Biodiesel
49
becomes insignificant. The understanding of the mixing effects on the kinetics of the transesterification process is a valuable tool in the process scale-up
and design. Generally, after adding the methanol and catalyst to the oil,
5–10 minutes stirring helps in a higher rate of conversion and recovery.
3.2.1.6 Purity of Reactants
The impurities present in the vegetable oil also affect ester conversion levels
significantly. The vegetable oil (refined or raw oil) is to be filtered before the
transesterification reaction. The oil settled at the bottom of the tank during storage would give lesser yield because of deposition of impurities like
wax.
3.2.1.7 Effect of Reaction Time
The conversion rate increases with reaction time. Transesterification of peanut, cottonseed, sunflower, and soybean oils under the condition of methanol
to oil ratio of 6:1, 0.5% sodium methoxide catalyst, and 60ºC the conversion
efficiency of about 80% was obtained after 1 minute for soybean and sunflower oils; and after completion of 60 minutes the conversions were almost
the same for all four oils (93–98%; Agarwal 2007).
Ma and Hanna (1999) studied the effect of reaction time on transesterification of beef tallow with methanol. The reaction was very slow during the
first minute due to the mixing and dispersion of methanol into beef tallow.
From 1 to 5 minutes, the reaction proceeded very fast and the conversion
efficiency peaked from 1 to 38%.
3.2.2 Acid Catalyst Transesterification
Alkaline esterification is suitable for the triglycerides having an acid value
less than four and all reactants should be substantially anhydrous. If the
acid value is greater than four the conversion efficiency would reduce drastically. The alkaline catalyst reacts with the high FFA feedstock to produce the
soap and water. Acid catalyzed process can be used for esterification of these
FFAs. The nonedible type oil, crude vegetable oils, and used cooking oils
typically contain more than 2% FFA, and the animal fats contain from 5 to
30% FFA. Low quality feedstock, such as trap grease, can contain FFA up to
100%. Moisture or water present in the vegetable oils increase the FFA value.
FFA content of rice bran rapidly increased within few hours, showing 5%
increase in FFA content per day. Heating of the rice bran immediately after
the milling inactivates the lipase and prohibits the formation of FFA.
Van Gerpen (2005) advocates that up to 5% FFA, alkaline catalyst can
be used for the transesterification reaction; however, additional catalyst
must be added to compensate for the catalyst lost to the soap. When FFA
value of the vegetable oil is more than 5%, the formation of soap inhibits
49
becomes insignificant. The understanding of the mixing effects on the kinetics of the transesterification process is a valuable tool in the process scale-up
and design. Generally, after adding the methanol and catalyst to the oil,
5–10 minutes stirring helps in a higher rate of conversion and recovery.
3.2.1.6 Purity of Reactants
The impurities present in the vegetable oil also affect ester conversion levels
significantly. The vegetable oil (refined or raw oil) is to be filtered before the
transesterification reaction. The oil settled at the bottom of the tank during storage would give lesser yield because of deposition of impurities like
wax.
3.2.1.7 Effect of Reaction Time
The conversion rate increases with reaction time. Transesterification of peanut, cottonseed, sunflower, and soybean oils under the condition of methanol
to oil ratio of 6:1, 0.5% sodium methoxide catalyst, and 60ºC the conversion
efficiency of about 80% was obtained after 1 minute for soybean and sunflower oils; and after completion of 60 minutes the conversions were almost
the same for all four oils (93–98%; Agarwal 2007).
Ma and Hanna (1999) studied the effect of reaction time on transesterification of beef tallow with methanol. The reaction was very slow during the
first minute due to the mixing and dispersion of methanol into beef tallow.
From 1 to 5 minutes, the reaction proceeded very fast and the conversion
efficiency peaked from 1 to 38%.
3.2.2 Acid Catalyst Transesterification
Alkaline esterification is suitable for the triglycerides having an acid value
less than four and all reactants should be substantially anhydrous. If the
acid value is greater than four the conversion efficiency would reduce drastically. The alkaline catalyst reacts with the high FFA feedstock to produce the
soap and water. Acid catalyzed process can be used for esterification of these
FFAs. The nonedible type oil, crude vegetable oils, and used cooking oils
typically contain more than 2% FFA, and the animal fats contain from 5 to
30% FFA. Low quality feedstock, such as trap grease, can contain FFA up to
100%. Moisture or water present in the vegetable oils increase the FFA value.
FFA content of rice bran rapidly increased within few hours, showing 5%
increase in FFA content per day. Heating of the rice bran immediately after
the milling inactivates the lipase and prohibits the formation of FFA.
Van Gerpen (2005) advocates that up to 5% FFA, alkaline catalyst can
be used for the transesterification reaction; however, additional catalyst
must be added to compensate for the catalyst lost to the soap. When FFA
value of the vegetable oil is more than 5%, the formation of soap inhibits