The effect of
agitation on the base-catalyzed Transesterification process
mass transfer of triglycerides from the oil phase towards the methanol-oil
This is a critical
step that limits the rate of alcoholysis reaction because the reaction
mixture is heterogeneous, consisting of two immiscible phases.
As a result, a
vigorous mixing is required to increase the area of contact between the
two immiscible phases and thus to produce an
irradiation is a useful tool for emulsification of immiscible
liquids (Colucci et al., 2005).
The collapse of
the cavitation bubbles disrupts the phase boundary and causes emulsification
that impinge one
liquid to another (Hanh et al., 2008).
can provide the mechanical energy
for mixing and
the required energy for initiating the transesterification reaction
Like any sound
wave, ultrasound alternately compresses and stretches the molecular spacing
of the medium through which it passes, causing a series of compression
and rarefaction cycles. If a large negative pressure gradient is applied
to the liquid so that the distance between the molecules exceeds
the critical molecular
distance necessary to hold the liquid intact, the liquid will break down
and voids (cavities) will be created, i.e., cavitation bubbles will form.
At high ultrasonic
intensities, a small cavity may grow rapidly through inertial effects.
As a result, some bubbles undergo sudden expansion to an unstable size
and collapse violently, generating energy for chemical and mechanical effects
and may increase the mass transfer rates by disrupting the interfacial
(known as the
liquid jet effect).
agitation is acoustic streaming mixing, in which a macroscopic
flow is induced in the liquid by the absorption of the ultrasonic
by the reactive medium (Colucci et al
that ultrasonic mixing produced smaller droplet sizes than conventional
leading to more
interfacial area for the reaction to occur.
By studying the
effect of ultrasonication
on droplet size in biodiesel mixtures, the authors
concluded that ultrasonication
can result in mean droplet sizes much lower
than those generated by conventional agitation,
and can be a more
powerful tool in breaking methanol
into small droplets.
in the chemical processing enhances both the mass transfer and chemical
It offers the
potential for shorter reaction times, cheaper reagents and less extreme
Leading to less
expensive and smaller chemical plants (Hanh et
Many studies have
investigated the effect of ultrasonication
on the transesterification
for producing biodiesel
and reported the optimum reaction conditions
(Stavarache et al
., 2005; Singh and Fernando, 2006; Stavarache et al
., 2007 a; Hanh et
., 2008; Kelkar et al
previous studies reported excellent ester yields (98-99 %) with a low amount
of catalyst in much shorter time than the mechanical stirring.
irradiation also proved suitable for community-scale continuous processing
of vegetable oils
since relatively simple devices can be used to perform
the reaction (Stavarache et al
Refaat and El Sheltawy (2008) compared the use of ultrasonication
for fast production
with the conventional base-catalyzed transesterification
and concluded that transesterification
by low frequency ultrasound
kHz) offered a lot of advantages over the conventional classical procedure.
proved to be efficient (biodiesel
yield up to 98-99 %), as well as time
and energy saving
(dramatic reduction of reaction time to 5 min, compared
to one hour or more using conventional batch reactor systems and remarkable
reduction in static separation time to 25 min, compared to 8 h).