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Biodiesel production from palm oil using combined mechanical stirred

and ultrasonic reactor

I. Choedkiatsakul a, K. Ngaosuwan b, G. Cravotto c, S. Assabumrungrat a,

a Centre of Excellence in Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering, Chulalongkorn University, Bangkok 10330, Thailandb Department of Chemical Engineering, Faculty of Engineering, Rajamangala University of Technology Krungthep, Bangkok 10120, Thailandc Dipartimento di Scienza e Tecnologia del Farmaco, University of Turin, Via P. Giuria 9, I-10125 Torino, Italy

a r t i c l e i n f o

Article history:

Received 5 October 2013

Received in revised form 24 December 2013

Accepted 25 December 2013

Available online 3 January 2014

Keywords:

Biodiesel

Combined mechanical stirred and ultrasonic

reactor

Dual frequency operation

Flow ultrasonic reactor

a b s t r a c t

This paper investigates the production of biodiesel from palm oil using a combined mechanical stirred

and ultrasonic reactor (MSUS). The incorporation of mechanical stirring into the ultrasonic reactor

explored the further improvement the transesterification of palm oil. Initial reaction rate values were

54.1, 142.9 and 164.2 mmol/L min for the mechanical-stirred (MS), ultrasonic (US) and MSUS reactors,

respectively. Suitable methanol to oil molar ratio and thecatalyst loading values were found to be 6 and 1

of oil, respectively. The effect of ultrasonic operating parameters; i.e. frequency, location, and number of

transducer, has been investigated. Based on the conversion yield at the reactor outlet after 1 h, the num-

ber of transducers showed a relevant role in the reaction rate. Frequency and transducer location would

appear to have no significant effect. The properties of the obtained biodiesel (density, viscosity, pour

point, and flash point) satisfy the ASTM standard. The combined MSUS reactors improved the reaction

rate affording the methyl esters in higher yield.

2013 Elsevier B.V. All rights reserved.

1. Introduction

Nowadays, there are many intensification technologies for bio-

diesel production that have been devised to solve the problems of

reaction reversibility and the immiscibility of the two reactants [1].

Many types of reactor, such as a membrane reactor [24], reactive

distillation [5]and reactive absorption[6]have been developed to

improve transesterification rate by removing the products during

the reaction, thus shifting the reaction equilibrium and facilitating

increased biodiesel yield. Rotating packed bed reactors[7], bubble

column reactors[8], jet flow stirred reactors[9], etc. have all been

applied to create a sufficient contact surface area between two

immiscible phases and assist the mixing of the two reactants.

Efficient multi-transducer sonochemical reactors (US) canstrongly promote the biodiesel production thanks to cavitational

phenomena which generate nucleation, growth, and consequent

collapse of microbubbles in the liquid medium, affecting positively

both heat and mass transfer. High power ultrasound waves cause a

series of compression and rarefaction determining intense cavita-

tion. The rarefaction cycle may exceed the attractive forces of li-

quid molecules and some microbubbles form. At this point, small

amount of vapor of the medium enters the bubbles during its

expansion phase and is not fully discharged during compression.

The microbubbles, hence, grow over a few cycles to reach an unsta-

ble size and then collapse in the successive compression cycles

[10,11]. This collapse can increase the interfacial area of the reac-

tants and enhance mass transfer[12]. Because ultrasound irradia-

tion influences the pressure variation in the liquid medium, it

induces the generation of a huge amount of microbubbles to form

an emulsion phase. When these microbubbles collapse, the interfa-

cial area of reactants boundary is largely increased [13]. The US

reactors have been further designed for improvement [14,15]and

their applications are widely studied [16]. Table 1 summarizes

some research studies into biodiesel production using US batch

reactors. Many types of feedstock have been investigated and their

optimal operating conditions are also summarized in this table. It

can be seen that a highmethyl ester yield can be achieved at a low-er reaction temperature [1721], alcohol to oil molar ratio [22] and

reaction time than those achieved by conventional biodiesel pro-

duction[23,24]. The lower reaction temperature and reaction time

give further advantages in terms of energy required, while the low-

er oil to methanol molar ratio creates benefits in terms of the cost

for product purification and product separation.

Low frequency ultrasound irradiation (in the range of 10

200 kHz, more specifically 2050 kHz for biodiesel production)

may be useful in providing the vigorous mechanical energy needed

for the transesterification of triglyceride with alcohol and enhanc-

ing the reaction rate [25]. Stavarache et al.[26]mentioned that the

cavitation bubble collapse provided by high frequency ultrasonics

1350-4177/$ - see front matter 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.ultsonch.2013.12.025

Corresponding author. Tel.: +66 2 218 6868; fax: +66 2 218 6877.

E-mail address: [email protected](S. Assabumrungrat).

Ultrasonics Sonochemistry 21 (2014) 15851591

Contents lists available at ScienceDirect

Ultrasonics Sonochemistry

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / u l t s o n
http://dx.doi.org/10.1016/j.ultsonch.2013.12.025mailto:[email protected]://dx.doi.org/10.1016/j.ultsonch.2013.12.025http://www.sciencedirect.com/science/journal/13504177http://www.elsevier.com/locate/ultsonhttp://www.elsevier.com/locate/ultsonhttp://www.sciencedirect.com/science/journal/13504177http://dx.doi.org/10.1016/j.ultsonch.2013.12.025mailto:[email protected]://dx.doi.org/10.1016/j.ultsonch.2013.12.025http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://crossmark.crossref.org/dialog/?doi=10.1016/j.ultsonch.2013.12.025&domain=pdfhttp://-/?-

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(in the range of 2001000 kHz) were not strong enough to homog-

enize the two liquids. Hence, the mixing or the emulsification can

take place only at the boundary between the two layers. Therefore,

low frequency ultrasound is more useful in biodiesel production

for the improvement of mass transfer. The difference in the

frequency of the transducer affects the reaction rate. Sutkar and

Gogate[10]indicated that multiple-frequency operation provided

high efficiency when compared to a single frequency operation

because it can generate more vigorous and uniform cavitational

activity in the reaction mixture. The recommended frequency isthe combination of low frequency ultrasound irradiation (typically

20 kHz) with other frequencies in the range of 50200 kHz. How-

ever, there is a lack of reports into multi-frequency ultrasound irra-

diation in continuous biodiesel production. Therefore, this study

designed a flow US reactor working at two different frequencies,

20 and 50 kHz, which has been used for biodiesel production from

palm oil and methanol with a sodium hydroxide (NaOH) catalyst.

In addition, the integration of horizontal mechanical stirrer mixing

and ultrasound irradiation (MSUS reactor) has been investigated

as a means to improve flow behavior in this reactor. Operating

parameters, such as the methanol to oil molar ratio and the

amount of catalyst loading have been studied so as to obtain a high

biodiesel yield for industrial applications. Moreover, the effects of

key flow US reactor operating parameters, such as the location, fre-

quency, and number of US transducers have also been investigated.

2. Experimental

2.1. Materials

Commercial refined palm oil was purchased from a local

department store in Thailand. 99.5% purity methanol, used for

transesterification, was purchased from Qrc. The NaOH catalyst

purchased from Loba Chemie was ground and dissolved in metha-

nol before use. Methyl heptadecanoate and heptane (internal stan-

dard and solvent for GC analysis, respectively) were high purity

chemicals from SigmaAldrich and were used as obtained.

2.2. Experimental setup

The schematic diagram of the MSUS flow reactor used in this

experiment is shown in Fig. 1. It can be divided into three main

parts, the reactant tank, the generator and the reactor, as can be

seen inFig. 2. A 6 L cylindrical reactant tank was used to preheat

the raw materials, oil and methanol, before feeding them into the

reactor (Fig. 2a). The preheat temperature and the feed flow rate

were fixed at about 45 C and 55 mL/min (corresponding to resi-

dence time of 4 min), respectively, and correspond to the best con-

ditions reported by Cintas et al. [27]. Stirrer speed in the preheat

tank was about 160 rpm. The NaOH catalyst was fed into the tank

after the reactant temperature (palm oil and methanol) was con-

stant. The experiment was carried out by circulating all productsand reactants from the US flow reactor outlet into the reactant

tank. Samples were taken at the reactor outlet. The generator unit

(Fig. 2b) was used for transducer power generation and for control-

ling the motor speed of the horizontal stirrer inside the reactor. The

rectangular shape (2.5 2.5 30 cm) stainless steel (316) reactor

is shown inFig. 2c. There were two different transducer frequen-

cies used in the experiments (20 and 50 kHz) and the location of

transducers was fixed along the length of the reactor, as shown

inFig. 1. The frequency of the transducers anchored to the reactor

wall on opposite sides was the same. The generator provided a

power of 50 W per transducer, hence, the total power of this reac-torwas 800 W when operating with all 16 transducers. This reactor

can be operated in 3 modes; the mechanical stirred mode using the

horizontal stirring rod inside the reactor with the maximum speed

of 25 rpm (MS reactor), ultrasound irradiation (US reactor) and the

integration of mechanical stirring and ultrasound irradiation

(MSUS reactor).

2.3. Analysis

The methyl ester yield samples were analyzed following the EN

14103 standard method using GC-2010 Plus Shimadzu gas chro-

matography equipment fitted with a flame ionization detector

and a capillary column DB-WAX (0.25 mm 30 m). Helium and

nitrogen were used as carrier gas and makeup gas. The oven tem-perature ramp program was started at 150 C and held for 5 min,

heated up to 190 C with a rateof 3 C/min and held for 5 min, then

heated up again to 220 C with a rate of 3 C/min and held for

5 min. Injector and detector temperatures were 250 C. Methyl

ester yield was calculated by Eq.(1):

%Yield

PA

AEI

AEICEI VEI

m 100% 1

whereP

A is total peak area,AEI is the peak area that corresponds to

methyl heptadecanoate,CEIis the concentration of the methyl hep-

tadecanoate solution (mg/mL), VEIis the volume of methyl heptade-

canoate (mL) and m is the mass of the biodiesel sample (mg).

The biodiesel sample was washed with distilled water at a

water; biodiesel ratio of 0.5:1 by volume, 3 times at ambient tem-

perature under a stirrer speed of 200 rpm[28]. The properties of

the purified biodiesel were then analyzed according to the ASTM

standard.

3. Results and discussion

3.1. Performance comparison of different reactors

The transesterification of palm oil operated at a methanol to oil

ratio of 6:1 and NaOH loading of 0.l5% wt of oil was used to com-

pare the reaction performance of the different reactors (MS, US and

MSUS). The mechanical stirrer operated at a speed of 25 rpm

while all 16 transducers (dual frequencies of 20 and 50 kHz) wereused to provide ultrasonic waves. FromFig. 3, it can be seen that a

Table 1

Research studies on biodiesel production by US batch reactor and optimal operating conditions.

Oil feedstocks Frequency (kHz)/Power (W) Temperature (C) Time (min) Alcohol:oil molar ratio Catalyst (wt%) Yield (%) Refs.

Soybean 19.7/150 45 30 6:1 1 NaOH 100 [12]

Rapeseed 40 40 25 6:1 0.5 KOH 97 [13]

Waste cooking 20/400 30 30 6:1 0.75 KOH 90 [14]

Waste cooking 20/200 45 40 6:1 1 KOH 89 [15]

Sunflower 40/150 33 60 7.5:1 0.7 KOH 88 [16]

Soybean 20/pulse mode 2030 1.5 3:1 1 NaOH 96 [17]Soybean 24/400 79 5 6:1 1 KOH 98 [18]

Coconut 24/200 60 7 6:1 0.75 KOH 98 [19]

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high yield value at the outlet (about 76%) can be rapidly achieved

in about 5 min with the MSUS flow reactor. The value is about

60% when using the US reactor. On the other hand, it took a muchlonger time (1 h) to obtain the same methyl ester yield under the

similar conditions when using the MS reactor. It should be noted

that in the case of US and MSUS reactors, after 5 min the good

yields reach a plateau that even with longer reaction times could

only slightly increase. The fast transesterification took place in con-

tinuous flow using a combined MSUS loop reactor where the

liquid mixture was pumped through the system. The circulation

rate was kept constant at 55 mL/min, corresponding to a residence

time of 4 min. The reported yields refers to samples collected at the

reactor outlet and values are calculated on the base of the initial

reactant concentration in the feed tank. On the other hand, in the

MS reactor, used in continuous flow, yields were much more time

dependent with a steady increase (Fig. 3). The initial rate of reac-

tion was also reported to express the speed of this reaction. Theinitial rate was calculated from the difference between the

concentrations of triglyceride at the initial point and at outlet after

5 min of reaction, divided by residence time in the reactor. It was

found that the initial rate of reaction in the US reactor(142.9 mmol/L min) was noticeably higher than that in MS reactor

(54.1 mmol/L min). This was because the US reactor can provide

the rapid emulsion of the reactants, as mentioned previously.

Therefore, it enhances the mass transfer process and a high methyl

ester yield can be achieved in a relatively short reaction time. This

result corresponds to the result from Chand et al. [22], which states

that the reaction time to reach a high methyl ester yield can be re-

duced from 60 min to only 1.5 min when using a probe US reactor

rather than a MS reactor. The results also indicated the advantage

of the combination of two different frequencies in biodiesel pro-

duction. Under the same conditions, Yin et al.[29] found that using

both a MS reactor and a single-frequency US reactor for biodiesel

production required more catalyst loading than 0.5% wt of oil.

The result showed that it provided about 60% biodiesel conversionin the MS reactor and only 50% biodiesel conversion in the US

Valve

Cover

Fan

Valve

Pump

Motor

6 L

Tank

Mechanical

stirrer

Reactor (2.5x2.5x30 cm3)

Pump Reactants tank

Generator

Transducer 20 kHz

Transducer 50 kHz

Fig. 1. Schematic diagram of the MSUS flow reactor system.

Fig. 2. Each part of the MSUS flow reactor system (a) reactant tank unit, (b) generator unit and (c) MSUS reactor.

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reactor at 50 min of reaction time. It can be concluded that thedual-frequency ultrasound irradiation for flow reactors exhibited

better mixing performance than conventional mechanical mixing.

The horizontal stirrer and dual frequency ultrasound irradiation

in the flow reactor (MSUS reactor) combined gave the highest

methyl ester yield. It was found that the methyl ester yield was

nearly 70% in 5 min of reaction time and it reached equilibrium

in the short reaction time of 30 min when using the same condi-

tions. Moreover, the initial rate of reaction was 164.2 mmol/L

min which was higher than US and MS reactor values. Gogate

et al.[30]have revealed that maximum energy is dissipated near

the irradiating surface in a cone like structure in low frequency

operation. This contributed to the maximum cavitational activity

very near the irradiating surface and the wide variation in energy

dissipation rates in the remaining reaction mixture bulk. The intro-duction of the horizontal mechanical stirrer may improve distribu-

tion of cavitational activity in this reactor. However, it should be

noted that the higher stirrer speed possibly also gave a negative

effect due to the fact that agitation strongly interferes with the

passage of incident ultrasound irradiation. The effect of the

MSUS reactor corresponds with the results published by Yin

et al. [29]. It was noticed that the combination of US and MS in

the reactor gave the high methyl ester conversion of 80%, while

the US and MS reactors at catalyst loading of 0.5% wt of oil

achieved only 50% and 60%, respectively. The next section will

discuss the influence that MSUS flow reactor operating parame-

ters, which are related to the energy consumption for biodiesel

production, had on the reaction.

3.2. Effect of MSUS reactor operating parameters

3.2.1. Methanol to oil molar ratio on methyl ester yield

In general, the theoretical methanol to oil molar ratio, as taken

from stoichiometric transesterification, is three. As previously

mentioned, this reaction is reversible. The higher concentration

of methanol can shift the reaction equilibrium to obtain higher

product yields [29]. Furthermore, as the amount of methanol

increases, more cavitational bubbles are obtained because ultra-

sound activity is easier in methanol than in oil. However, some

methanol can dissolve into the glycerol by-product phase during

the reaction and only the remaining methanol can be reacted with

oil in the transesterification[31]. It was found that methanol can

completely dissolved in the glycerol phase [32]. Therefore, theeffect of the methanol to oil molar ratio on the methyl ester yield

should be optimized. The previous section confirmed that the

application of the MSUS reactor gave more advantages than the

use of MS and US reactors separately. The effects of the methanol

to oil molar ratios of 3, 6, 9, and 12 on methyl ester yield were

studied along the reaction period. The results are presented in

Fig. 4. When using a methanol to oil molar ratio of 3, the methyl

ester yield increases more than for ratios of 9 and 12 in the first

20 min of reaction time. The initial reaction rate also shows a sim-

ilar trend; a higher initial reaction rate of 127.5 mmol/L min was

obtained when using a methanol to oil molar ratio of 3, whereas

only 58.3 and 22.4 mmol/L min were obtained from methanol to

oil molar ratios of 9 and 12, respectively. However, the methyl

ester yield obtained after 1 h of reaction time was lower. When

using a methanol to oil molar ratio of 6, not only was the highest

initial reaction rate, of 142.9 mmol/L min, achieved but also the

highest methyl ester yield of 75%. This was due to the fact that

some methanol is able to dissolve in the glycerol product, as

previously described, when using a methanol to oil molar ratio of

3. Therefore, only the remaining methanol can react giving a low

yield. Under US irradiation, the excess methanol was more likely

to contribute to the cavitational activity resulting in a higher

emulsion quality (smaller drop sizes) and hence increased yield.

However, methyl ester yield also decreased when using a molar

ratio of methanol to oil over 6. This was because only a limited

portion of the methanol is able to act in the transesterification

[33]. Moreover, the large amount of methanol that persists after

the reaction also leads to a complicated product separation step,

since methanol can work as an emulsifier that creates an emulsion

between glycerol and biodiesel [34]. Therefore, increasing the

molar ratio of methanol to oil beyond 6 was not only detrimental

to biodiesel yield, but also caused difficulties in the product

separation step. The optimal methanol to oil molar ratio was 6.

3.2.2. Effect of catalyst loading

This parameter is more significant, especially when a feedstock

is composed of high amount of free fatty acids (FFAs). Since FFAs

can easily react with a base catalyst and undergo saponification

when using a large amount of the catalyst. This resulted in a lower

methyl ester yield and complications in product separation and

purification [35]. The effect of catalyst loading on methyl ester

yield is exhibited in Fig. 5 as a function of reaction time. The

amount of NaOH was varied and 5 concentrations were used;

0.15%, 0.30%, 0.50%, 0.70% and 1% wt oil. It was found that methyl

ester yield increased with increasing catalyst loading. However, it

Time (min)

0 10 20 30 40 50 60

Yie

ld(%)

0

20

40

60

80

100

MS

US

US+MS

Fig. 3. Effect of differenttypesof mixing on methyl esters yield at thereactor outlet

within 60 min (methanol/oil molar ratio = 6:1, NaOH = 0.15% wt oil, feed flow

rate = 55 mL/min).

Time (min)

0 10 20 30 40 50 60

Yield(%)

0

20

40

60

80

100

Oil:MeOH molar ratio 1:3




Fig. 4. Effect of methanol to oilmolar ratio onmethyl ester yield at reactor outlet at

various reaction times using the MSUS reactor (NaOH = 0.15% wt oil, feed flowrate = 55 mL/min).


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increased only 4% when the amount of catalyst increased from

0.15% to 0.30% wt oil. The methyl ester yield also increased only2% when the amount of catalyst was increased from 0.50% to

0.70% wt oil. This result corresponds with the initial rate of reac-

tion. It can be seen in Table 2that the value of the initial reaction

rate at a catalyst loading level of 0.15% wt is similar to the rate at

0.3% wt, and that the value of the initial reaction rate at a catalyst

loading level of 0.5 was the same as at 0.7% wt. The highest methyl

ester yield of 94% and the highest initial rate of reaction, of

217.3 mmol/L min, were both obtained at a catalyst loading level

of 1% wt of oil. On the other hand, a catalyst loading of 0.15% wt

of oil provided the highest initial TOF (2.90 min1) which was cal-

culated by dividing the initial rate per gram of catalyst by the basic

site concentration of the catalyst per gram. This was because of the

lower amount of catalyst used for the reaction, hence lower basic

site concentration. This reaction, therefore, reached a lower methylester yield compared to the others. It can be seen from the results

that the importance of the catalyst loading variable was more pro-

nounced in this process than the methanol to oil molar ratio. This

conclusion corresponds with the results published by Choudhury

et al. [36]. The influence of these two parameters was studied

experimentally and in a simulation model where it was clearly

indicated that the amount of catalyst loading had more effect on

the methyl ester yield than the methanol to oil molar ratio. There-

fore, the optimal operating conditions for biodiesel production in

this study are a methanol to oil molar ratio of 6 and catalyst load-

ing of 1% wt of oil. The effect of the application of dual frequency

ultrasound irradiation in any location along the length of reactor

is investigated in the next section.

3.3. Effect of the operating parameters of MSUS flow reactor

A dual frequency MSUS reactor showed the best biodiesel effi-

ciency as demonstrated in Section 3.1. However, it also required

the highest energy consumption to supply the transducers and

horizontal stirrer. Therefore, this section investigates the effect of

location, frequency, and the number of transducers, which are

related to power consumption, on the methyl ester yield in the

transesterification process. Differences in frequency and trans-

ducer location are the significant operating parameters in this

reactor. The effect of the sound wave is more intense in the areas

closest to the irradiating surface and rapidly decreases further

away from the source[10]. Moreover, the combination of two fre-

quencies might enhance the reaction rate by providing better cavi-

tational phenomena from the resonance effect, as compared to a

single frequency[3739]. The reactor was divided into 4 positions

according to the location of the transducer attached to the reactor

wall, as shown inFig. 6. As mentioned previously, the frequency of

the transducers on the opposite side of the wall was the same and

the opposite transducer pairs with the same location and fre-

quency were operated together. There were six experiments where

the locations and the frequencies of the transducers used for the

transesterification of palm oil were varied, and results are shown

inTable 3. The total number of transducers and operating power

for four experiments (14) were 8 and 400 W, respectively (50 W

for each transducer). The first four experiments were compared

to the best operating conditions (integrated US reactor with max-

imum power, fifth experiment) and the conventional mechanical

stirrer (MS reactor, sixth experiment).

In order to investigate the effect of MSUS flow reactor operat-

ing parameters on the reaction, the methanol to oil molar ratio and

NaOH loading was kept constant at 6:1 and 0.15% wt oil, respec-

tively for all experiments in this section. The first and the second

experiments were carried out using a single transducer frequency,

20 and 50 kHz, respectively. It was found that both experiments

provided similar results. The acquired methyl ester yield after 1 h

was about 62%. However, the higher frequency (50 kHz) reached

the final methyl ester yield more quickly than the lower frequency

(20 kHz). This result corresponds to results published by Stavar-

ache et al.[26]and Encinar et al.[40]. For the third and the fourth

experiments, different transducer frequencies were used. Theexperiment was done by switching the location of the two frequen-

cies. In the third experiment, the first and the third 20 kHz trans-

ducer locations and the second and the fourth location of the

50 kHz transducers were activated. The opposite 20 and 50 kHz

transducer locations were used in the fourth experiment. The

results showed a similar methyl ester yield value, of about 62%,

as the first and the second experiment. The location of the trans-

ducer had no significant effect on methyl ester yield in this study

as the identical distance of each transducer from the irradiating

surface for both experiments would give uniform cavitational

intensity (US power per irradiation area) and resulted in the same

effect at the different locations [41]. The use of dual frequencies

(20 and 50 Hz) for all transducers was studied in the fifth experi-

ment which saw a total operating power of 800 W. It gave thehighest methyl ester yield of about 76% while the MS reactor gave

only 57%. It seems that the number of transducers plays a more

important role than differences in transducer frequency and

location on methyl ester yield. The higher number of transducers

was directly related to the intensity of the cavitational activity.

More transducers give increased cavitational intensity resulting

in a higher oilmethanol interfacial area. Bubble collapse generates

high jet velocity and micro-mixing at the boundary layer between

the two phases. Finally, it forms a fine emulsion and gives higher

mass transfer and hence, higher biodiesel formation [40,42].

However, the influence of frequency in this study is different to

what has been published in other reports [40,42]. This might be

because the transducers generate uniform cavitational intensity

in this reactor at both, low frequency range, frequencies, aspreviously mentioned.

Time (min)

0 10 20 30 40 50 60

Yie

ld(%)

0

20

40

60

80

100

NaOH 0.15%wt oil

NaOH 0.30%wt oil

NaOH 0.50%wt oil

NaOH 0.70%wt oil

NaOH 1%wt oil

Fig. 5. Effect of catalyst loading on methyl ester yield at reactor outlet at various

reaction times using the MSUS reactor (methanol to oilmolar ratio = 6:1, feed flow

rate = 55 mL/min).

Table 2

Methyl ester yield at reactor outlet, initial reaction rate and initial TOF as obtained

from the MSUS reactor at different catalyst loading amounts (methanol to oil molar

ratio = 1:6, frequency = 20 + 50 kHz and reaction time = 1 h).

NaOH loading (%wt

of oil)

Yield

(%)

Initial rate of reaction

(mmol/L min)

Initial TOF

(min1)

0.15 75.68 164.20 2.8987

0.3 79.81 155.35 0.6908

0.5 88.91 183.11 0.2931

0.7 91.10 183.22 0.1496

1 93.78 217.27 0.0869

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It should be noted that the experimental results obtained from

this study indicate that the combined MSUS reactor is a promising

technology for biodiesel production. It offers higher biodiesel yield

than the US reactor and the MS reactor. However, further investi-

gation will probably optimize the reaction condition by varying

the liquid feed rate and other parameters with the aim to achieve

full conversion even with shorter residence time and low energy

consumption. The reactors design play an important role. The ini-

tial rates of the MS, US and MSUS reactors were 54.1, 142.9, and

164.2 mmol/L min, respectively. The initial rate of the MSUS reac-

tor was found to be less than the additiveeffects of MSand US indi-vidual reactors (197 mmol/L min). Therefore, more in depth energy

analysis should be taken into account in the design of the MSUS

reactor.

3.4. Biodiesel chemical composition and physical properties

The properties of palm oil derived biodiesel were evaluated

according to the ASTM standard. The density, viscosity, flash point,

pour point, and heating value of biodiesel were all estimated from

ASTM D4052, ASTM D445, ASTM D93, ASTM D6749/D97, and

ASTM D240, respectively. The standard values and the values of

biodiesel produced in this work are presented in Table 4. It was

found that the heating value was slightly lower than the standard

value. However, the other properties of the biodiesel produced inthis work (density, viscosity, flash point, and pour point) met the

ASTM standard. Therefore, this MSUS reactor is a promising reac-

tor that can produce biodiesel in accordance with the ASTM

standard.

4. Conclusion

A dual frequency US reactor integrated with a horizontal

mechanical stirrer has been designed to produce biodiesel frompalm oil and methanol in the presence of NaOH as a catalyst.

Two transducer frequencies, 20 and 50 kHz, were employed in four

possible locations along the length of the reactor. The results

showed that this reactor provided high methyl ester yield in only

5 min reaction time, whereas the MS reactor required 60 min.

The optimal operating conditions were; a methanol to oil molar

ratio of 6 and a NaOH catalyst loading value of 1% wt of oil as

operated in the circulation of reaction mixture with feed flow rate

55 mL/min. The influence of transducer number on methyl ester

yield was more important than differences in transducer frequency

and location along the length of the reactor. The identical distance

of each transducer along the length of the reactor was able to

provide uniform cavitational intensity and resulted in the same

effect at the different locations. Moreover, the properties ofbiodiesel produced from this reactor conform to the ASTM

standard. The combined MSUS reactor enhanced the biodiesel

production with better performance than the MS and US reactors

alone.

Acknowledgements

The Ph.D. student (I. Choedkiatsakul) is financially supported by

the Royal Golden Jubilee Ph.D. program under the Thailand

Research Fund and Chulalongkorn University. The authors grate-

fully acknowledge the Ratchadaphiseksomphot Endowment Fund

of Chulalongkorn University (RES560530134-EN) for financial sup-

port and Mr. Jutipong Poosumas and Mr. Bamrung Sungnoen fortechnical support.

20

kHz20

kHz

20

kHz

20

kHz

50 kHz50 kHz50 kHz50 kHz

50 kHz

20

kHz

50 kHz 50 kHz

20

kHz

50 kHz

20

kHz

20

kHz

Position 1 Position 2 Position 3 Position 4

Fig. 6. The position on the reactor wall of the two transducer frequencies.

Table 3

Methyl ester yield at reactor outlet obtained from the use of two frequencies in each location.

Experiment The position of 20 kHz transducer The position of 50 kHz transducer Yield (%)

1 2 3 4 1 2 3 4

1 d d d d 62.23

2 d d d d 62.78

3 d d d d 62.80

4 d d d d 61.68

5 d d d d d d d d 75.68

6 57.31

Table 4

Biodiesel properties in comparison with ASTM standard (ASTM 6751).

Properties Value Unit

Biodiesel ASTM 6751 Biodiesel (this work)

Density @ 15 C 0.860.89 0.8755 g/cm3

Viscosity @ 40 C 1.96.0 4.46 mm2/s

Flash point P130 254 C

Pour point 10 to 12 10 CHeating value 9940 9527.1 cal/g


8/11/2019 1-s2.0-S1350417713003325-main

7/7

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