Page 302 - Synthetic Fuels Handbook

288 CHAPTER NINE

The Effect of Reaction Temperature and Time. Transesterification can occur in differ-
ent temperatures depending on the type of oil employed (Ma and Hanna, 1999). A few
works reported the reaction at room temperature (Marinetti, 1962, 1966; Graboski and
McCormick, 1998; Encinar et al., 2002).
The effect of reaction temperature on production of propyl oleate was examined at the
temperature range from 40 to 70°C with free P. fluorescens lipase (Iso et al., 2001). The
conversion ratio to propyl oleate was observed highest at 60°C, whereas the activity highly
decreased at 70°C.
The conversion rate increases with reaction time. The transesterification of rice bran
oil with methanol was studied at molar ratios of 4:1, 5:1, and 6:1 (Gupta et al., 2007). At
molar ratios of 4:1 and 5:1, there was significant increase in yield when the reaction time
was increased from 4 to 6 hours. Among the three molar ratios studied, ratio 6:1 gave the
best results.

The Effect of Molar Ratio and Alcohol Type. One of the most important factors that
affect the yield of ester is the molar ratio of alcohol to triglyceride. Although the stoichio-
metric molar ratio of methanol to triglyceride for transesterification is 3:1, higher molar
ratios are used to enhance the solubility and to increase the contact between the triglyceride
and alcohol molecules (Noureddini et al., 1998). In addition, investigation of the effect of
molar ratio on the transesterification of sunflower oil with methanol showed that when the
molar ratio varied from 6:1 to 1:1 and concluded that 98 percent conversion to ester was
obtained at a molar ratio of 6:1 (Freedman et al., 1986).
Another important variable affecting the yield of methyl ester is the type of alcohol to
triglyceride. In general, short-chain alcohols such as methanol, ethanol, propanol, and buta-
nol can be used in the transesterification reaction to obtain high methyl ester yields. Çanakci
and van Gerpen (1999) investigated the effect of different alcohol types on acid-catalyzed
transesterification of pure soybean oil. They obtained yields from 87.8 to 95.8 percent after
48 and 96 hours of reaction.
The Effect of Catalyst. Catalysts used for the transesterification of triglycerides are clas-
sified as alkali, acid, and enzyme. Alkali-catalyzed transesterification is much faster than
acid-catalyzed transesterification and is most often used commercially (Ma and Hanna,
1999) and, quite often, for the base-catalyzed transesterification the best yields were
obtained when the catalyst was used in small concentration, that is, 0.5 percent wt/wt of oil
(Stavarache et al., 2005). On the other hand, data show that during the production of free
and bound ethyl ester (FAEE) from castor oil, hydrochloric acid is much more effective
than sodium hydroxide at higher reaction temperatures (Meneghetti et al., 2006).

9.5.6 Properties

Biodiesel is a liquid which varies in color between golden and dark brown depending on
the feedstock from which it is produced. It is practically immiscible with water, has a high
boiling point and low vapor pressure. Typical methyl ester biodiesel has a flash point of
approximately 150°C (302°F), making it rather nonflammable. Biodiesel has a density of
approximately 0.88 g/cm³, less than that of water. Biodiesel uncontaminated with starting
material can be regarded as nontoxic but it is recommended that no one drink any!
Biodiesel has a viscosity similar to diesel produced from petroleum (petrodiesel). It can
be used as an additive in formulations of diesel to increase the lubricity of pure ultra-low
sulfur diesel (ULSD) fuel, which is advantageous because it has virtually no sulfur content.
Much of the world uses a system known as the “B” factor to state the amount of biodiesel
in any fuel mix, in contrast to the “BA” or “E” system used for ethanol mixes. For example,