DSpace at VNU: A two-step continuous ultrasound assisted production of biodiesel fuel from waste cooking oils: A practical and economical approach to produce high quality biodiesel fuel

Bioresource Technology 101 (2010) 5394–5401

Contents lists available at ScienceDirect

Bioresource Technology
journal homepage: www.elsevier.com/locate/biortech

A two-step continuous ultrasound assisted production of biodiesel fuel
from waste cooking oils: A practical and economical approach to produce
high quality biodiesel fuel
Le Tu Thanh a,b,*, Kenji Okitsu c, Yasuhiro Sadanaga a, Norimichi Takenaka a, Yasuaki Maeda a,
Hiroshi Bandow a
a

Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
Faculty of Environmental Sciences, University of Science, Vietnam National University, Ho Chi Minh City, 227 Nguyen Van Cu St., Dist. 5, Ho Chi Minh City, Vietnam
c
Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
b

a r t i c l e

i n f o

Article history:
Received 28 December 2009
Received in revised form 9 February 2010
Accepted 11 February 2010
Available online 9 March 2010
Keywords:
Biodiesel production

Transesterification
Waste cooking oils
Ultrasonic reactor

a b s t r a c t
A transesterification reaction of waste cooking oils (WCO) with methanol in the presence of a potassium
hydroxide catalyst was performed in a continuous ultrasonic reactor of low-frequency 20 kHz with input
capacity of 1 kW, in a two-step process. For the first step, the transesterification was carried out with the
molar ratio of methanol to WCO of 2.5:1, and the amount of catalyst 0.7 wt.%. The yield of fatty acid
methyl esters (FAME) was about 81%. A yield of FAME of around 99% was attained in the second step with
the molar ratio of methanol to initial WCO of 1.5:1, and the amount of catalyst 0.3 wt.%. The FAME yield
was extremely high even at the short residence time of the reactants in the ultrasonic reactor (less than
1 min for the two steps) at ambient temperature, and the total amount of time required to produce biodiesel was 15 h. The quality of the final biodiesel product meets the standards JIS K2390 and EN 14214 for
biodiesel fuel.
Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction
Biodiesel, a liquid fuel consisting of mono-alkyl esters of longchain fatty acids derived from vegetable oils or animal fats, can
be used as a substitute for diesel fuel (Hu et al., 2004; Veljkovíc
et al., 2006). Some of the advantages of using biodiesel fuel are
its renewability, easy biodegradability, non-toxicity and safer handling due to its higher flash point compared to those of fossil fuels
(Wang et al., 2006). In addition, biodiesel fuel is also primarily free
of sulfur and aromatics, producing more tolerable exhaust gas
emissions than conventional fossil diesel (Demirbas, 2009).
Biodiesel produced from virgin vegetable oils costs much more
than petro-diesel; this is a major drawback to the commercialization of biodiesel in the market. Therefore, it is necessary to find the
ways to minimize the production cost of biodiesel. In this context,
methods that can reduce the costs of raw materials as well as the
energy consumption are of special concern. The use of waste cooking oils (WCO) is one of the more attractive options to reduce the
raw material cost (Encinar et al., 2005; Kulkarni and Dalai, 2006).

* Corresponding author. Address: Department of Applied Chemistry, Graduate
School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai,
Osaka 599-8531, Japan. Tel./fax: +81 72 254 9326.
E-mail address: (L.T. Thanh).
0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biortech.2010.02.060

Biodiesel is synthesized by the transesterification of triglycerides (TG), the main components of vegetable oils and animal fats,
with mono-alcohol in the presence of a catalyst, into fatty acid alkyl esters. The TG is converted stepwise to diglycerides (DG),
monoglyceride (MG) intermediates and finally to glycerin (GL)
(Darnoko and Cheryan, 2000).
The transesterification can be carried out in batch or continuous
reactors (Meher et al., 2006a; West et al., 2008; Zhang et al., 2003).
The batch transesterification process requires large reactors and
longer reaction and separation times because the reaction and the
separation stages are usually carried out in the same tank. In contrast, the reactor for the continuous process can be smaller than that
of the batch process for the same production capacity. Several types
of continuous reactors have been studied and applied for biodiesel
production (Lertsathapornsuk et al., 2008; Zhang et al., 2010). On
the laboratory scale, continuous reactor systems assisted by microwave have been demonstrated. Other continuous-flow processes
using a rotating packed bed, supercritical methanol or gas–liquid
reactor have been found to be more effective for the transesterification (Chen et al., 2010; He et al., 2007; Behzadi and Farid, 2009).
It is believed that the transesterification of TG with methanol is
an equilibrium reaction system. Therefore, the equilibrium can be
shifted to the right, i.e., the formation of FAME, by performing a
multi-step transesterification processes. To minimize the influence


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L.T. Thanh et al. / Bioresource Technology 101 (2010) 5394–5401

of glycerin on the back reaction, the glycerin in the reaction mixture should be taken out after each step.
Over the past two decades, applications of sonochemistry have
been widely developed in many areas of chemical technologies.
Ultrasound energy is well known as a useful tool to make fine
emulsions from immiscible liquids. Owing to this aspect, the
transesterification reaction of vegetable oil and alcohol can reach
equilibrium in a short reaction time with a high yield of alkyl esters
even at low temperatures (Stavarache et al., 2003, 2006; Hanh
et al., 2008; Georgogianni et al., 2008a, 2009). In the previous work,
a pilot plant using ultrasound irradiation method for biodiesel production from canola oil and methanol was developed by our group
(Thanh et al., 2010), in which the transesterification was carried
out by a circulation process at room temperature. The high yield
of FAME can be obtained even in a short reaction time under the
molar ratio of methanol to oil 5:1, and potassium hydroxide
(KOH) catalyst, at 0.7 wt.%.
In this work, the transesterification of WCO with methanol in
the presence of KOH catalyst was carried out in the continuous
ultrasonic reactor by a two-step process. The effects of the residence time of reactants in the reactor, molar ratio of methanol to
WCO and separation time of glycerin from the reaction mixture
in each step were investigated. The objective of this work is to produce biodiesel of high quality meeting the specifications of the
standard for B100 (pure 100% biodiesel) fuel with minimal costs
of materials and energy.

2. Methods
2.1. Materials
The WCO used were those after domestic use, collected by municipal activities, and then filtered and settled in a drum to remove
particles remaining in the oils. The physical and chemical properties of WCO are shown in Table 1. KOH (grade 95.5%) and methanol

(grade 99%) were purchased from Wako Pure Chemical Industries,
Osaka, Japan, and used without further purification. Chemical standards such as methyl oleate, methyl linoleate, methyl linolenate,
methyl palmitate, methyl stearate, monoolein, diolein, and triolein,
were obtained from Sigma–Aldrich, Tokyo, Japan.
2.2. Apparatus
The major units of the pilot plant include the liquid pumps, flow
meters and ultrasonic reactors with a working volume of 0.8 L, and
separation and purification tanks. An ultrasound source was a horn
type transducer generating low-frequency ultrasounds of 20 kHz
with an input capacity of 1 kW. The experimental setup for the
transesterification and purification of crude biodiesel using the pilot plant is schematically depicted in Fig. 1. This system was described in more detail in the previous paper (Thanh et al., 2010).
2.3. Procedures
KOH was pre-mixed with a known amount of methanol adapted
to each experiment and kept at ambient temperature (20–25 °C).
In the first step of the transesterification, 120 L of WCO was fed
with methanol, in the desired molar ratios 2.5:1, 3:1, 3.5:1 or
4:1, to the reactor. The feeding of WCO and methanol was carried
out by piston and peristaltic pumps, respectively, and both were
connected to flow meters to control the mixing ratio of the reactants accurately. The flow rate of the reaction mixture was set in
the range of 0.5–2.5 L minÀ1. After passing through the reactor,
the reaction mixture was transferred to the separation tank, where
the transesterification and phase separation of glycerin from the
reaction mixture proceeded simultaneously. It took 4 h to complete the phase separation. The lower layer, containing glycerin,
catalyst and excess methanol, was drained from the separation
tank. On the other hand, the upper layer, mainly FAME, TG and
small amounts of DG and MG, was used for the second-step
transesterification.

Table 1
Chemical and physical properties of WCO used in this study. (five samples were

analyzed, n = 5).
Properties

À3

Density
Acid value
Iodine value
Water content
Oleic acid (C18:1)a
Linoleic acid (C18:2)a
Linolenic acid (C18:3)a
Palmitic acid (C16:0)a
Stearic acid (C18:0)a
Other fatty acids
Mean molecular weight of WCO
a
b

Average ± SDb

Unit
g cm
mg KOH/g oil
g I2/100 g oil
mg gÀ1
wt.%
wt.%
wt.%
wt.%

wt.%
wt.%
g molÀ1

0.918 ± 0.002
1.07 ± 0.10
112.5 ± 0.5
0.15 ± 0.03
47.02 ± 0.51
31.42 ± 0.48
10.21 ± 0.18
7.42 ± 0.44
2.77 ± 0.21
1.15 ± 0.16
876.60 ± 15.76

Carbon atoms number: double bond number.
SD: one standard deviation of five samples.

V

US2

US1

S1
P

V

S2

F

P

V

W1

P’

F

M2

M1

V
V

O
P

V

F

G1

P

V

F

V

V

V

P
G2

B

O: Oil tank; M1, M2: Methanol and catalyst tanks; P: Liquid pumps; V: Valves; F: Flow meters
US1, US2: Ultrasonic reactors; S1, S2: Separation tanks; G1, G2: Glycerin tanks
P’: Purification tank; B: Biodiesel product tank; W1, W2: fresh and waste water tanks
Fig. 1. Flow diagram of ultrasound assisted continuous reactor for biodiesel production at the pilot plant.

W2


L.T. Thanh et al. / Bioresource Technology 101 (2010) 5394–5401

The second-step transesterification was performed in the same
manner as the first step, except that the molar ratios of methanol
to initial WCO that is 1:1, 1.5:1 or 2:1 for the second step. After

the transesterification and the phase separation were completed,
the crude FAME was transferred to the purification tank. Here,
the KOH catalyst, excess methanol and glycerin remaining in the
crude FAME were removed by washing three times with tap water
of the ratio of 20% by weight to crude FAME for each washing. After
washing, the water content in the FAME was effectively eliminated
by heating the FAME to 70 °C under reduced pressure around
500 torr while flushing with a small amount of dried air for 3 h.
All experiments were performed at ambient temperature of 20–
25 °C. After passing through the reactor, the temperatures of the
reaction mixtures were in the range of 30–32 °C and 27–29 °C for
the first and second steps, respectively, due to the heating effect
of the ultrasound.
2.4. Analysis
A 200 mL sample of the reaction mixture was withdrawn from
the pipe connecting the ultrasonic reactor and the separation tank,
and the sample was stored in a 250 mL beaker. The time zero of the
reaction was defined when the reactants, including WCO, methanol and KOH were introduced to the ultrasonic reactor. Five milliliters of samples were taken from the beaker in prescribed time
intervals and were immediately neutralized by the addition of
1 mL of 5% phosphoric acid aqueous solution to stop the reaction.
The samples were left to settle for 3 h for phase separation before
analysis of the samples. The concentrations of the reactants such as
TG, DG, MG and FAME, were quantified by a high performance liquid chromatograph connected to a refractive index detector. The
analytical method employed in this study is described in more detail in the previous paper (Thanh et al., 2010).
The FAME yields of each transesterification step were calculated
from the weight of FAME in the FAME phase and the theoretical
material balance of the transesterification reaction, as shown in
Eq. (1):

wFAME =MFAME

FAME yield ð%Þ ¼
 100;
3wWCO =MWCO

ð1Þ

where wFAME and wWCO are the weight of FAME in the FAME phase
and the weight of WCO used, respectively, MFAME and MWCO are the
average molecular weights of the FAME and the WCO, respectively,
and the factor 3 indicates that one mole of triglyceride yields three
moles of FAME.
The amount of the glycerin phase obtained from phase separation was determined by the gravity method and was calculated by
Eq. (2):

GL ðwt:%Þ ¼

wGL
 100;
wm

(FFA) content and water, the acid-catalyst transesterification process is preferable. However, this process requires higher temperatures and longer reaction times, in addition to causing undesired
corrosion of the equipment. Therefore, to reduce the reaction time,
the process with an acid-catalyst is adapted as a pretreatment step
only when necessary to convert FFA to esters, and is followed by an
alkaline-catalyst addition for the transesterification step to transform triglycerides to esters (Leung and Guo, 2006). In contrast,
when the FFA content in the oils is less than 1 wt.%, many researchers have recommended that only an alkaline-catalyst assisted process should be applied because this process requires fewer and
simpler equipment than that mentioned above (Meher et al.,
2006b; Freedman et al., 1984). Among alkaline-catalysts, sodium
and potassium hydroxide have most often been used in industrial
biodiesel production, both in the concentration range from 0.4 to

2 wt.% of the oil (Meher et al., 2006b). Encinar et al. (2005) studied
the effects of alkaline-catalyst types, such as sodium hydroxide,
potassium hydroxide, sodium alkoxide and potassium alkoxide,
on the methanolysis of WCO. They concluded that the best yield
of methyl esters was obtained at KOH concentration of 1 wt.%. In
our previous work, the transesterification of canola oil, containing
0.4 wt.% of FFA, with methanol, was assisted by ultrasound irradiation in the circulation process. The optimal FAME yield was observed at a KOH concentration of 0.7 wt.% (Thanh et al., 2010). In
another previous study, the transesterification of WCO containing
1.7 wt.% of FFA was conducted with the same system mentioned
above. The best yield of FAME was attained when the amount of
KOH catalyst was 1.0 wt.% (Thanh et al., 2008).
Generally, as noted above, KOH is an effective catalyst for the
transesterification, and as such, it was chosen for this study. As
shown in Table 1, the acid value of the WCO was 1.07, corresponding to FFA 0.54 wt.%. Based on the previous work, the total KOH
concentration of 1.0 wt.%, i.e. 0.7 and 0.3 wt.% for the first and
the second steps, respectively, was conservatively used for all of
the transesterification experiments on the WCO.
3.2. Effect of flow rate
In the continuous reactor, the flow rate is one of the most
important parameters affecting the reaction yield. Lower flow rates
lead to longer residence times of the reaction mixture in the reactor. One could expect a low flow rate to enhance the emulsification
efficiency of the reactants, resulting in increased FAME yield. In the
present study, with the reactor volume of 0.8 L, the flow rates were

100
95

ð2Þ

where wGL and wm were the weights of the glycerin phase and the

reaction mixture, respectively. The weight of the reaction mixture
was the sum of the weights of the raw materials, including the
WCO, methanol and catalyst used for the transesterification.
In this study, each experiment used 120 L of WCO; thus, a limited number of experiments were performed in triplicate, and the
results are shown as average values with one standard deviation.
3. Results and discussion

FAME yield/ %

5396

90
85
80
75
70
0.0

First-step
Second-step
0.5

1.0

1.5

2.0

2.5

-1

Flow rate/ L min
3.1. Choice of type and amount of catalyst
The choice of a catalyst for the transesterification depends on
the quality of raw materials. If the oils have high free fatty acid

Fig. 2. Effect of flow rate on the FAME yields for methanolysis of WCO in the
continuous ultrasonic reactor. The molar ratio of methanol to WCO and the amount
of KOH catalyst were (2.5:1 and 0.7 wt.%), and (1:1 and 0.3 wt.%) for the first and
second steps, respectively.


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L.T. Thanh et al. / Bioresource Technology 101 (2010) 5394–5401

90
80
70
FAME yield/ %

varied from 0.5 to 2.5 L minÀ1, corresponding to residence times of
the reactants in the reactor from 1.60 to 0.32 min. In the first step,
the molar ratio of methanol to WCO and the catalyst amount were
2.5:1 and 0.7 wt.%, respectively. When the first step was completed
and phase separation accomplished, the FAME phase was used for
the second step. In the second step, the molar ratio of methanol to
initial WCO and the catalyst amount were 1:1 and 0.3 wt.%, respectively, added to the FAME phase. As shown in Fig. 2, the FAME yield
value increased, from 72.3% to 81.0% and from 95.3% to 97.5% for

the first and second steps, respectively, as the flow rate decreased
from 2.5 to 0.5 L minÀ1. The maximum FAME yields were 81.0%
and 97.5%, which were obtained at the flow rates less than 1.5
and 2.0 L minÀ1 for the first and second steps, respectively. Even
with a short residence time of 0.93 min and a small molar ratio
of methanol to WCO of 3.5:1, for the sum of the two steps, the
FAME yield was 97.5%. As demonstrated in the literature (Stavarache et al., 2007; Georgogianni et al., 2008b; Ramachandran
et al., 2006), ultrasonic irradiation is a tremendously useful tool
for forming fine emulsions of immiscible liquids.

60
50
40
30
20
10

2.5:1

3:1

3.5:1

4:1

0
0

10

20
30
Time/ min

40

50

Fig. 3a. Effect of molar ratio of methanol to WCO on the FAME yield in the presence
of KOH catalyst 0.7 wt.% for the first step of the transesterification.

3.3. Effect of the molar ratio of methanol to WCO

3.3.1. The first transesterification step
The first step of transesterification was conducted with molar
ratios of methanol to WCO in the range from 2.5:1, 3.0:1, 3.5:1
or 4:1 in the presence of KOH 0.7 wt.% of WCO. The flow rate of
the reactants was fixed at 1.5 L minÀ1, corresponding to a residence
time of 0.53 min. After passing through the reactor, the reaction
mixture became a fine emulsion, and thus the reaction proceeded
efficiently. After 10 min of reaction time, the yield of FAME reached
about 80% for all cases, and thus the reaction mixture had become
homogeneous. To determine the amount of time required to reach
equilibrium and the yield of FAME during the experiments, the
reaction mixture was analyzed for FAME content at every sampling
interval. As shown in Fig. 3a at the initial 5 min of the reaction
time, the conversion rate of FAME was found to be faster at the
lower molar ratios of methanol to WCO. This can be explained by

FAME

Product composition/ wt.%

The molar ratio of methanol to oil is also other the important
factor affecting the yield of FAME. Although the molar ratio of
methanol to oil necessary to complete the transesterification is
3:1, an excess amount of methanol is helpful to shift the reaction
toward the FAME formation. Thus in practice, the molar ratio of
methanol to oil used is usually more than 6:1.
Because methanol and oil are immiscible liquids, the transesterification reaction occurs on the interface between the oil and the
methanol. As a result, only methanol on the surface of droplets is
effective for the transesterification reaction if there is droplet formation in the reaction mixture, whether the reaction takes place
using the conventional stirring method or the ultrasound assisted
method such as the present study. Additionally, glycerin is also
formed as a by-product. Because glycerin and methanol are polar
compounds, they can dissolve each other at any ratio. Therefore,
the presence of glycerin absorbs significant amounts of methanol,
requiring large amounts of methanol for the transesterification.
However, the use of large amounts of excess methanol has adverse
effects on the phase separation of glycerin and FAME, and increases
the energy and time consumption for the recovery of excess methanol. Moreover, as mentioned above, using the ultrasonic reactor
reactants form a fine emulsion, which increases the interface area
between methanol and oil. Therefore, in this case, the rate of the
transesterification can be enhanced, and it can reduce the amount
of excess methanol required. Overall, to enhance the effectiveness
of methanol, the transesterification was carried out by a two-step
process. A proper amount of methanol was used, and the glycerin
and excess methanol were removed after each step.

MG

DG

TG

90
80
70
60
50
40
30
20
10
0
2.5:1

3:1
3.5:1
4:1
Molar ratio of methanol to WCO

Fig. 3b. Composition of products in the FAME phase of the first step of the
transesterification of WCO with various molar ratios in the presence of KOH catalyst
0.7 wt.% after the reaction and phase separation were completed.

the fact that the concentration of catalyst was lower in the cases
with larger amounts of methanol because the same amount of catalyst was used based on the amount of WCO. As reported by Vicente et al. (2004), the transesterification is initiated by attacks of
methoxide ions (CH3OÀ) on the carbonyl carbon atoms of TG, DG
and MG molecules. Because the KOH catalyst is a strong base, its

dissociation constant is very large. Therefore, higher concentrations of methoxide ions on the surface of the methanol droplets
were obtained when lower molar ratios were used. As a result,
the lower molar ratios of methanol to WCO increased the reaction
rate during initial stage of the reaction. This result agrees with the
previous work, where the same reactor was used in the circulation
process (Thanh et al., 2010). However, after 5 min of the reaction,
higher conversion of FAME was achieved when higher molar ratios
were employed. The equilibrium state of the reaction was reached
at 25, 30, and 40 min with the molar ratios of 2.5:1, 3:1, and 4:1,
respectively.
As shown in Fig. 3a, when the molar ratio of methanol to WCO
increased from 2.5:1 to 4:1, the yield of FAME increased from 81.0%
to 90.1%. Although the addition of methanol was increased significantly by 60%, the yield of FAME increased only by 10%. This result
can be explained as follows: methanol and glycerin are structurally
similar molecules, containing hydroxyl groups, which can easily
stimulate the intermolecular H-bonding between glycerin and
methanol, and thus dissolve each other well. Therefore, even


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L.T. Thanh et al. / Bioresource Technology 101 (2010) 5394–5401

though excess methanol is added to the reaction mixture, larger
proportions of the excess methanol could be removed from the
reaction zone by dissolution into the glycerin phase once the glycerin phase has formed during the transesterification reaction. In
other words, a very limited portion of the methanol added could
act as the reactant for the transesterification. This phenomenon
may be the reason why the mechanical stirring method applied
for the transesterification needs a higher molar ratio i.e., at least

6:1, a higher temperature, and a longer reaction time to enhance
the effect of methanol.
Fig. 3b shows the composition of products of the first step of
transesterification after the phase separation was completed. The
concentrations of TG, DG, and MG changed insignificantly when
the molar ratio increased from 2.5:1 to 4:1. The concentrations of
MG and DG were in the range from 4 to 6 wt.%, obtained at the molar ratio from 4:1 to 2.5:1, and the concentration ratios of DG and
MG changed slightly in all the molar ratios used. The concentration
of TG remaining in the FAME phase was 7.6, 6.9, 4.5, and 4.1 wt.%,
acquired at the molar ratios of 2.5:1, 3:1, 3.5:1, and 4:1, respectively. This result agrees with the conversion of FAME described
above. Consequently, the molar ratio of 2.5:1 between methanol
and WCO has been judged as the best compromise point between
the acceptable FAME conversion for the first step and the least
methanol usage. Therefore, the crude FAME consisting of FAME
81 wt.%, TG 7.6 wt.%, and the rest of TG and DG was used for the
second transesterification step.
3.3.2. The second transesterification step
The molar ratio of methanol to initial WCO was in the range
from 1:1; 1.5:1 or 2:1, and the amount of KOH catalyst was
0.3 wt.% of initial WCO. The flow rate of reactants was fixed at
2 L minÀ1, corresponding to the residence time of 0.4 min in the
reactor. After 3 min of reaction time, the reaction mixture attained
homogeneity by emulsification. As shown in Fig. 4a, when the molar ratio was increased from 1:1 to 2:1, the yield of FAME was increased from 97.2% to 99.3%. It should be noted that the conversion
of FAME became extremely high, and the equilibrium was almost
reached after around 20 min of reaction in all cases. Because the
average concentration of TG in the crude FAME was 7.6 wt.%, the
molar ratios of methanol to initial WCO of 1:1, 1.5:1 and 2:1, corresponded to the ratios of methanol to TG in the crude FAME phase
of 12.7:1, 19.1:1 and 25.5:1, respectively. These ratios are much
higher than the theoretical molar ratio of methanol to TG, i.e.,
3:1. Furthermore, the starting material containing 81 wt.% of FAME

has low viscosity. Therefore, methanol can easily diffuse in the

100

FAME phase to facilitate the reaction between the methanol and
the TG as well as the DG and MG remaining in the FAME phase.
These effects may be the main cause of the outstandingly high
yield of FAME.
Fig. 4b shows the composition of the products in the FAME
phase of the second step of the transesterification. To demonstrate
the changes in the concentrations of the products more clearly, the
concentrations of TG, DG, and MG shown in Fig. 4b are plotted
along a scale multiplied by 10. The concentrations of TG, DG, and
MG were 1.1, 1.0, and 0.7 wt.%, respectively, at the molar ratio of
methanol to initial WCO 1:1. At the molar ratios from 1.5:1 to
2:1, TG was not detected in the FAME phase, indicating that TG
was converted completely to the products, and the concentrations
of DG and MG were also less than 0.2 and 0.8 wt.%, respectively.
Compared to the biodiesel standard, JIS K 2390 and EN 14214,
the concentrations of TG, DG, and MG should be less than 0.2,
0.2, and 0.8 wt.%, respectively. Therefore, the optimal molar ratio
for the second step of the transesterification was 1.5:1.

3.4. Glycerin separation
Separation of glycerin is an important factor to determine the
final product quality and FAME recovery, as well as the time necessary for the full process of the biodiesel production. Therefore, the
separation of glycerin was investigated intensely. In this discussion, the time for the separation of the glycerin phase was defined
as zero when the reaction mixture of the first step of the transesterification was completely transferred to the separation tank. The
glycerin phase was drained from the bottom of the separation tank
every 0.5 h. As shown in Fig. 5, at the initial stage of separation

within 1 h, the higher the molar ratios of methanol to WCO, the
faster the glycerin separation took place. This fact can be elucidated as follows: at higher molar ratios, higher FAME yield could
be attained. In this case, the amount of excess methanol remaining
in the reaction mixture was large. As a result, the viscosity of the
reaction mixture was reduced. Furthermore, as mentioned in the
effect of molar ratio, when a larger amount of methanol was used,
the gathering probability of methanol in the glycerin phase was
large. Therefore, methanol and glycerin easily encounter each
other to form a large droplet, resulting in the faster separation of
glycerin and methanol from the reaction mixture. This phenomenon agrees with the conversion of FAME, which only rose by 10%
when the molar ratio increased from 2.5:1 to 4:1. On the other
hand, in the case of settling for more than 1 h, the time would be
long enough for methanol and glycerin dissolving each other; the
smaller the amount of methanol, the faster the glycerin separation
took place. Because glycerin has a much higher density

Product composition/ wt.%

FAME yield/ %

FAME

95

90

85
1:1

1.5:1

2:1

MGx10

DGx10

TGx10

100
80
60
40
20
0

80
0

10

20

30

40

50

Time/ min

Fig. 4a. Effect of molar ratio of methanol to initial WCO on the FAME yield in the
presence of KOH catalyst 0.3 wt.% for the second step of the transesterification.

1:1

1.5:1
2:1
Molar ratio of methanol to initial WCO

Fig. 4b. Composition of products in the FAME phase of the second step of the
transesterification of WCO with various molar ratios in the presence of KOH catalyst
0.3 wt.% after the reaction and phase separation were completed.


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L.T. Thanh et al. / Bioresource Technology 101 (2010) 5394–5401

12

WCO
100 ± 2.0

Glycerin phase/ wt.%

10
8

Methanol

First-step

KOH

9.13 ± 0.10

transesterificationm

0.70 ± 0.01

6
2.5:1

4

3:1
3.5:1

2

Glycerin separation

Glycerin phase

1

9.75 ± 0.30

4:1

0
0

1

2

3

4

5

Time/ h
Fig. 5. The amount of glycerin phase separation in the first step of the transesterification with the molar ratios of 2.5:1, 3:1, 3.5:1, and 4:1 and the KOH catalyst
0.7 wt.%.

(d20 = 1.26 g cmÀ3) than methanol (d20 = 0.79 g cmÀ3), the density
of the glycerin phase decreases as the amount of methanol in the
glycerin phase increases. Therefore, a slower acceleration of phase
separation between the FAME layer (typical density is ca.
0.885 g cmÀ3 for WCO in this study) and the glycerin layer takes
place owing to the larger difference in the densities of the two
layers.
Due to the presence of excess methanol in the glycerin phase,
the weight of glycerin phase separated increased from 8.3 to
12.5 wt.% when the molar ratio increased from 2.5:1 to 4:1. Practically, phase separation could be completed within 4 h after settling
the reaction mixture in the separation tank.
The behavior of the glycerin separation in the second step was
the same as in the first step. However, the time required for entire

separation in the second step was 3 h.
Table 2 shows the product compositions and the distribution of
methanol in both phases at different molar ratios of five runs.
When the total molar ratios of methanol to WCO used for the
two steps were all higher than 4:1, the concentrations of the impurity components TG, DG and MG in the FAME phase of the second
step met the biodiesel standards, JIS K 2390 or EN 14214. In these
cases, of course, the lowest total molar ratio of 4:1 would be favorable for biodiesel production, and this ratio was applied in runs #2
and #3. The molar ratios for the first and second steps of runs #2

Methanol

FAME 1

Methanol

0.16 ± 0.10

98.76 ± 1.8

0.26 ± 0.10

Methanol

Second-step

KOH

5.48 ± 0.10

transesterificationm

0.30 ± 0.01

Glycerin separation

Glycerin phase

2

5.81 ± 0.25

Methanol

FAME 2

Methanol

1.50 ± 0.22

96.23 ± 2.0

1.71 ± 0.10

Tap water
60 ± 2

Wastewater

Purification

63 ± 2

FAME product
93.83 ± 2.1
Fig. 6. Material balance for the full process under the optimal conditions. (The
materials are shown in proportions by weight, over three runs, n = 3).

Table 2
Methanol content in the FAME and glycerin phases from each step after phase separation.
Run

1
2
3
4
5

Step

First
Second
First
Second
First
Second
First
Second
First
Second

Molar ratio
CH3OH:WCO

2.5:1
1:1
2.5:1
1.5:1
3:1
1:1
3.5:1
1:1
4:1
1:1

Product composition (wt.%) of FAME phase

FAME phase

FAME

MG

DG

TG

Weight
(kg)

CH3OH

(wt.%)

CH3OH
(kg)

Weight
(kg)

CH3OH
(wt.%)

CH3OH
(kg)

80.52
97.10
81.61a ± 1.52b
98.65 ± 1.30
83.25
98.73
86.07
98.91
89.30
99.20

6.31
1.16
5.93 ± 1.00
0.63 ± 0.15
5.55

0.75
5.13
0.16
3.32
0.40

5.45
1.02
4.85 ± 0.70
0.20 ± 0.10
4.83
0.21
4.31
0.11
3.52
0.09

8.72
0.72
7.61 ± 0.90
ND
6.47
ND
4.49
ND
3.81
ND

106.5
104.3

105.3 ± 1.8
104.9 ± 1.3
106.9
103.7
107.1
106.3
106.3
104.8

0.15
0.87
0.17 ± 0.10
1.61 ± 0.32
0.94
1.2
1.23
1.41
1.77
1.59

0.16
0.91
0.18 ± 0.10
1.65 ± 0.34
1.00
1.24
1.32
1.50
1.88
1.67

10.06
4.35
9.85 ± 0.27
6.40 ± 0.24
11.02
4.39
12.93
4.9
14.58
4.85

3.06
18.85
2.91 ± 0.55
29.35 ± 1.73
8.65
16.44
18.17
21.98
20.37
25.81

0.31
0.82
0.29 ± 0.05
1.88 ± 0.11
0.95
0.72
2.35

1.08
2.97
1.25

Notes:
ND: not detectable.
a
Average value of three runs.
b
One standard deviation of three runs.

Glycerin phase


5400

L.T. Thanh et al. / Bioresource Technology 101 (2010) 5394–5401

Table 3
Properties of biodiesel produced from WCO under the optimal conditions (2.5:1 and 1.5:1 of molar ratio of methanol to initial WCO for the first and second steps, respectively;
KOH catalyst 1.0 wt.%).
Test parameter

Unit

Total ester
Density (15 °C)
Viscosity (40 °C)
Flash point
Sulfur

10% carbon residue
Cetane number
Sulfated ash
Water content
Particulate
Copper corrosion (3 h, 50 °C)
Oxidation stability
Acid value
Iodine value
Methyl linolenate
Methanol
Total glycerin
Free glycerin
Monoglyceride
Diglyceride
Triglyceride
Total Na + K
Total
Phosphorus
Pour point
CFPP

Result

mass%
g cmÀ1
mm2 sÀ1
°C
mg kgÀ1
mass%

À
mass%
mg kgÀ1
mg kgÀ1
À
hours
mg KOH gÀ1
I2/100 g
mass%
mass%
mass%
mass%
mass%
mass%
mass%
mg kgÀ1
mg kgÀ1
mg kgÀ1
°C
°C

JIS K2390

98.2
0.8845
4.452
180
2
0.03
57.2

<0.01
470
12
1
3.0
0.23
111
9.7
<0.01
0. 1
<0.01
0.27
0.11
0.07
<1
1.6
<2
À5.0
À5.0

Test method

Minimum

Maximum

96.5
0.860
3.50
120

À
À
51.0
À
À
À
À
À
À
À
À
À
À
À
À
À
À
À
À
À
À
À

À
0.900
5.00
À
10
0.3
À

0.02
500
24
1
–
0.5
120
12.0
0.20
0.25
0.02
0.80
0.20
0.20
5.0
5.0
10.0
À
À

EN 14103
JIS K2249
JIS K2283
JIS K2265
JIS K2541
JIS K2270
JIS K2280
JIS K2272
JIS K2275
EN 12662

JIS K2513
EN 14112
JIS K2501
JIS K0070
EN 14103
EN 14110
EN 14105
EN 14105
EN 14105
EN 14105
EN 14105
EN 14538
EN 14538
EN 14107
JIS K2261
JIS K2288

Notes:
This table gives the certified quality of our biodiesel product as analyzed by an authorized analysis organization.
CFPP: cold filter plugging point.
JIS: Japanese Industrial Standard for biodiesel (B100) and testing method.
EN: European Standard for testing method.

and #3 were (2.5:1, 3:1) and (1.5:1, 1:1), respectively. It is worth
noting that for the first step, the methanol content in the FAME
and glycerin phases in run #2 was lower than that in run #3. On
the other hand, in the second step of run #2, significantly excess
methanol remained in both phases. Therefore, to reduce the processing time and to save energy consumption, the recovery of excess methanol from the FAME and glycerin phase should be
carried out for the second step of run #2. We can conclude that
the optimal molar ratios of methanol to WCO with the two-step

process are 2.5:1 and 1.5:1 for the first and the second steps of
transesterification, respectively.

tion of FAME recovered in this process was 93.8 ± 2.1 wt.% (one
standard deviation for three runs).
To confirm the quality of the final product obtained under the
optimal conditions, biodiesel samples were analyzed by an authority certified to analyze the characteristics of commercial biodiesel
fuels (Nippon Kaiji Kentei Kyokai, Osaka Laboratory). The physical
properties and chemical compositions of the product are given in
Table 3. The testing results show that the FAME product in the
present study fulfills the standards JIS K2390 and EN 14214.
3.6. Energy and time consumption for full process
The continuous ultrasonic reactor is very efficient for the
transesterification of WCO. Moreover, the reaction was carried
out at ambient temperature, and this fact gives this method a large
advantage in that the electricity consumption by the transesterification can be greatly reduced. As shown in Table 4, the overall electricity needed to produce 116 L of biodiesel from 120 L of WCO was
8.35 kWh; thus, the average energy paid for 1 L of biodiesel was

3.5. Material balance and biodiesel quality
Fig. 6 shows the material balance by weight based for the full
process under the optimal conditions (the molar ratios of methanol
to WCO were 2.5:1, and 1.5:1 for the first and second steps, respectively; and KOH was 1.0 wt.% for both steps.). The average produc-

Table 4
The average electricity and time consumption for the full process under optimal conditions (with five runs, n = 5).
Process

First step
transesterification

Power (W)

1300

Glycerin separation
1

Second-step
transesterification
1300

0
Time consumption (h)

1.5

1

1.95

Purification

Total

Washing

Drying

200

1200

4000

1.5

4

15

0.30

4.80

8.35

0

4
Electricity consumption
(kWh)

Glycerin separation
2

3
1.30

0

Notes: Relative standard deviation for the electricity consumption for each runs was less than 5%.

0


L.T. Thanh et al. / Bioresource Technology 101 (2010) 5394–5401

0.072 kWh. The time consumption for the full process was 15 h,
and most of the time, the separation and purification steps took
12 h. Therefore, if these steps are simultaneously continuous with
the transesterification step, one dramatically reduced the time
consumption for the full process.
4. Conclusion
The continuous ultrasonic reactor with a two-step process is a
beneficial technique for the production of biodiesel from WCO.
The use of WCO reduces the product cost of the raw material.
The optimal conditions for the transesterification are the total molar ratio 4:1, KOH 1.0 wt.%, and the residence time in the reactor of
0.93 min for the entire process. Under these conditions, the recovery of biodiesel from WCO is 93.8 wt.%. The properties of the product satisfy the Japanese Industrial Standard (JIS K2390) and
European Committee Standard (EN14214). This process significantly reduces the use of methanol compared to conventional
methods (the mechanical stirring and supercritical methanol
methods).
Acknowledgements
This study was supported in part by the Grant-in-Aid for Cooperative R&D Project under Industry-University-Government Partnerships between Osaka Prefecture University and Sakai
Municipal Government, FY 2008–2009. The author, L.T. Thanh,
would like to thank the Vietnam Government Support Scholarship
for the PhD course.
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