The orientation of the dipoles changes with the magnitude and the direction of the electric field. Molecules that have a permanent dipole moment are able to align themselves through rotation, completely or at least partly, with the direction of the field. Therefore, energy is lost in the form of heat through molecular friction and dielectric loss (Loupy, 2002). The amount of heat produced by this process is directly related to the capability of the matrix to align itself with the frequency of the applied electric field. If the dipole does not have enough time to realign, or reorients too rapidly with the applied field, no heating occurs (Kappe, 2004).

The production of biodiesel via the conventional heating system appears to be inefficient due to the fact that the heat energy is transferred to the reactants through conduction, convection and radiation from the surface of the reactor. Hence, conventional heating requires longer reaction time and a larger amount of heat energy to obtain a satisfactory biodiesel. The replacement of conventional heating by microwave radiation for the transesterification process is expected to shorten the reaction time due to the transfer of heat directly to the reactants. The microwave radiation during the transesterification process is expected to create (i) an alignment of polar molecules such as alcohols with a continuously changing magnetic field generated by microwaves and (ii) molecular friction due to which heat will be generated (Yaakob et al., 2009).

The involvement of such heterogeneous catalytic systems under microwave conditions represents an innovative approach with processing advantages. These solid-state catalysts find scope in the context of green chemistry development as they are active in solvent free or dry media synthesis, with potential advantages in terms of separation, recovery postreaction and recycling assays. The creation of hot spots, specific under MW conditions, is typically utilized for energy saving as improved yields and selectivities are recorded after shorten reaction times at lower nominal temperatures. These hot spots may induce a reorganization of the catalyst under microwave conditions and are probably responsible for reaction rates and selectivity enhancement (compared to conventional heating at the same nominal temperature) (Richela et al., 2011)

#### **2.1 Esterification reactions assisted by MW**

The esterification reaction is a slow equilibrium, and can be catalyzed by Brønsted acids such as sulfuric acid. The main problem is the generation of highly acidic waste causing a serious environmental problem, and to reduce this problem have been used alternative heterogeneous catalysts and microwaves as a heating source to promote and increase the yielding. Algunos catalizadores empleados son: scandium triflate and bismuth triflate (Socha and Sello, 2010), sulfated zirconia (Kim et al., 2011a), niobium oxide (Melo et al., 2010), entre otros.

The temperature presented a pronounced effect on the conversion, following an exponential dependence. The results for a distinct molar ratio of alcohol/fatty acid indicated that the

Alternative Methods for

**FFA Catalyst** 

Oleic Sulfated

Oleic Amberlyst 15 dry

Oleic Niobium Oxide Sulfated Zirconia

> Sc(OTf)3 Bi(OTf)3 Sc(OTf)3 Bi(OTf)3 Sc(OTf)3 Bi(OTf)3 Sc(OTf)3 Bi(OTf)3

H2SO4 2.5%wt Oil MeOH

Dowex 50X2 10%wt Oil MeOH

Amberlyst15 10%wt Oil MeOH

Table 1. Microwave assisted esterification.

Linoleic Linoleic Oleic Oleic Myristic Myristic Palmitic Palmitic

FFA Palm Oil

FFA Palm Oil

FFA Palm Oil

FFA Palm Oil

FFA Palm Oil

Amberlite IR120

Zirconia

Oleic - - MeOH

**Catalyst amount (%)** 

**Alcohol** 

EtOH

Linoleic - - MeOH 49.6

IsoprOH IsoBuOH 2-BuOH IsopentOH

H2SO4 4% wt FFA EtOH 1:24 Domestic MW

IsoprOH IsoBuOH 2-BuOH IsopentOH

IsoprOH IsoBuOH 2-BuOH IsopentOH

IsoprOH IsoBuOH 2-BuOH IsopentOH

10%wt Oil MeOH

Fatty Acid Alkyl-Esters Production: Microwaves, Radio-Frequency and Ultrasound 273

**Oil to alcohol molar ratio** 

5 wt MeOH 1:20 Experimental MW

10 wt MeOH 1:20 Experimental MW

5 wt MeOH 1:10 Synthos 3000-

1%mol MeOH 48 eq Biotage MW

1:8

1:20

1:20

1:20

1:10 Synthos 3000- Anton Paar. 1400W. 30min, 200C

**Microwave reaction conditions** 

heating system 20min, 60C

heating system. Pulsed MW. 15min, 60C

Anton Paar. 1400W. 20min, 200C

reactor. 1min, 150C

Domestic MW 1000W 60 min, 60C 60 min, 75C 60 min, 105C 60 min, 90C 60 min, 115C

800W 60 min, 70C

Domestic MW 1000W 60 min, 60C 60 min, 75C 60 min, 105C 60 min, 90C 60 min, 115C

Domestic MW 1000W 60 min, 60C 60 min, 75C 60 min, 105C 60 min, 90C 60 min, 115C

Domestic MW 1000W 60 min, 60C 60 min, 75C 60 min, 105C 60 min, 90C 60 min, 115C

**Ester conversion (%)** 

51.8 31.5

68.0 68.7

97.0 98.0 100.0 88.0 98.0 90.0 100.0 99.0

99.8 99.8 96.2 95.5 90.8

95.6 86.2 82.8 78.5 77.7

91.3 85.1 81.4 74.8 74.1

91.4 84.7 80.6 66.8 73.5 **Ref.** 

(Melo et al., 2009)

(Melo et al., 2010)

(Socha and Sello,

(Mazo and Rios, 2010a)

al., 2010a and b)

(Mazo and Rios, 2010b)

(Mazo and Rios, 2010b)

(Mazo and Rios, 2010b)

90.0 (Kim et al., 2011a)

66.1 (Kim et al., 2011b)

2010)

87.7 (Suppalakpanya et

increase of this parameter lead to a decrease on the reaction conversion. In general, the esterification reaction under microwave irradiation yielded similar results to those obtained with the conventional heating but with very fast heating rates (Melo et al., 2009). The pulsed microwaves with repetitive strong power could enhance the efficiency of biodiesel production relative to the use of continuous microwave with mild power (Kim et al., 2011b). Electric energy consumption for the microwave heating in this accelerated esterification was only 67% of estimated minimum heat energy demand because of significantly reduced reaction time (Kim et al., 2011a).

For oils with a high content of free fatty acid FFA as palm oil, has been proposed obtain alkyl ester from crude palm oil (CPO), using microwaves like heating source, in a process of two stages by means of homogeneous and heterogeneous catalysis; the first stage (esterification), was made using sulfuric acid and Dowex 50X2, Amberlyst 15 and Amberlite IR-120 resin catalysts, to diminish the acid value of the oil, avoiding the soap formation and facilitating the separation of the phases. In these works has been reported the obtaining of alkyl ester using alcohols non-conventional such as: ethanol (EtOH) (Suppalakpanya et al., 2010a, 2010b), isopropyl (IsoprOH), isobutyl (IsobuOH), 2-butyl (2- BuOH) and Isopentyl (IsopentOH) alcohols (Mazo and Rios, 2010a; Mazo and Rios, 2010b), where was found that that the acidity order obtained for the catalysts is Dowex < Amberlite < Amberlyst, and the order for the alcohols: Methanol < isopropyl alcohol < isobutyl alcohol < 2-butyl alcohol < isopentyl alcohol, because Dowex microreticular resin presents the lowest divinylbenzene (2%), which has a lower cross-linking that produces a high expansion of the resin in a polar medium, and the resin can expand their pores up to 400%, enabling the income of the voluminous substrate (FFA) and its protonation. Amberlyst 15 macroreticular resin is activated due to its surface area, and the protons located on the outer surface seem that catalyse the esterification because the interiors are inaccessible due to high cross-linking. The reaction is favoured with the increasing of polarity of solvents.

Table 1 shows the work carried out for bio-diesel production by esterification of FFA under different conditions using microwave irradiation.

#### **2.2 Transesterification reactions assisted by MW**

Vegetable oils are becoming a promising alternative to diesel fuel because they are renewable in nature and can be produced locally and in environmentally friendly ways. Edible vegetable oils such as canola and soybean oil in the USA, palm oil in Malaysia, rapeseed oil in Europe and corn oil have been used for biodiesel production and found to be good diesel substitutes. Non-edible vegetable oils, such as Pongamia pinnata (Karanja or Honge), Jatropha curcas (Jatropha or Ratanjyote), Madhuca iondica (Mahua) and Castor Oil have also been found to be suitable for biodiesel production (Yusuf et al., 2011).

Transesterification (also called alcoholysis) is the reaction of a fat or oil with an alcohol (with or without catalyst) to form esters and glycerol. Since the reaction is reversible, excess alcohol is used to shift the equilibrium to the product side (Fangrui and Milford, 1999). Under Transesterification reaction with alcohol the first step is the conversion of triglycerides to diglycerides, which is followed by the subsequent conversion of higher glycerides to lower glycerides and then to glycerol, yielding one methyl ester molecule from each glyceride at each step (Hideki et al., 2001).

increase of this parameter lead to a decrease on the reaction conversion. In general, the esterification reaction under microwave irradiation yielded similar results to those obtained with the conventional heating but with very fast heating rates (Melo et al., 2009). The pulsed microwaves with repetitive strong power could enhance the efficiency of biodiesel production relative to the use of continuous microwave with mild power (Kim et al., 2011b). Electric energy consumption for the microwave heating in this accelerated esterification was only 67% of estimated minimum heat energy demand because of significantly reduced

For oils with a high content of free fatty acid FFA as palm oil, has been proposed obtain alkyl ester from crude palm oil (CPO), using microwaves like heating source, in a process of two stages by means of homogeneous and heterogeneous catalysis; the first stage (esterification), was made using sulfuric acid and Dowex 50X2, Amberlyst 15 and Amberlite IR-120 resin catalysts, to diminish the acid value of the oil, avoiding the soap formation and facilitating the separation of the phases. In these works has been reported the obtaining of alkyl ester using alcohols non-conventional such as: ethanol (EtOH) (Suppalakpanya et al., 2010a, 2010b), isopropyl (IsoprOH), isobutyl (IsobuOH), 2-butyl (2- BuOH) and Isopentyl (IsopentOH) alcohols (Mazo and Rios, 2010a; Mazo and Rios, 2010b), where was found that that the acidity order obtained for the catalysts is Dowex < Amberlite < Amberlyst, and the order for the alcohols: Methanol < isopropyl alcohol < isobutyl alcohol < 2-butyl alcohol < isopentyl alcohol, because Dowex microreticular resin presents the lowest divinylbenzene (2%), which has a lower cross-linking that produces a high expansion of the resin in a polar medium, and the resin can expand their pores up to 400%, enabling the income of the voluminous substrate (FFA) and its protonation. Amberlyst 15 macroreticular resin is activated due to its surface area, and the protons located on the outer surface seem that catalyse the esterification because the interiors are inaccessible due to high cross-linking. The reaction is favoured with the increasing of

Table 1 shows the work carried out for bio-diesel production by esterification of FFA under

Vegetable oils are becoming a promising alternative to diesel fuel because they are renewable in nature and can be produced locally and in environmentally friendly ways. Edible vegetable oils such as canola and soybean oil in the USA, palm oil in Malaysia, rapeseed oil in Europe and corn oil have been used for biodiesel production and found to be good diesel substitutes. Non-edible vegetable oils, such as Pongamia pinnata (Karanja or Honge), Jatropha curcas (Jatropha or Ratanjyote), Madhuca iondica (Mahua) and Castor Oil have also been found to be suitable for biodiesel production

Transesterification (also called alcoholysis) is the reaction of a fat or oil with an alcohol (with or without catalyst) to form esters and glycerol. Since the reaction is reversible, excess alcohol is used to shift the equilibrium to the product side (Fangrui and Milford, 1999). Under Transesterification reaction with alcohol the first step is the conversion of triglycerides to diglycerides, which is followed by the subsequent conversion of higher glycerides to lower glycerides and then to glycerol, yielding one methyl ester molecule from

reaction time (Kim et al., 2011a).

polarity of solvents.

(Yusuf et al., 2011).

different conditions using microwave irradiation.

each glyceride at each step (Hideki et al., 2001).

**2.2 Transesterification reactions assisted by MW** 


Table 1. Microwave assisted esterification.

Alternative Methods for

NaOH KOH

KOH

0.5 1.0

Soybean Nano CaO 3.0 MeOH 1:7 ETHOS900

Soybean Novozym 435 3.0 MeOH 1:6 MCR-3

1.5 2.0

0.5 1.0 1.5 2.0 2.0 2.0

Safflower NaOH 1.0 MeOH 1:10 Start

NaOH 0.6 EtOH 1:5 ETHOS E-

ButOH ButOH

NaOH 1.0 MeOH 1:12 MW650

IsoprOH IsoBuOH 2-BuOH IsopentOH 1:30

Rapeseed KOH 1.0 ButOH 1:4 MARS CEM

5.0 1.0

Palm H2SO4 3.0 MeOH

Canola ZnO/La2O2CO3 1.0 MeOH 1:1 wt Biotage MW

Pongamia Pinnata

Rapeseed NaOH

Camelina BaO

Camelina NaOH

Soybean Rice Bran

Jatropha Waste frying

Soybean H2SO4

KOH

SrO

KOH BaO SrO BaCl2/AA SrCl2/AA

Fatty Acid Alkyl-Esters Production: Microwaves, Radio-Frequency and Ultrasound 275

1.0 MeOH - Start Synth-

MeOH 1:9

MeOH 1:9

1:12

MeOH 1:6 Start Synth-

Milestone 1200W 60C, 7 min

Milestone 1200W 60C, 5 min

Milestone 900W 65C, 60 min

Shanghai JieSi 800W 40C, 12h

Domestic MW 800W 100C, 4 min 60C, 4 min

Domestic MW 800W 60C, 60 s 60C, 60 s 60C, 4 min 60C, 4 min 60C, 5 min 60C, 5 min

labstation-Milestone 60C, 6 min

Milestone 73C, 10 min

Corp. 117C, 30 min

1:6 CEM Discover 300W CEM MARS 1600W 100C, 15 min 120C, 1 min

> Aurora Instruments MW discovery 65C, 7 min

Domestic MW 1000W 60C, 5h 75C, 5h 105C, 5h 90C, 5h 115C, 5h

95 78

99.25 99.34

93 93

89.7 88.63

49.40 62.39 67.39 62.39 75.00

reactor. <100C, 5 min 95.3 96.0

91.7 90.8 (Kumar et al., 2011)

(Azcan and Danisman, 2008)

96.6 (Hsiao et al., 2011)

94.0 Yu et al., 2010)

>95 (Jin et al., 2011)

98.4 (Duz et al., 2011)

100 (Geuens et al., 2008)

2008)

(Patil et al., 2011)

(Patil et al., 2010)

(Terigar et al., 2010)

(Leadbeater et al.,

(Yaakob et al., 2009)

(Mazo and Rios, 2010a)

Several examples of microwave irradiated transesterification methods have been reported using homogenous alkali catalyst (Kumar et al., 2011; Azcan and Danisman, 2008), acid catalyst (Mazo and Rios, 2010a) and heterogeneous alkali catalyst (Patil et al., 2011), heterogeneous acid catalyst (Yuan et al., 2009) and enzymatic (Yu et al., 2010).

Microwave synthesis is not easily scalable from laboratory small-scale synthesis to industrial multi kilogram 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 reason for rejecting microwave reactors in industry (Groisman and Aharon, 2008).

The preparation of biodiesel using a scientific microwave apparatus offers a fast, easy route to this valuable biofuel with advantages of a short reaction time, a low oil/methanol ratio, and an ease of operation. The methodology allows for the reaction to be run under atmospheric conditions; it is complete in a matter of a few minutes and can be performed on batch scales up to 3 kg of oil at a time (Leadbeater and Stencel, 2006).

The continuous-flow preparation of biodiesel using a commercially available scientific microwave apparatus offers a fast, easy route to this valuable biofuel. The methodology allows for the reaction to be run under atmospheric conditions and performed at flow rates of up to 7.2 L/min using a 4 L reaction vessel. Energy consumption calculations suggest that the continuous-flow microwave methodology for the transesterification reaction is more energy-efficient than using a conventional heated apparatus (Barnard et al., 2007).

Few studies report the use of alcohols different to methanol. Alcohols more used are ethanol and butanol, and the latter is a versatile and sustainable platform chemical that can be produced from a variety of waste biomass sources. The emergence of new technologies for the production of fuels and chemicals from butanol will allow it to be a significant component of a necessarily dynamic and multifaceted solution to the current global energy crisis. Recent work has shown that butanol is a potential gasoline replacement that can also be blended in significant quantities with conventional diesel fuel (Harvey and Meylemans, 2011).

Table 2 shows the work carried out for bio-diesel production from various feedstocks, catalysis and alcohols under different conditions using microwave irradiation.


Several examples of microwave irradiated transesterification methods have been reported using homogenous alkali catalyst (Kumar et al., 2011; Azcan and Danisman, 2008), acid catalyst (Mazo and Rios, 2010a) and heterogeneous alkali catalyst (Patil et al., 2011),

Microwave synthesis is not easily scalable from laboratory small-scale synthesis to industrial multi kilogram 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

The preparation of biodiesel using a scientific microwave apparatus offers a fast, easy route to this valuable biofuel with advantages of a short reaction time, a low oil/methanol ratio, and an ease of operation. The methodology allows for the reaction to be run under atmospheric conditions; it is complete in a matter of a few minutes and can be performed on

The continuous-flow preparation of biodiesel using a commercially available scientific microwave apparatus offers a fast, easy route to this valuable biofuel. The methodology allows for the reaction to be run under atmospheric conditions and performed at flow rates of up to 7.2 L/min using a 4 L reaction vessel. Energy consumption calculations suggest that the continuous-flow microwave methodology for the transesterification reaction is more

Few studies report the use of alcohols different to methanol. Alcohols more used are ethanol and butanol, and the latter is a versatile and sustainable platform chemical that can be produced from a variety of waste biomass sources. The emergence of new technologies for the production of fuels and chemicals from butanol will allow it to be a significant component of a necessarily dynamic and multifaceted solution to the current global energy crisis. Recent work has shown that butanol is a potential gasoline replacement that can also be blended in significant quantities with conventional diesel fuel (Harvey and

Table 2 shows the work carried out for bio-diesel production from various feedstocks,

1:6

1.5 MeOH 1:4 wt Domestic MW

**Oil to alcohol molar ratio** 

**Microwave reaction conditions** 

Shanghai Sineo MW 65C, 60 min

Milestone 1200W 65C, 2 min

900W 60C, 40 s

Domestic MW 540W 60C, 30 min 60C, 20 min 60C, 5 min

95 95 95

99 97

**Ester conversion (%)** 

**Ref.** 

(Perin et al., 2008)

(Koberg et al., 2011)

94 (Yuan et al., 2009)

99 (Shakinaz et al., 2010)

heterogeneous acid catalyst (Yuan et al., 2009) and enzymatic (Yu et al., 2010).

reason for rejecting microwave reactors in industry (Groisman and Aharon, 2008).

energy-efficient than using a conventional heated apparatus (Barnard et al., 2007).

catalysis and alcohols under different conditions using microwave irradiation.

**Alcohol** 

MeOH EtOH MeOH

**Catalyst amount (%wt)** 

Castor 50% H2SO4/C 5 MeOH 1:12 MAS-1

Jatropha KOH 1.5 MeOH 1:7.5 Start Synth-

10 10 10

batch scales up to 3 kg of oil at a time (Leadbeater and Stencel, 2006).

Meylemans, 2011).

**Oil Catalyst** 

Castor SiO2/50%H2SO4

SrO Sr(OH)2

Waste frying SiO2/50%H2SO4 Al2O3/50%KOH


Alternative Methods for

RF heating apparatus (Lui et al., 2008).

**3.1 Esterification reactions assisted by RF** 

**3.2 Transesterification reactions assisted by RF** 

described below:

observed (Lui et al., 2010).

Fig. 1.

Fatty Acid Alkyl-Esters Production: Microwaves, Radio-Frequency and Ultrasound 277

dielectric heating technology, RF heating systems are simpler to configure and have a higher conversion efficiency of electricity to electromagnetic power (Wang et al., 2003). Moreover, RF energy has deeper penetration into a wide array of materials than microwave energy,

Very few publications have been obtained by this alternative heating method, which use a RF heating apparatus (SO6B; Strayfield Fastran, UK). The distance between the two electrodes was fixed at 15 cm. A 150-mL conical flask coupled with a water-cooling reflux condenser was used as a reactor. Schematic diagram and photograph are shown in

Fig. 1. a) Schematic diagram of RF heating apparatus (Lui et al., 2010) and b) Photograph of

Applications to obtaining biodiese using different oils, reaction conditions and catalysts are

Efficient biodiesel conversion from waste cooking oil with high free fatty acids (FFAs) was achieved via a two-stage procedure (an acid-catalyzed esterification followed by an alkalicatalyzed transesterification) assisted by radio frequency (RF) heating. In the first stage, with only 8-min RF heating the acid number of the waste cooking oil was reduced from 68.2 to 1.64 mg KOH/g by reacting with 3.0% H2SO4 (w/w, based on oil) and 0.8:1 methanol (weight ratio to waste oil). Then, in the second stage, the esterification product (primarily consisting of triglycerides and fatty acid methyl esters) reacted with 0.91% NaOH (w/w, based on triglycerides) and 14.2:1 methanol (molar ratio to triglycerides) under RF heating for 5 min, and an overall conversion rate of 98.8 ± 0.1% was achieved. Response surface methodology was employed to evaluate the effects of RF heating time, H2SO4 dose and methanol/oil weight ratio on the acid-catalyzed esterification. A significant positive interaction between RF heating time and H2SO4 concentration on the esterification was

Efficient biodiesel production from beef tallow was achieved with radio frequency (RF) heating. A conversion rate of 96.3% was obtained with a NaOH concentration of 0.6% (based

increasing feasibility of RF heating for industrial scale applications.


Table 2. Microwave assisted transesterification.
