Fuel Processing Technology 116 (2013) 241–249

Contents lists available at ScienceDirect

Fuel Processing Technology

journal homepage: www.elsevier.com/locate/fuproc

Ultrasound assisted interesterification of waste cooking oil and methyl

acetate for biodiesel and triacetin production
Ganesh L. Maddikeri, Aniruddha B. Pandit, Parag R. Gogate ⁎
Chemical Engineering Department, Institute of Chemical Technology, Matunga, Mumbai 40019, India

a r t i c l e i n f o a b s t r a c t

Article history: Intensification of the interesterification reaction of waste cooking oil with methyl acetate using potassium
Received 21 February 2013 methoxide as a catalyst has been carried out using ultrasonic horn (frequency of irradiation of 22 kHz and
Received in revised form 14 July 2013 rated power of 750 W). Experiments have been performed at different operating parameters viz. reaction tem-
Accepted 14 July 2013
perature (30, 40 and 50 °C), oil to methyl acetate molar ratio (over the range of 1:4 to 1:14), catalyst concentra-
Available online 7 August 2013
tion (0.5, 1.0 and 1.5% by weight of oil) and amplitude of ultrasound (40, 50, 60 and 70%) with an objective of
Keywords:
understanding the effect of important operating parameters on the extent of conversion of waste cooking oil
Biodiesel to the ester. It has been observed that maximum yield (90%) of biodiesel from waste cooking oil using
Waste cooking oil sonochemical reactors was observed at a molar ratio of 1:12, catalyst concentration of 1.0% and temperature of
Interesterification 40 °C. It is also observed that higher conversion was obtained in the presence of ultrasound as compared to
Process intensification the conventional method. Kinetic studies have been carried out to determine the rate constant by fitting the
Ultrasound obtained experimental data to a second-order rate equation. It has been observed that rate constant increases
with an increase in temperature and the activation energy is found to be 56.97 kJ/mol.
© 2013 Elsevier B.V. All rights reserved.

1. Introduction than pure vegetable oil and also by employing interesterification pro-
cess instead of the more commonly used transesterification process
Biodiesel is generally referred to as fatty acid esters, which can be for biodiesel production. The interesterification of oils and fats with
essentially synthesized from oils or fats using the esterification based methyl acetate provides a promising alternative to transesterification
reactions. Biodiesel has several advantages over petroleum derived because of the formation of triacetin instead of glycerol. This complex
diesel such as biodegradability, non-toxicity, lower harmful emissions, reaction is composed of three consecutive reversible reactions, which
higher flash point, excellent lubricity and superior cetane number [1]. are shown in Scheme 1 [6]. Triacetin is used mainly as a plasticizer
Moreover, biodiesel is essentially free of sulfur and the engines fuelled and a gelatinizing agent in polymers and explosives and as an additive
by biodiesel emit significantly fewer particulate matters, residual hy- in tobacco, pharmaceutical industries, and cosmetics. Recent studies
drocarbons (due to near complete combustion) and less carbon monox- have also shown that triacetin may be added to the formulation of bio-
ide as compared to the engines operating on conventional petrobased diesel (up to 10% by weight) and the blended biodiesel still meets the
diesel [2]. Due to the depletion of petroleum reserves and increased quality standards set by ASTM D6451 and EN 14214 because of its mu-
environmental concerns, synthesis of alternative fuels has been a signif- tual solubility [7]. Interesterification has been mostly studied in the
icant point of interest to the researchers. Overall, biodiesel has a huge presence of enzymes [8–10] or under supercritical conditions [3,11–14].
potential to replace exhaustible fossil fuel, ensuring the sustainability Supercritical and enzymatic methods of interesterification have their
of human development and energy sources [3]. It is, however, estimated own disadvantages. The main disadvantages of supercritical method
that the cost of biodiesel is approximately 1.5 to 2 times higher than include (a) operations at very high pressures (20–40 MPa); (b) require-
that of the petroleum based diesel fuel [4]. The reasons for the higher ments of high temperatures (350–400 °C) resulting in much higher
price of biodiesel are as follows: a) 70–95% cost of biodiesel synthesis heating and cooling costs; (c) high oil: methyl acetate ratios (usually
process is from the cost of raw materials such as food grade vegetable set at 1:42) [12] and finally the supercritical method entails higher
oils [5] b) low rates of reaction due to mass transfer limitations in the costs for the evaporation of the unreacted methyl acetate. The draw-
heterogeneous reaction system and c) difficulty in the separation of bio- backs of enzymatic route of interesterification are significantly higher
diesel from the side product and unreacted alcohols giving lower yields. production costs [15] as well as difficulty in manufacturing at larger
The production cost of biodiesel can be reduced by using waste scales due to the need for a careful control of the reaction parameters
cooking oil (WCO) as a starting raw material, which is less expensive and inherent slowness of the reaction [16].
Based on this analysis, it can be established that there is a need to
⁎ Corresponding author. Tel.: +91 22 33612024; fax: +91 22 3361 1020. develop sustainable process intensification technology for biodiesel
E-mail address: pr.gogate@ictmumbai.edu.in (P.R. Gogate). production from WCO sources based on interesterification with an

0378-3820/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.fuproc.2013.07.004
242 G.L. Maddikeri et al. / Fuel Processing Technology 116 (2013) 241–249

O O

O C R1 O C CH3
H2C O O H2C O O
C
HC O C R2 + H3C OCH 3
HC O C R2 + R1
C
OCH3
H2C O H2C O
O C R3 O C R3
Triglyceride Methyl acetate Monoacetindiglyceride Fatty acid methyl ester

O O

O C CH3 O C CH3
O O H2C O O
H2 C
C
HC O C R2 + H3C OCH3
HC O C CH3 + R2
C
OCH3
H2C O H2C O
O C R3 O C R3
Monoacetindiglyceride Methyl acetate Diacetinmonoglyceride Fatty acid methyl ester

O O

O C CH 3 O C CH3
O O H2C O O
H2C
HC O C CH 3 + H 3C
C
OCH 3
HC O C CH3 + C
OCH 3
O R3
H2C O H2C
O C R3 O C CH3
Diacetinmonoglyceride Methyl acetate Triacetin Fatty acid methyl ester

O O

O C R O C CH3
O O H2C O O
H2C
C
HC O C R + 3
H3C OCH3
HC O C CH3 + 3
R
C
OCH3
O H2C O
H2C
O C R O C CH3

Triglyceride Methyl acetate Triacetin Fatty acid methyl ester

Scheme 1. Interesterification reaction (Initial 3 equations show the individual steps whereas final equation gives the overall reaction).

objective of reducing the cost of processing. Cavitational reactors can concentration and amplitude to investigate the dependency of biodiesel
offer a useful energy-efficient process intensification approach for bio- yield from WCO. Kinetic constant as well as activation energy for the
diesel production as compared to other approaches for intensification interesterification reaction have been also determined at optimum
such as microwave irradiation, oscillatory flow reactor, microchannel operating conditions. Also, the properties of the synthesized biodiesel
reactor, addition of co-solvent and supercritical uncatalyzed transester- from these methods have been evaluated in order to match with
ification [17]. However, a careful study of the existing literature indi- ASTM standards.
cates that there has been absolutely no study related to the use of
cavitational reactors for intensification of synthesis of biodiesel and 2. Material and methods
triacetin using the interesterification reaction route. With this back-
ground, the present work deals with the intensification of interester- 2.1. Materials
ification reaction using sonochemical reactors, which are based on the
generation of cavitation events due to the pressure fluctuations induced Waste cooking oil was procured from a local restaurant (Garnish
by the incident ultrasound waves. The interesterification reaction of Restaurant, King's Circle, Mumbai, India). Analysis of the WCO
pretreated WCO has been carried out in the presence of potassium (Table 1) indicates that it is mainly composed of 91% unsaturated
methoxide as a catalyst using ultrasonic horn. Potassium methoxide fatty acids (linoleic and oleic acids) and 9% saturated fatty acids
has been selected as the catalyst for interesterification reaction due to (palmitic and stearic acid). Table 1 also shows the properties of waste
the fact that it gives higher yield of biodiesel in interesterification reac- cooking oil used as the starting raw material. Methyl acetate, potassium
tion as compared to other catalysts such as potassium hydroxide, or hydroxide pellets (LR grade), ortho-phosphoric acid, molecular sieves
PEGK (Polyethylene glycol complex with potassium). Experiments (3°A) used in the experimental work were procured from S.D. Fine
have been performed in the presence of potassium methoxide as a cat- Chem. Ltd., Mumbai. The weak anion-exchange resin (Indion 860)
alyst at different temperature, methyl acetate to oil molar ratio, catalyst was obtained from Ion Exchange Ltd., Mumbai. Acetonitrile and acetone
 G.L. Maddikeri et al. / Fuel Processing Technology 116 (2013) 241–249 243

Table 1
Composition and properties of waste cooking oil.

Property Value
2
Linoleic acid (%) 73.4 4
Oleic acid (%) 18.3
Palmitic acid (%) 6.7
Stearic acid (%) 1.6
Saponification value (mg KOH/g of oil) 198
Density (kg/m3) 930
Acid value (mg KOH/g oil) 4.3 1
Viscosity (mm2/s) 54.3
5

(HPLC grade) used as solvent for HPLC analysis were procured from Hi 3
Media, Mumbai. Methanolic potassium methoxide (33% by weight) was 6
procured from Spectrochem Pvt. Ltd., Mumbai whereas methyl oleate
and methyl linoleate standards were procured from Sigma-Aldrich. All
the chemicals were used as received from the supplier without any pu-
Fig. 1. Experimental setup for interesterification reaction. 1. Stand support. 2. Condenser.
rification, except for the waste cooking oil used in the experiments.
3. Ultrasonic generator. 4. Ultrasonic horn. 5. Reactor. 6. Temperature controller.

2.2. Experimental methodology

charged oil), temperature (30, 40 and 50 °C) and percentage amplitude
2.2.1. Pretreatment of waste cooking oil of ultrasonic horn (40, 50, 60 and 70%). Experiments were also performed
A series of pretreatment processes were undertaken prior to the using conventional stirring approach under optimized set of operating
interesterification reaction of the WCO for biodiesel production. Pre- parameters to establish the degree of intensification obtained due to
treatment of WCO consists of physical treatment for removing the the use of ultrasonic irradiations. The setup for stirring is very similar to
suspended solid contaminants by filtration and repeated water washing that shown in Fig. 1, except for the fact that in the conventional method
for the separation of water soluble salt impurities present in the WCO. an overhead stirrer has been used instead of an ultrasonic horn. A six-
After the water washing, dehydration was carried out by heating the blade turbine with a diameter of 1.5 cm was used as the stirrer and the
sample at 80 °C under a vacuum of 25 mmHg for 10 min to remove operating speed used in the work was 1000 rpm. All experiments were
the traces of water. Deacidification of WCO has been then carried out carried out in duplicate and the reported values are the average of the
by adsorption using ion exchange resins to reduce the acid value of individual runs. The experimental errors were within 2% of the reported
the WCO from 4.3 to 0.4. Experiments have been carried out by adding average value of biodiesel yield.
1 wt.% of weak anion exchange resin (Indion 860) to the WCO in the
presence of stirring at 800 rpm maintained for 3 h at 273 K. Weak
2.3. Analytical methods
anion exchange resin (Indion 860) was used for deacidification of WCO
because it has excellent adsorption capacity of 491 and 532 mg/g of
The acid value of the waste cooking oil in the first stage (pretreat-
resin for unsaturated and saturated fatty acids respectively as
ment) was determined by the acid-base titration technique. For this
established in our previous work [18]. After the adsorption treat-
analysis, 5 g of sample is transferred into a conical flask containing
ment, cooking oil was separated from the anion exchange resin and
20 ml of neutralized ethanol. The mixture was titrated with 0.003 N
this pretreated waste cooking oil has been used as the raw material
KOH using phenolphthalein as an indicator to determine the free fatty
in the interesterification reaction for the biodiesel production.
acid present in the solution. The observed end point is colorless to pink.
The fatty acid compositions in the WCO were determined by
2.2.2. Interesterification of pretreated waste cooking oil
converting all the fatty acids into the corresponding fatty acid methyl
The interesterification experiments were performed in a 100-mL
esters followed by gas chromatography (GC) analysis using BP-X70
three-neck batch reactor. The three openings in the reactor are used for
column. Fatty acid methyl ester content in the reaction mixture was
accommodating the condenser (to condense the methyl acetate evapo-
analyzed on HPLC using Agilent Eclipse XDB C-18 column, with dimen-
rated during reaction), for the introduction of ultrasonic horn or stirrer
sions of 4.6 × 250 mm using RI detector. The samples were analyzed
(based on the approach of ultrasound assisted or the conventional ap-
isocratically using mobile phase of acetonitrile: acetone (70:30) with
proach) and for inserting the temperature sensor to monitor the temper-
1.5 ml/min flow rate. Samples were prepared by using 10 μl of the reac-
ature during the reaction. The schematic of the experimental setup has
tion mixture diluted with 10 ml of mobile phase. A typical HPLC
been shown in Fig. 1. The reactor was initially charged with vegetable
chromatogram for interesterification product identification has been
oil and methyl acetate and heated to the desired reaction temperature.
shown in Fig. 2. The concentration was calculated based on the area
Once the desired temperature was reached, catalyst was added to the re-
under the peak for the retention time of standard samples of methyl ole-
actor, at which point the reaction can be considered to start. Several
ate and methyl linoleate procured from Sigma-Aldrich. Based on the
samples were collected at different pre-designated times and were im-
reaction stoichiometry, the percentage biodiesel yield was calculated
mediately quenched using equimolar amount of ortho-phosphoric acid
from the product formation using the extent of completion of reaction
to arrest the progress of the reaction in the withdrawn samples. After
under equilibrium conditions. Density [ASTM D7777], flash point, kine-
this treatment with ortho-phosphoric acid, the samples were washed
matic viscosity [ASTM D445], and acid value [ASTM D664] of final inter-
with water in order to remove the excess catalyst from the mixture.
esterified product have also been evaluated.
Finally, a molecular sieve (3°A) was added to each sample to adsorb
the trace amount of moisture. Samples were then refrigerated until the
final analysis was carried out using HPLC. 2.4. Kinetic constant and activation energy
The different operating parameters used in the present work, to
optimize the yield of biodiesel from waste cooking oil, include oil to The interesterification reaction progresses in three consecutive
methyl acetate molar ratio (OMAMR) (1:4, 1:6, 1:8, 1:10, 1:12 and reversible reaction steps [19]. As shown in Scheme 1, interesterification
1:14), catalyst concentration (0.5, 0.75, 1 and 1.25% by weight of the begins with the reaction of triglyceride and methyl acetate to produce
244 G.L. Maddikeri et al. / Fuel Processing Technology 116 (2013) 241–249

Fig. 2. HPLC chromatogram for identification of interesterification products.

monoacetindiglycerides and one mole of methyl ester. The 3. Results and discussion
monoacetindiglycerides further react with methyl acetate to yield
diacetinmonoglycerides and another mole of methyl ester. The 3.1. Effect of oil to methyl acetate molar ratio
diacetinmonoglycerides finally react with methyl acetate to produce
one mole of methyl ester and triacetin each. As interesterification reac- In interesterification, as per the reaction stoichiometry, three moles
tion is reversible in nature, all the experiments were carried out at of methyl acetate are needed for each mole of triglycerides, implying an
higher methyl acetate to oil molar ratio than that required as per the OMAMR of 1:3. Interesterification reaction is a reversible reaction and
reaction stoichiometry. Due to the high methyl acetate: oil molar to drive the reaction in forward direction usually an excess of methyl ac-
ratio, the reverse reaction in Scheme 1 can be minimized and the con- etate is used as compared to the stoichiometric requirement. Consider-
centration of methyl acetate can be considered as invariant. So, the ing this aspect, experiments were conducted with OMAMR ranging
overall interesterification has been assumed to follow a second-order from 1:4 to 1:14 and the obtained results have been shown in Fig. 3. It
reaction, which is also reported in the literature [20,21]. The governing has been observed that with an increase in the molar ratio from 1:4 to
rate equation may be expressed as: 1:12, the biodiesel yield increases from 45% to 90%. This is because as
the interesterification reaction is reversible in nature, an excess of
dCTG 2 methyl acetate drives the interesterification reaction in the forward
− ¼ rTG ¼ k  CTG ð1Þ
dt direction resulting in an increase in the conversion of WCO to biodiesel.
It was also observed that further increase in the OMAMR from 1:12 to
where CTG is the molar concentration (mol L− 1) of the triglycer-
1:14, did not result in any significant increase in the conversion of oil
ides, t is the reaction time (min), rTG is the reaction rate of the tri-
to biodiesel. The obtained results can be attributed to the fact that the
glycerides (mol L− 1 min− 1) and k is the reaction rate constant
use of higher molar ratio results in diluted products of biodiesel and
(L mol− 1 min− 1). Integration of the above equation yields:
triacetin which might initiate the reverse reaction to reduce the conver-
sion [3].
1 1
¼ktþ ð2Þ The yield of biodiesel obtained (90%) at OMAMR of 1:12 was higher
CTG CTG0
than that obtained under the conventional approach (76.7%) as report-
ed by Casas et al. [6]. It is also interesting to know that the OMAMR used
Eq. (2) can be rearranged in terms of the conversion and can be for this conventional approach is significantly higher at 1: 50. In the
expressed as follows: present study, higher yield of 90% is obtained at significantly lower ex-
cess of methyl acetate, which can be attributed to the cavitational
X
¼ CTG0  k  t ð3Þ events releasing large magnitude of energy due to the violent collapse
1−X
of the cavities. Cavitation occurs at millions of locations in the reactor si-
where C TG0 is the initial triglyceride molar concentration (mol L−1) and multaneously and generates conditions of very high local temperatures
X is the conversion of triglyceride at any time t. The actual triglyceride and pressures (few thousand atmosphere pressure and few thousands
concentration during the interesterification process can be calculated
from the knowledge of the biodiesel yield and used to fit Eq. (2). A
plot of 1/CTG versus t will be a straight line if the model is valid, and k
will be the value of the slope.
The Arrhenius equation gives a relationship between the specific
reaction rate constant (k), absolute temperature (T) and the energy of
activation (Ea) and can be given as follows:
 
−Ea
k ¼ A exp ð4Þ
RT

where A is the frequency factor and R is the universal gas constant

(J mol−1 K−1). The equation can be rewritten as:

−Ea
ln ðkÞ ¼ þ ln ðAÞ ð5Þ
RT
Fig. 3. Effect of oil to methyl acetate molar ratio on the yield of biodiesel. [Reaction condi-
A plot of ln (k) vs. 1/T (the Arrhenius plot) gives slope equal to (−Ea/R) tions: Catalyst concentration = 1.00% (weight); Temperature = 40 °C; Percentage am-
from which the activation energy can be determined. plitude = 60%].
 G.L. Maddikeri et al. / Fuel Processing Technology 116 (2013) 241–249 245

Kelvin temperature) with bulk ambient conditions. These cavitational

effects are mainly responsible for the intensification of physical and
chemical processing applications limited by mass transfer [22]. Consid-
ering the specific case of biodiesel synthesis, it can be said that the phys-
ical effects in terms of intense turbulence, liquid micro-circulation
(acoustic streaming) and micro-emulsion formation giving enhanced
areas for reaction, play a dominating role in intensification. Usai et al.
[8] also carried out interesterification reaction of methyl acetate with
triglycerides such as olive oils resulting into the formation of corre-
sponding fatty acid methyl esters and triacetyl glycerol (triacetin).
The reaction was catalyzed by lipase enzyme and effect of operating
parameters such as type of catalyst, enzyme hydration and immobiliza-
tion support has been investigated. It was reported that by using the
immobilized lipase Candida antarctica, yields as high as 80% of both
Fig. 4. Effect of catalyst concentration on the yield of biodiesel. [Reaction conditions:
fatty acid esters and triacetin can be achieved at OMAMR of 1:20. Simi-
OMAMR = 1:12; Temperature = 40 °C; Percentage amplitude = 60%].
larly, Xu et al. [9] obtained a FAME yield of 67% at 40 °C from refined
soybean oil during interesterification reaction carried out at atmospher-
ic pressure, with an OMAMR of 1:12 and a reaction time of 36 h using reaction between refined sunflower oil and methyl acetate for biodiesel
Novozym enzyme (0.1 g of enzyme per 1 g of oil). A major drawback production at different catalyst ratio (0.1:1, 0.15:1 and 0.2:1) and it was
of the use of the enzymes as catalysts is the long reaction times needed reported that a maximum conversion of 76.7% was obtained at optimized
for high conversions as observed in the work of Xu et al. [9]. Campanelli catalyst to oil molar ratio of 0.2:1 (i.e. 1.04% by weight of the catalyst).
[12] used methyl acetate instead of methanol for supercritical synthesis
of glycerol-free biodiesel from edible oils (soybean and sunflower seed 3.3. Effect of temperature
oil) and non-edible oils (Jatropha curcas) with different fatty acid com-
position and the supercritical process was also applied to waste oil with The effect of operating temperature on the yield of biodiesel from
higher free fatty acid content. Experiments were carried out to investi- WCO in interesterification reaction has been investigated over a temper-
gate the influence of temperature, pressure and molar ratio of reactants ature range from 30 to 50 °C under the fixed conditions of reactant
on the conversion and it has been reported that the oil composition molar ratio and catalyst loading (1:12 of OMAMR and 1 wt.% catalyst
does not significantly influence the biodiesel yield. All the oils gave concentration). The obtained results have been presented in Fig. 5
complete conversion after 50 min of experiment at 345 °C and which indicate that with an increase in temperature from 30 to 40 °C,
20 MPa pressure with oil: methyl acetate molar ratio equal to 1:42. a corresponding increase in the biodiesel yield from 78 to 88% is obtained
It is observed that significantly higher molar ratios of the reactants and a further increase in the reaction temperature from 40 to 50 °C re-
and conditions of high pressure/temperature are usually required sults only in a marginal increase in the biodiesel yield from 88 to 93%.
in supercritical methyl acetate interesterification reaction to shift The obtained results can be explained on the basis of an enhanced solu-
the equilibrium in the forward direction. bility of methyl acetate in the other phase due to increase in the reaction
A detailed comparison with literature indicates that as per the re- temperature resulting into better contact of the reactants leading to
sults obtained in the present work, WCO can be considered a potential higher yield of biodiesel from WCO. Any further increase in the temper-
good feedstock for the biodiesel manufacture. Use of ultrasound during ature has no detectable effect on the final conversion to ester which can
the interesterification reaction reduces the requirement of methyl ace- be also attributed to lower extent of cavitational effects at higher operat-
tate and also requires considerably lower treatment times as well as ing temperatures. Thus, an optimum temperature for efficient process-
milder ambient conditions, which should strongly influence the energy ing exists. It is also important to note here that higher temperatures
requirement for the overall synthesis process. also decrease the time required to reach the maximum conversion.
Casas et al. [24] reported similar results for interesterification of sunflow-
3.2. Effect of catalyst concentration er oil with methyl acetate for biodiesel and triacetin production over the
operating temperature range from 30 to 50 °C. Tan et al. [3] also reported
The effect of catalyst i.e. potassium methoxide concentration on the that the yield of biodiesel increased with an increase in the temperature
interesterification reaction of WCO has been investigated over a concen- from 340 to 420 °C at a pressure of 220 bar in the case of supercritical
tration range of 0.5% to 1.25% (by weight based on the weight of WCO). interesterification reaction of purified palm oil. The observed results
The obtained results have been depicted in Fig. 4. It can be seen from the have been explained on the basis of enhanced reactivity between methyl
figure that an increase in the catalyst concentration from 0.5% to 1% re-
sults in an increase in the biodiesel yield from 45% to 90%. Initially, an in-
sufficient amount of potassium methoxide (0.5%) results in incomplete
conversion of triglycerides into the corresponding methyl esters i.e. bio-
diesel, as indicated from its lower ester content. However, a further in-
crease in the catalyst concentration from 1 to 1.25 wt.% does not show
a significant increase in the biodiesel yield. Suppalakpanya et al. [23] re-
ported similar results for transesterification reaction of crude palm oil
where an addition of excess amount of catalyst (potassium hydroxide)
gave rise to the formation of an emulsion which in turn led to the forma-
tion of gels and a decreased percentage of ethyl ester content in the final
mixture. Formation of gel hinders the glycerol separation from the mix-
ture and hence reduces the obtained ester yield. The optimum amount
of catalyst required in the present case was established at 1 wt.% level,
which is based on the fact that a further increase in the catalyst concen-
tration will not increase the conversion and will lead to expensive cata- Fig. 5. Effect of temperature on the yield of biodiesel. [Reaction conditions: OMAMR =
lyst recovery costs. Casas et al. [24] carried out interesterification 1:12; Catalyst concentration = 1.00% (weight); Percentage amplitude = 60%].
246 G.L. Maddikeri et al. / Fuel Processing Technology 116 (2013) 241–249

acetate and triglycerides which leads to a rapid reaction rate. Beyond the that maximum energy transfer efficiency is obtained at percentage
optimum temperature, the yield remained constant as the high temper- amplitude equal to 60%.
ature had an adverse effect on the FAME which has relatively low ther- Chand et al. [25] have also reported similar results for biodiesel pro-
mal stability at extreme conditions. Campanelli et al. [12] reported that duction from soybean oil using ultrasonic irradiations. Experiments car-
the yield of FAME declined in the case of prolonged interesterification re- ried out at three levels of amplitude (60, 120, and 180 μmpp) in pulse
action at 345 °C possibly attributed to the thermal decomposition of the mode (5 s on/25 s off) indicated that highest yield was obtained with
FAME (especially the polyunsaturated ones) occurring beyond an oper- 120 μmpp amplitude. It has been reported that at the higher amplitude
ating temperature of 300 °C. Similarly, Saka el al. [14] reported less of 180 μmpp, excessive cavitation or fine bubble formation occurs at
yield of FAME at 380 °C compared with 350 °C due to the breakdown the interface between the horn and the liquid, resulting in the formation
of unsaturated fatty acids at the higher temperature. Thus, it is very im- of an intense near-field cavitation region near the horn. Suslick and
portant that an optimum operating temperature is selected and compar- Nyborg [26] describe such near-fields as having high attenuation, thus
ison of the literature has also confirmed that similar conversions have inhibiting penetration of sound waves throughout the reaction mixture,
been obtained at much lower operating temperatures in the case of ul- which may be responsible for lowering the product yield at higher
trasound assisted synthesis process. power dissipations beyond the optimum value. Mahamuni et al. [27]
have also observed similar effects of power on biodiesel yield in the
presence of ultrasound. It has been reported that the biodiesel yield in-
3.4. Effect of percentage amplitude of ultrasound creased to 92.5% in 30 min with an initial increase in the ultrasonic
power from 46 to 143 W. Kumar et al. [28] investigated the effect of
The variation in the percentage amplitude of ultrasonic horn de- ultrasonic amplitude on the enzymatic transesterification of Jatropha
cides the ultrasonic power input into the system and hence it is curcas oil for biodiesel production at four levels of 30, 40, 50 and 60%.
also an important influencing factor for the degree of intensification It has been reported that a maximum of 84% conversion was obtained
of interesterification reactions. The power input to the system de- at an ultrasound amplitude of 50% (i.e. 375 W) using a molar ratio of
cides the extent of cavitational activity in the reactor and hence the oil to methanol as 1:4 catalyst concentration as 5 wt.% of oil, and reac-
extent of biodiesel yields. The effect of percentage amplitude of ul- tion time of 30 min. Ji et al. [29] studied the effect of ultrasonic power
trasonic horn on the yield of biodiesel from WCO has been studied on biodiesel yield at three levels of 100, 150, and 200 W and found
at different values such as 40, 50, 60 and 70% corresponding to 300, that optimum conversion of 100% was obtained at the intermediate
375, 450 and 525 W respectively. The obtained experimental data power level of 150 W using the 1:6 molar ratio of soybean oil/methanol
has been presented in Fig. 6. It can be seen from the figure that the in the presence of base catalyst at 45 °C.
biodiesel yield increases from 68 to 90% with an increase in the per-
centage amplitude of ultrasonic horn from 40 to 60% (actual power
from 300 to 450 W) and no significant change is observed for a 3.5. Reaction kinetics
further increase in the percentage amplitude to 70% (actual power
of 525 W). The obtained results can be attributed to enhanced Time history of biodiesel yield from triglycerides in the reaction
cavitational effects due to an increase in the percentage amplitude. mixture at different temperatures has been shown in Fig. 5 for alcohol
Higher degree of cavitational effects also ensures sufficient mixing to oil molar ratio of 1:12, catalyst concentration of 1% and percentage
and emulsification of two immiscible reaction layers. Beyond the op- amplitude of 60% (i.e. 450 W). The obtained data for the conversion
timum percentage amplitude, the yield slightly decreased from 90 to with time have been analyzed using a second-order kinetic model to
88%, which can be attributed to the fact that at very high amplitudes, determine the rate constants at different temperatures. The consider-
higher extents of cavitation events can result in cushioning of the ation of irreversibility of this reaction is favored with an excess of meth-
cavity collapse which results in decreased energy transfer into the yl acetate that induces low concentrations of triglycerides in the
system giving lower cavitational activity and hence biodiesel yields. equilibrium composition, similar to an irreversible reaction. Fig. 8
Calorimetric study has been carried out with fixed amount of methyl shows these plots of X/(1-X) vs. time at different reaction temperatures
acetate (50 ml) measuring the time required to increase the temper- of 30, 40 and 50 °C and the obtained rate constants from these plots
ature by 15 °C. The energy transfer efficiency has been calculated have been given in Table 2. It has been observed that the rate constant
based on the input energy (percentage amplitude × rated power) for interesterification reaction increased from 0.22 to 0.93 L/(mol min)
and actual energy dissipation (m × Cp × ΔT) into the methyl acetate with an increase in the temperature from 30 to 50 °C. Similar results in
solution. The obtained results for the different percentage amplitude terms of second-order kinetic fitting has been reported by Casas et al.
from 30 to 90% are shown in Fig. 7. From Fig. 7, it can be established [24] for interesterification of sunflower oil at different temperatures.
Musavi et al. [30] also observed that rate constant increased with an

12.0

10.0
Efficiency (%)

8.0

6.0

4.0

2.0

0.0
30 40 50 60 70 80 90
Fig. 6. Effect of percentage amplitude of ultrasound on the yield of biodiesel. [Reaction % Amplitude
conditions: OMAMR = 1:12; Catalyst concentration = 1.00% (weight); Tempera-
ture = 40 °C]. Fig. 7. Calorimetric study for finding the energy efficiency at different amplitudes.
 G.L. Maddikeri et al. / Fuel Processing Technology 116 (2013) 241–249 247

increase in the temperature of interesterification reaction of soybean oil Table 2

over the temperature range of 10–80 °C. The reaction rate for biodiesel Reaction rate constant and activation energy of interesterification reaction.

synthesis is also increased due to enhanced interfacial area between oil Temperature Rate constant (k) Correlation factor Activation Energy
and methyl acetate phase, due to cavitational effects, mainly in terms of (°C) (L/mol min) R2 (J/mol)
the intense levels of turbulence and mixing generated in the reactor. 30 0.22 0.987 58,170
The physical effects contribute to intensification because of the genera- 40 0.66 0.978
tion of microemulsions between the two phases taking part in the reac- 50 0.93 0.998
tion, and hence the available interfacial area between the reactants
increases enormously, giving faster reaction rates and the requirement
of less-severe conditions, in terms of the operating temperature [31]. from ultrasound (155 °C) has been found to be lower than that obtained
The activation energy was also obtained using rate constant using the conventional method (190 °C) which can be attributed to
data for the controlled temperature experiments at 30, 40, and higher purity of the final product. Flash point of biodiesel obtained in
50 °C. The obtained result is shown in Fig. 9, with the activation the present study is also less as compared to biodiesel produced from
energy calculated to be 58.170 kJ/mol and the pre-exponential other sources (Karanja (230 °C) [32] and Madhuca indica (208 °C) [33]).
factor found to be 2.67 × 109 L/(mol min). These values of activa- Based on these properties, the superiority of the ultrasound assisted syn-
tion energy for interesterification of WCO using ultrasound are thesis can be clearly established.
in the same range as those reported by Casas et al. [24] in the
study of kinetics of sunflower oil interesterification. The value of 4. Comparison of acoustic cavitation with conventional stirring
activation energies ranged from 24.318 to 88.551 kJ/mol for the
sunflower oil interesterification. The resulting Arrhenius expres- The optimized parameters for the interesterification of waste
sion for biodiesel synthesis using ultrasound based on current cooking oil have been established as OMAMR of 1:12, catalyst concen-
work is given by tration of 1% and temperature of 40 °C. The comparison of the obtained
results using conventional method and ultrasound assisted technique
k ¼ 2:67  10  eð RT Þ
−58:170
9
under these optimized conditions has been presented in Fig. 10. It
can be seen that the biodiesel yield is higher for ultrasonic technique
Mahamuni and Adewuyi [27] carried out transesterification of soybean oil (90%) as compared to conventional method (70%). The obtained superi-
using high frequency ultrasound and reported that ultrasonication pro- ority of ultrasound assisted approach can also be confirmed from litera-
vides an effective way to attain the required mixing while providing the ture illustrations. Casas et al. [6] carried out interesterification of
necessary activation energy. Campanelli et al. [12] synthesized biodiesel sunflower oil using conventional approach and reported maximum bio-
from edible, non-edible and waste cooking oils via supercritical methyl diesel yield of 76.7% under conditions of molar ratio of 1:50 and, catalyst
acetate transesterification and reported that activation energies depend to oil molar ratio 0.2 (~1.04 wt.%) at 50 °C. The current work has shown
on the vegetable oil and the employed supercritical alcohol; a value of much higher conversions for the interesterification reaction at much
56 kJ/mol was found for supercritical methanol transesterification of soy- lower excess of reactants which also should reduce the separation
bean oil over the temperature range of 240 to 280 °C at 28 MPa, with oil: loads. The higher conversion obtained due to the use of ultrasonic irra-
methanol molar ratio equal to 1:42. It was also reported that apparent diations is attributed to the physical effects of the cavitation phenome-
activation energies for the supercritical methyl acetate process are higher na. It is important to note here that only the physical effects in terms of
showing a lower reactivity of this solvent compared to methanol. intense turbulence and microstreaming generated during the cavitation
plays a dominating role in intensification. The higher biodiesel produc-
3.6. Biodiesel properties tion obtained using ultrasonic irradiations is due to local turbulence,
liquid micro-circulation (acoustic streaming) and micro-emulsion for-
Biodiesel properties i.e. density, flash point, kinematic viscosity, and mation giving enhanced areas for reaction. Because of the generation
acid value of the final dried product obtained using both methods i.e. of microemulsions between the two phases taking part in the reaction,
conventional as well as ultrasound assisted synthesis are given in the the available interfacial area between the reactants increases enor-
Table 3. It is observed that the properties of biodiesel obtained by both mously, giving faster reaction rates and the requirement of less-severe
methods match with the ASTM D 6751 standards. The flash point is conditions, in terms of the operating temperature and also lowering
one of the key parameters for biodiesel, which indicate the working fea- the degree of excess reactants for similar yields.
sibility and engine performance and depends on the quality of separa- Similar superiority of use of ultrasound has been observed in the
tion of the unreacted triglyceride from the final product, fatty acid literature for the transesterification reactions though not much illus-
methyl esters and triacetin. The flash point of the biodiesel synthesized trations can be obtained for the interesterification reaction. A few

0.00

-0.40
ln(k)

-0.80

-1.20 y = -6972.x + 21.55

R² = 0.991

-1.60
1/T (1/kelvin)

Fig. 8. Kinetic study for establishing the rate constants. Fig. 9. Arrhenius plot for estimation of activation energy.
248 G.L. Maddikeri et al. / Fuel Processing Technology 116 (2013) 241–249

Table 3 5. Conclusions
Biodiesel properties.

Property Conventional Ultrasound ASTM D6751 The present work has established the utility of ultrasound assisted
interesterification process for the production of biodiesel and triacetin
Density, g/cm3 0.8637 0.871 0.86–0.9
Kinematic viscosity, 40 °C, mm2/s 3.64 3.78 1.9–6.0 from WCO. The process can be a useful alternative to the more com-
Flash point, °C 190 155 130 min monly used transesterification as it generates triacetin instead of glycer-
Acid value, mg of KOH/g of oil 0.71 0.28 0.5 max ol which is an important additive to improve the properties of biodiesel
particularly in cold conditions. The presented results are very important
in the current context as it has focused on one of the cheaper synthesis
illustrative examples have been discussed here just to give some de- routes for biodiesel production based on the use of waste oils with
gree of confidence for the use of ultrasound based synthesis approach. significant degree of intensification using ultrasound. Higher biodiesel
Babajide et al. [34] conducted transesterification of waste cooking oil yield has been observed in the case of ultrasonic technique as compared
using ultrasonic homogenizer with 20 kHz frequency and power output to the conventional method and also the properties of the synthesized
of 400 W at different catalyst loadings and reaction times. It has been re- biodiesel are better. The excess of reactants required is also lower as
ported that ultrasonic homogenization proved suitable for large-scale compared to the conventional routes and supercritical oxidation route
processing of waste cooking oils resulting in a better yield of 90% and which gives benefits in terms of lower separations costs. The work has
higher conversion efficiency of 98%. The effective mass transfer in the confirmed for the first time that significant process intensification
ultrasonic field enhanced the rate of transesterification reaction com- is obtained for the synthesis of biodiesel based on the interesterification
pared to mechanical mixing (stirring conditions). Hingu et al. [35] inves- approach. To summarize, the main advantages offered due to the use of
tigated the transesterification of used frying oil using low-frequency sonochemical reactors, as revealed by optimization studies related to
ultrasonic reactor (20 kHz) and conventional stirring approach based the various operating parameters such as molar ratio, catalyst concen-
on the use of a six-blade turbine with diameter of 1.5 cm operating at tration, and reaction temperature, are a reduced reaction temperature
1000 rpm. It has been reported that acoustic cavitation results in 89.5% (savings in the energy required for heating of the process streams), a
conversion whereas the conventional stirring method results in much lower reaction time (energy savings in reactor), and the requirement
lower extent of conversion (57.5%) over similar time of operation as of a smaller amount of excess methyl acetate for equivalent levels
40 min. The obtained results have been attributed to the fact that as of equilibrium conversion (considerable energy savings in the separa-
the reaction is mass transfer controlled, the micro-level turbulence gen- tion units). The kinetics of the conversion of triglycerides through
erated due to the oscillation and collapse of cavitational bubbles results interesterification follows a second-order reaction scheme and it was
into the higher interfacial area and hence higher conversion. observed that the rate constant increased with an increase in the oper-
Another advantage offered by the use of the ultrasound approach is ating temperature. The properties of biodiesel produced from both
in terms of the requirement of lower excess of methyl acetate (lower methods match the ASTM standards with superior properties for the ul-
molar ratios) for achieving a similar or higher level of conversions. A trasound assisted approach. Overall, it can be said that use of ultrasonic
lower requirement of excess methyl acetate will certainly reduce the irradiations considerably enhances the rates of biodiesel synthesis and
energy requirements for the overall process, because methyl acetate would also lead to a substantial energy savings, because of various pro-
separation using distillation is a significantly energy-intensive opera- cess improvements such as use of lower temperature and lower excess
tion, controlling the overall economics of the biodiesel synthesis pro- of reactants, as observed in the present work.
cess. The ultrasound-based process also offers easy purification of the
product and the final product properties are more suitable, compared
to the conventional approach as shown in Table 3. The energy require- Acknowledgment
ment for conventional method and ultrasonic technique is 22,154 and
4780 kJ/kg respectively. The overall production costs of the fatty acid GLM is thankful to the University Grants Commission, Government
methyl esters depend strongly on the total energy requirement for the of India for the financial assistance. GLM would also like to acknowledge
process in addition to the cost of raw materials. Use of sustainable the help of B.N. Rekha in the preparation of the manuscript.
feed stock in terms of WCO indeed gives an advantage and the use of ul-
trasonic technique also gives additional advantage in terms of the lower
energy requirements and operations at ambient conditions. References

[1] C.Y. Lin, H.A. Lin, L.B. Hung, Fuel structure and properties of biodiesel produced by
the peroxidation process, Fuel 85 (2006) 1743–1749.
[2] Z. Yang, W. Xie, Soybean oil transesterification over zinc oxide modified with alkali
earth metals, Fuel Processing Technology 88 (2007) 631–638.
[3] K.T. Tan, K.T. Lee, A.R. Mohamed, Prospects of non-catalytic supercritical methyl acetate
process in biodiesel production, Fuel Processing Technology 92 (2011) 1905–1909.
[4] S.H. Liu, Y.C. Lin, K.H. Hsu, Emissions of regulated pollutants and PAHs from
waste-cooking-oil biodiesel-fuelled heavy-duty diesel engine with catalyzer, Aerosol
and Air Quality Research 12 (2012) 218–227.
[5] M.C. Math, S.P. Kumar, S.V. Chetty, Technologies for biodiesel production from used
cooking oil — A review, Energy for Sustainable Development 14 (2010) 339–345.
[6] A. Casas, M.J. Ramos, A. Perez, New trends in biodiesel production: chemical
interesterification of sunflower oil with methyl acetate, Biomass and Bioenergy 35
(2011) 1702–1709.
[7] A. Casas, J.R. Ruiz, M.J. Ramos, A. Perez, Effects of triacetin on biodiesel quality, Energy
& Fuels 24 (2010) 4481–4489.
[8] E.M. Usai, E. Gualdi, V. Solinas, E. Battistel, Simultaneous enzymatic synthesis of
FAME and triacetyl glycerol from triglycerides and methyl acetate, Bioresource
Technology 102 (2010) 7707–7712.
[9] Y. Xu, W. Du, D. Liu, Study on the kinetics of enzymatic interesterification of triglyc-
erides for biodiesel production with methyl acetate as the acyl acceptor, Journal of
Fig. 10. Comparison of biodiesel yield in the ultrasound assisted and conventional methods. Molecular Catalysis B: Enzymatic 32 (2005) 241–245.
[Reaction conditions: Molar ratio (OMAMR) = 1:12; Catalyst concentration = 1.00% (by [10] W. Du, Y. Xu, D. Liu, J. Zeng, Comparative study on lipase-catalyzed transformation
weight); Temperature = 40 °C; Ultrasound method: Percentage amplitude = 60%; Con- of soybean oil for biodiesel production with different acyl acceptors, Journal of
ventional method: stirrer speed = 1000 rpm]. Molecular Catalysis B: Enzymatic 30 (2004) 125–129.
 G.L. Maddikeri et al. / Fuel Processing Technology 116 (2013) 241–249 249

[11] K.T. Tan, K.T. Lee, A.R. Mohamed, A glycerol-free process to produce biodiesel by [23] K. Suppalakpanya, S. Ratanawilai, C. Tongurai, Production of ethyl ester from esterified
supercritical methyl acetate technology: an optimization study via response surface crude palm oil by microwave with dry washing by bleaching earth, Applied Energy 87
methodology, Bioresource Technology 101 (2010) 965–969. (2010) 2356–2359.
[12] P. Campanelli, M. Banchero, L. Manna, Synthesis of biodiesel from edible, non-edible [24] A. Casas, M.J. Ramos, A. Parez, Kinetics of chemical interesterification of sunflower
and waste cooking oils via supercritical methyl acetate transesterification, Fuel 89 oil with methyl acetate for biodiesel and triacetin production, Chemical Engineering
(2010) 3675–3682. Journal 171 (2011) 1324–1332.
[13] S. Saka, Y. Isayama, Z. Ilham, X. Jiayu, New process for catalyst-free biodiesel production [25] P. Chand, V. Reddy Chintareddy, J.G. Verkade, D. Grewell, Enhancing biodiesel pro-
using subcritical acetic acid and supercritical methanol, Fuel 89 (2010) 1442–1446. duction from soybean oil using ultrasonics, Energy & Fuels 24 (2010) 2010–2015.
[14] S. Saka, Y. Isayama, A new process for catalyst-free production of biodiesel using su- [26] K.S. Suslick, W.L. Nyborg, Ultrasound: its chemical, physical and biological effects,
percritical methyl acetate, Fuel 88 (2009) 1307–1313. The Journal of the Acoustical Society of America 87 (1990) 919–920.
[15] L.C. Meher, D. Vidya Sagar, S.N. Naik, Technical aspects of biodiesel production by [27] N.N. Mahamuni, Y.G. Adewuyi, Application of Taguchi method to investigate the
transesterification—a review, Renewable and Sustainable Energy Reviews 10 (2006) effects of process parameters on the transesterification of soybean oil using high
248–268. frequency ultrasound, Energy & Fuels 24 (2010) 2120–2126.
[16] J.M. Cervero, J. Coca, S. Luque, Production of biodiesel from vegetable oils, Grasas y [28] G. Kumar, D. Kumar, Poonam, R. Johari, C.P. Singh, Enzymatic transesterification of
Aceites 59 (2008) 76–83. Jatropha curcas oil assisted by ultrasonication, Ultrasonics Sonochemistry 18 (2011)
[17] G.L. Maddikeri, A.B. Pandit, P.R. Gogate, Intensification approaches for biodiesel 923–927.
synthesis from waste cooking oil: a review, Industrial and Engineering Chemistry [29] J. Ji, J. Wang, Y. Li, Y. Yu, Z. Xu, Preparation of biodiesel with the help of ultrasonic
Research 51 (2012) 14610–14628. and hydrodynamic cavitation, Ultrasonics 44 (2006) 411–414.
[18] G.L. Maddikeri, A.B. Pandit, P.R. Gogate, Adsorptive removal of saturated and unsat- [30] A. Musavi, A. Tekin, M. Kaya, I. Sanal, Interesterification kinetics of soybean oil, Journal
urated fatty acids using ion-exchange resins, Industrial and Engineering Chemistry of Food Lipids 10 (2003) 277–284.
Research 51 (2012) 6869–6876. [31] V.G. Deshmane, P.R. Gogate, A.B. Pandit, Ultrasound assisted synthesis of isopropyl
[19] A. Kalva, T. Sivasankar, V.S. Moholkar, Physical mechanism of ultrasound-assisted syn- esters from palm fatty acid distillate, Ultrasonics Sonochemistry 16 (2009) 345–350.
thesis of biodiesel, Industrial and Engineering Chemistry Research 48 (2009) 534–544. [32] P.K. Srivastava, M. Verma, Methyl ester of karanja oil as an alternative renewable
[20] B. Freedman, R.O. Butterfield, E.H. Pryde, Transesterification kinetics of soybean oil source energy, Fuel 87 (2008) 1673–1677.
1, Journal of the American Oil Chemists' Society 63 (1986) 1375–1380. [33] H. Raheman, S.V. Ghadge, Performance of compression ignition engine with mahua
[21] P.C. Narváez, S.M. Rincón, F.J. Sánchez, Kinetics of palm oil methanolysis, Journal of (Madhuca indica) biodiesel, Fuel 86 (2007) 2568–2573.
the American Oil Chemists' Society 84 (2007) 971–977. [34] O. Babajide, L. Petrik, B. Amigun, F. Ameer, Low-cost feedstock conversion to biodiesel
[22] P.R. Gogate, Cavitational reactors for process intensification of chemical processing via ultrasound technology, Energies 3 (2010) 1691–1703.
applications: a critical review, Chemical Engineering and Processing Process Intensi- [35] S.M. Hingu, P.R. Gogate, V.K. Rathod, Synthesis of biodiesel from waste cooking oil
fication 47 (2008) 515–527. using sonochemical reactors, Ultrasonics Sonochemistry 17 (2010) 827–832.