6 Analysis of Reactor Configurations for Process
Intensification
Biodiesel production from microalgae has been quite successful under laboratory
scales and can be under serious consideration for commercialization. The biodiesel
production can also be significantly improved based on the concept of process
intensification that focuses on achieving shorter reaction time and high conversion
with lower molar ratio of alcohol to oil and low catalyst concentration, also possibly
giving lower operating cost and energy consumption for biodiesel purification with
recovery of glycerol, catalyst, and excess alcohol. Reactor configurations which can
be utilized for intensification of biodiesel production process are now discussed.
6.1 Cavitational Reactors
Application of cavitation in the field of biodiesel production has gained interest
lately. Cavitation helps the reaction by providing mechanical energy for mixing and
enhanced surface area for the transesterification reaction resulting in reduced
reaction time and increased yield (Gogate and Pandit 2004). The main effects which
are generated due to cavitation consist of (1) chemical effect which is produced due
to generation of radicals (H
+
, OH
− , and HO 2
+
) from transient implosive collapse of
the bubbles though this is not dominating in the case of biofuel production,
(2) homogenization of the mixture, which is caused by micro-turbulence generated
due to the collapse of bubbles. Due to the formation of fine emulsion, the interfacial
region is increased between oil and alcohol which leads to increased reaction rate
and high yield. There are mainly two types of cavitational reactors, ultrasonic
(US) and hydrodynamic (HC). The ultrasonic reactors are operated in the frequency
range of 20–40 kHz with lower range of power (120–220 W) (Gupta and Verma
2015), giving dominant physical effects controlling biodiesel production.
Utilization of 40 kHz frequency has been reported to reduce the time required for
reaction drastically (Stavarache et al. 2005). Ultrasound-assisted transesterification
reactions are generally performed with reaction parameters as: molar ratio
(1:6–1:10), catalyst loading (0.5–2 wt% of oil), and reaction time (15–20 min) with
temperature over the range of 30 to 60 °C as observed in the literature (Gole and
Gogate 2012). It is important to understand that most of the applications have been
based on the use of ultrasonic horn and bath at the laboratory scale but application
of ultrasound on continuous mode has not been reported. More research needs to be
performed to utilize ultrasound effectively at commercial scale especially using
continuous operation. Hydrodynamic cavitation (HC) produces similar effects to
that of ultrasonic cavitation; only difference is in the method of generation of
cavities. Cavity generation is due to sudden pressure drop with help of constriction
introduced in the flow of the liquid. These reactors are generally more energy
efficient and can work with large quantity reaction batch as compared to US and
78
S. Joshi and P. Gogate
Intensification
Biodiesel production from microalgae has been quite successful under laboratory
scales and can be under serious consideration for commercialization. The biodiesel
production can also be significantly improved based on the concept of process
intensification that focuses on achieving shorter reaction time and high conversion
with lower molar ratio of alcohol to oil and low catalyst concentration, also possibly
giving lower operating cost and energy consumption for biodiesel purification with
recovery of glycerol, catalyst, and excess alcohol. Reactor configurations which can
be utilized for intensification of biodiesel production process are now discussed.
6.1 Cavitational Reactors
Application of cavitation in the field of biodiesel production has gained interest
lately. Cavitation helps the reaction by providing mechanical energy for mixing and
enhanced surface area for the transesterification reaction resulting in reduced
reaction time and increased yield (Gogate and Pandit 2004). The main effects which
are generated due to cavitation consist of (1) chemical effect which is produced due
to generation of radicals (H
+
, OH
− , and HO 2
+
) from transient implosive collapse of
the bubbles though this is not dominating in the case of biofuel production,
(2) homogenization of the mixture, which is caused by micro-turbulence generated
due to the collapse of bubbles. Due to the formation of fine emulsion, the interfacial
region is increased between oil and alcohol which leads to increased reaction rate
and high yield. There are mainly two types of cavitational reactors, ultrasonic
(US) and hydrodynamic (HC). The ultrasonic reactors are operated in the frequency
range of 20–40 kHz with lower range of power (120–220 W) (Gupta and Verma
2015), giving dominant physical effects controlling biodiesel production.
Utilization of 40 kHz frequency has been reported to reduce the time required for
reaction drastically (Stavarache et al. 2005). Ultrasound-assisted transesterification
reactions are generally performed with reaction parameters as: molar ratio
(1:6–1:10), catalyst loading (0.5–2 wt% of oil), and reaction time (15–20 min) with
temperature over the range of 30 to 60 °C as observed in the literature (Gole and
Gogate 2012). It is important to understand that most of the applications have been
based on the use of ultrasonic horn and bath at the laboratory scale but application
of ultrasound on continuous mode has not been reported. More research needs to be
performed to utilize ultrasound effectively at commercial scale especially using
continuous operation. Hydrodynamic cavitation (HC) produces similar effects to
that of ultrasonic cavitation; only difference is in the method of generation of
cavities. Cavity generation is due to sudden pressure drop with help of constriction
introduced in the flow of the liquid. These reactors are generally more energy
efficient and can work with large quantity reaction batch as compared to US and
78
S. Joshi and P. Gogate