Page 175 - Biofuels Refining and Performance

158 Chapter Five

new structure indices termed allylic and bis-allylic position equivalents
(APE and BAPE), which are based on the number of such positions in
a fatty acid chain and are independent of molecular weight, are likely
more suitable than the IV [64]. The BAPE index distinguishes mixtures
having nearly identical IV correctly by their OSI times. Note that the
BAPE index is the decisive index compared to the APE because it relates
to the more reactive bis-allylic positions. Engine performance tests with
a mixture of vegetable oils of different IVs did not yield results that
would have justified a low IV [65, 66]. No relationship between the IV
and oxidative stability has been observed in another investigation on
biodiesel with a wide range of IV [52].

5.5 Viscosity
Viscosity affects the atomization of a fuel upon injection into the com-
bustion chamber and, thereby, ultimately the formation of engine
deposits. The higher the viscosity, the greater the tendency of the fuel to
cause such problems. The viscosity of a transesterified oil, i.e., biodiesel,
is about an order of magnitude lower than that of the parent oil [1, 2].
High viscosity is the major fuel property why neat vegetable oils have
been largely abandoned as alternative DF. Kinematic viscosity has been
included in most biodiesel standards. It can be determined by standards
such as ASTM D445 or ISO 3104. The difference in viscosity between the
parent oil and the alkyl ester derivatives can be used in monitoring
biodiesel production [67]. The effect on viscosity of blending biodiesel and
petrodiesel has also been investigated [68], and an equation has been
derived, which allows calculating the viscosity of such blends.
The prediction of viscosity of fatty materials has received considerable
attention in the literature. Viscosity values of biodiesel/mixtures of fatty
esters have been predicted from the viscosities of the individual components
by a logarithmic equation for dynamic viscosity [10]. Viscosity increases
with chain length (number of carbon atoms) and with increasing degree
of saturation. This holds also for the alcohol moiety as the viscosity of
ethyl esters is slightly higher than that of methyl esters [11]. Factors such
as double bond configuration influence viscosity (cis double bond con-
figuration giving a lower viscosity than the trans configuration), while
the double bond position affects viscosity less [11]. Thus, a feedstock
such as used frying oils, which is more saturated and contains some
amounts of trans fatty acid chains, has a higher viscosity than its parent
oil. Branching in the ester moiety, however, has little or no influence on
viscosity, again showing that this is a technically promising approach for
improving low-temperature properties without significantly affecting
other fuel properties. Values for dynamic viscosity and kinematic viscosity
of neat fatty acid alkyl esters are included in Table 5.1.