Singh 2007
Singh 2007
Singh 2007
ReceiVed October 10, 2006. ReVised Manuscript ReceiVed December 19, 2006
There is an increasing demand for alternative fuels that are environmentally friendly, especially because of
the fact that crude petroleum reserves are dwindling. Also, research on alternative fuels is essential for increased
energy security. Biodiesel is a renewable, biodegradable, and nontoxic fuel. At present, biodiesel is primarily
produced in batch reactors in which the required energy is provided by heating accompanied by mechanical
mixing. Alternatively, ultrasonic processing is an effective way to attain required mixing while providing the
necessary activation energy. We found that, using ultrasonication, a biodiesel yield in excess of 99% can be
achieved in a remarkably short time duration of 5 min or less in comparison to 1 h or more using conventional
batch reactor systems.
Figure 2. Effect of input energy on the fatty acid methyl esters yield.
the results, data on input energy, temperature, and yield for slices
across Figure 1 at 5, 10, 15, and 20 min are depicted in Table
1.
At 5 min after the initiation of the reaction, it was clearly
established that increasing wave amplitude resulted in an
increase of chamber temperature as well as biodiesel yields. At
the 100% amplitude level, the ester yield was >99% (highest
Figure 1. Biodiesel yield, input energy, and temperature variation with in all the 16 combinations) and the corresponding input energy
time and sonic wave amplitude. and temperature was 131 177 J and 89 °C, respectively (Table
1). Also, all four amplitudes generated >95% biodiesel yield
10, 15, and 20 min. The pulse of the reaction was kept constant in 5 min.
for all combinations at 100%. After completion of the reaction, Subjecting ultrasonication for 10 min produced high ester
the solution was treated with concentrated sulfuric acid in order yields only at lower amplitudes. For example, an increase in
to neutralize the potassium hydroxide and to immediately stop amplitude from 25 to 50% resulted in an ester yield increase
the reaction. The product, a mixture of fatty acid methyl esters from 95 to 97%. However, at higher amplitudes, ester yields
(FAMEs) and glycerol, was then transferred to a freezer before reduced drastically. This was attributable to cracking followed
sending it for gas chromatography (GC) analysis. by oxidation of the fatty acid methyl esters to aldehydes,
Gas Chromatography Analysis. Samples obtained from the ketones, and lower-chained organic fractions. It was observed
top layers of the mixture (after stabilization) were sent to that the ester yields were maximized at an optimum energy level.
Mississippi State Chemical Laboratory, Mississippi State Uni- Similar trends were observed for 15 and 20 min of ultrasoni-
versity, for GC analysis. cation at different amplitude levels.
Effect of Input Energy. The data for input energy (i.e., sound
Results and Discussion energy) and yield of FAMEs are shown in Figure 2.
According to Figure 2, it is evident that, as the input energy
Figure 1 depicts overlaid images of biodiesel yield, input increased, the FAME yield increased, reached a maximum, and
energy, and reactant temperature variation with sonic amplitudes started to decline. Accordingly, for input energies less than ∼150
and time. kW, yields are relatively constant and maximized near 97%.
Effect of Amplitude. The amplitude of sound waves had a Higher input energies tend to decrease yields, mainly because
large effect on the transesterification reaction. To better explain of cracking and degradation. It was observed that, in order to
1164 Energy & Fuels, Vol. 21, No. 2, 2007 Singh et al.
obtain biodiesel yields >97%, the range for input energy to the amplitudes, respectively, gave biodiesel yields that met the
transesterification should be maintained between 125 and 215 ASTM D 6751 standard. It was observed that, in order to obtain
kJ. biodiesel yields >97%, the range for input energy should be
Conclusions maintained between 125 and 215 kJ. Since ultrasonication could
reduce the transesterification retention times to 5 min compared
The base (KOH) catalyzed transesterification of soybean oil to over 1 h or more necessary for conventional batch processing,
using ultrasonic mixing produced high biodiesel yields at this method could be effectively used for continuous production
significantly shorter times in comparison to those reported of biodiesel using plug-flow or continuous stirred tank reactor
previously using batch systems with external heating and
systems.
mechanical stirring. At a 24 kHz frequency, combinations of 5
min/75%, 5 min/100%, and 15 min/25%, reaction time/ EF060507G