Page 298 - Synthetic Fuels Handbook

284 CHAPTER NINE

the resulting over-fertilization, pesticide use, and land use conversion that they say would
be needed to produce the additional vegetable oil. Many advocates suggest that waste veg-
etable oil is the best source of oil to produce biodiesel. However, the available supply is
drastically less than the amount of petroleum-based fuel that is burned for transportation
and domestic heating in the world.

9.5.2 Transesterification (Alcoholysis)

Transesterification is the conversion of triacylglycerol lipids by alcohols to alkyl esters
without first isolating the free fatty acids (May, 2004).
Transesterification of a vegetable oil was conducted as early as 1853 many years before
the first diesel engine became functional. Rudolf Diesel’s prime model, a single 10-ft(3-m)-iron
cylinder with a flywheel at its base, ran on its own power for the first time in Augsburg,
Germany, on August 10, 1893. Diesel later demonstrated his engine and received the Grand
Prix (highest prize) at the World Fair in Paris, France in 1900. This engine stood as an
example of Diesel’s vision because it was powered by peanut oil, which is a biofuel, though
not biodiesel, since it was not transesterified. He believed that the utilization of biomass
fuel was the real future of his engine and that the use of vegetable oils for engine fuels
would become as important as petroleum and the coal-tar products of the present time.
The purpose of transesterification of vegetable oils to their methyl esters (biodiesel)
process is to lower the viscosity of the oil. The transesterification reaction is affected by
alcohol type, molar ratio of glycerides to alcohol, type and amount of catalyst, reaction tem-
perature, reaction time, and free fatty acids and water content of vegetable oils or animal
fats. The transesterification reaction proceeds with or without a catalyst by using primary
or secondary monohydric aliphatic alcohols having one- to eight-carbon atoms as follows
(Demirbas, 2006a; 2006b):
Triglycerides + Monohydric alcohol ↔ Glycerin + Mono-alkyl esters

Generally, the reaction temperature near the boiling point of the alcohol is recommended
(Çanakçi and Özsezen, 2005). The reactions take place at low temperatures (approximately
65°C) and at modest pressures (2 atm, 1 atm = 14.7 psi = 101.325 kPa). Biodiesel is fur-
ther purified by washing and evaporation to remove any remaining methanol. The oil
(87 percent), alcohol (9 percent), and catalyst (1 percent) are the inputs in the production
of biodiesel (86 percent), the main output (Lucia et al., 2006). Pretreatment is not required
if the reaction is carried out under high pressure (9000 kPa) and high temperature (240°C),
where simultaneous esterification and transesterification take place with maximum yield
obtained at temperatures ranging from 60 to 80°C at a molar ratio of 6:1 (Barnwal and
Sharma, 2005). The alcohols employed in the transesterification are generally short chain
alcohols such as methanol, ethanol, propanol, and butanol. It was reported that when trans-
esterification of soybean oil using methanol, ethanol, and butanol was performed, 96 to
98 percent of ester could be obtained after 1 hour (Dmytryshyn et al., 2004).

9.5.3 Catalytic Transesterification
Transesterification reactions can be catalyzed by alkalis (Fig. 9.3) (Korytkowska et al.,
2001; Dmytryshyn et al., 2004; Stavarache et al., 2005; Varghaa and Truterb, 2005; Meher
et al., 2006a), acids (Lee et al., 2000; Goff et al., 2004; Lopez et al., 2005; Liu et al., 2006),
or enzymes (Watanabe et al., 2000, 2002; Ghanem, 2003; Reyes-Duarte et al., 2005; Royon
et al., 2007; Shah and Gupta, 2007; Bernardes et al., 2007). The catalytic transesterification