The effect of
agitation on the base-catalyzed
Transesterification process causes the
mass transfer of triglycerides from the oil phase towards the methanol-oil
interface.
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
emulsion.
Low frequency
ultrasonic 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
by
ultrasonic jets
that impinge one
liquid to another (Hanh et al., 2008).
Hence,
ultrasonication 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
boundary layers
(known as the
liquid jet effect).
Another effect
of
ultrasound agitation is acoustic streaming mixing, in which a macroscopic
flow is induced in the liquid by the absorption of the
ultrasonic
wave by the reactive medium (Colucci
et al.,
2005).
Wu
et
al. (2007) reported
that ultrasonic mixing produced smaller droplet sizes than conventional
agitation,
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.
The
ultrasound in the chemical processing enhances both the mass transfer and chemical
reactions.
It offers the
potential for shorter reaction times, cheaper reagents and less extreme
physical conditions,
Leading to less
expensive and smaller chemical plants (Hanh
et
al., 2008).
Many studies have
investigated the effect of
ultrasonication on the
transesterification
process for producing
biodiesel and reported the optimum reaction conditions
(Stavarache
et al., 2005; Singh and Fernando, 2006; Stavarache
et al., 2007 a; Hanh
et
al., 2008; Kelkar
et al., 2008).
These
previous studies reported excellent ester yields (98-99 %) with a low amount
of catalyst in much shorter time than the mechanical stirring.
Ultrasonic 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., 2007b).
Refaat and El Sheltawy (2008) compared the use of
ultrasonication for fast
production
of biodiesel from
waste
vegetable oil with the conventional base-catalyzed
transesterification and concluded that
transesterification by low frequency
ultrasound (20
kHz) offered a lot of advantages over the conventional classical procedure.
It
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).
