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Alternative Fuels for Transportation
Biodiesel is mainly produced in batch reactors commercially. Ultrasonic
biodiesel processing permits for continuous inline processing. Ultrasonication
can achieve a biodiesel yield in excess of 99%. Ultrasonic reactors reduce
the processing time remarkably. The conventional esterification reaction
in a batch is slow and phase separation of the glycerin is time-consuming,
often taking more than 4 hours. Ultrasonic process reduces the separation
time from 4 hours to less than an hour. Due to the increased chemical activity in the presence of cavitations, the ultrasonic transesterification helps to
decrease the amount of catalyst required by up to 50%. Ultrasonic transesterification reduce the amount of excess methanol required and also to improve
the purity of glycerin.
Stavarache et al. (2005) used an ultrasonic method for the preparation of the
emulsification in the alkaline-catalyzed esterification process. The collapse of
cavitation bubbles disrupts the phase boundary and causes the emulsification, by ultrasonic jets that impinge one liquid to another. With increasing
the chain length, the miscibility between the oil and alcohol increases, thus,
decreasing the reaction time (10–20 minutes) but also making the separation
of the esters difficult. The normal chain alcohols react quite rapidly under
the ultrasonic irradiation. This behavior is due to increased mass transfer in
the presence of ultrasound.
The velocity of an ultrasonic wave through a material depends on its
physical properties and hence, the ultrasonic velocity decreases with the
increasing density. The droplets of more dense oil move upward and form a
cream layer, while the alcohol moves downward, facilitating the mixing and
increasing the contact surface between alcohol and oil.
The reaction time at 40 kHz is shorter than that of a reaction at 28 kHz,
however the yield was lower. At 40 kHz, the soap is formed in higher amount
and acts as phase transfer catalyst leading to formation of the esters more
rapidly than at 28 kHz. But during the washing, the soap hinders the separation and some ester is trapped into the soap micelles and thus the yield in the
isolated product is decreased.
Table 3.2 describes the comparison of the yield of methyl esters with the
mechanical stirring and ultrasonic irradiation (Stavarache et al. 2005). At
higher frequencies, the collapse of cavitation bubbles is not very strong and
impingement of one liquid to the other is poor. Thus, the mixing between the
two immiscible layers (alcohol and oil) is very poor and the emulsification
does not occur. The transesterification takes place mainly at the boundary
between the two layers.
Low frequency ultrasound is efficient, time saving, and economical offering advantages over the conventional procedure. The ultrasonic aided biodiesel production method can be a valuable tool for the transesterification of
fatty acids.
Ji et al. (2006) developed an alkali-catalyzed biodiesel production method
with power ultrasonic (19.7 kHz) that allows a short reaction time and high
yield because of emulsification and cavitations of the liquid–liquid immiscible
Alternative Fuels for Transportation
Biodiesel is mainly produced in batch reactors commercially. Ultrasonic
biodiesel processing permits for continuous inline processing. Ultrasonication
can achieve a biodiesel yield in excess of 99%. Ultrasonic reactors reduce
the processing time remarkably. The conventional esterification reaction
in a batch is slow and phase separation of the glycerin is time-consuming,
often taking more than 4 hours. Ultrasonic process reduces the separation
time from 4 hours to less than an hour. Due to the increased chemical activity in the presence of cavitations, the ultrasonic transesterification helps to
decrease the amount of catalyst required by up to 50%. Ultrasonic transesterification reduce the amount of excess methanol required and also to improve
the purity of glycerin.
Stavarache et al. (2005) used an ultrasonic method for the preparation of the
emulsification in the alkaline-catalyzed esterification process. The collapse of
cavitation bubbles disrupts the phase boundary and causes the emulsification, by ultrasonic jets that impinge one liquid to another. With increasing
the chain length, the miscibility between the oil and alcohol increases, thus,
decreasing the reaction time (10–20 minutes) but also making the separation
of the esters difficult. The normal chain alcohols react quite rapidly under
the ultrasonic irradiation. This behavior is due to increased mass transfer in
the presence of ultrasound.
The velocity of an ultrasonic wave through a material depends on its
physical properties and hence, the ultrasonic velocity decreases with the
increasing density. The droplets of more dense oil move upward and form a
cream layer, while the alcohol moves downward, facilitating the mixing and
increasing the contact surface between alcohol and oil.
The reaction time at 40 kHz is shorter than that of a reaction at 28 kHz,
however the yield was lower. At 40 kHz, the soap is formed in higher amount
and acts as phase transfer catalyst leading to formation of the esters more
rapidly than at 28 kHz. But during the washing, the soap hinders the separation and some ester is trapped into the soap micelles and thus the yield in the
isolated product is decreased.
Table 3.2 describes the comparison of the yield of methyl esters with the
mechanical stirring and ultrasonic irradiation (Stavarache et al. 2005). At
higher frequencies, the collapse of cavitation bubbles is not very strong and
impingement of one liquid to the other is poor. Thus, the mixing between the
two immiscible layers (alcohol and oil) is very poor and the emulsification
does not occur. The transesterification takes place mainly at the boundary
between the two layers.
Low frequency ultrasound is efficient, time saving, and economical offering advantages over the conventional procedure. The ultrasonic aided biodiesel production method can be a valuable tool for the transesterification of
fatty acids.
Ji et al. (2006) developed an alkali-catalyzed biodiesel production method
with power ultrasonic (19.7 kHz) that allows a short reaction time and high
yield because of emulsification and cavitations of the liquid–liquid immiscible