Page 172 - Biofuels Refining and Performance

Fuel and Physical Properties of Biodiesel Components 155

2 petrodiesel. Low-temperature filterability tests were stated to be nec-
essary because of better correlation with operability tests than the CP
or PP test [31]. However, for fuel formulations containing at least 10
vol% methyl esters, both LTFT and CFPP are linear functions of the CP
[32]. Additional statistical analysis have shown a strong 1:1 correlation
between LTFT and CP [32].
Several approaches to low-temperature problems of esters have been
investigated, including blending with petrodiesel, winterization, additives,
branched-chain esters, and bulky substituents in the chain. The latter
approach may be considered a variation of the additive approach, as the
corresponding compounds have been investigated in biodiesel at additive
levels. Blending of esters with petrodiesel will not be discussed here.
Numerous, usually polymeric, additives were synthesized and
reported mainly in the patent literature to have the effect of lowering
the PP or sometimes even the CP. A brief overview of such additives has
been presented [33]. Similarly, the use of fatty compound-derived mate-
rials with bulky moieties in the chain [34] at additive levels has been
investigated. The idea associated with these materials is that the bulky
moieties in these additives would destroy the harmony of the crystal-
lizing solids. The effect of some additives appears to be limited because
they more strongly affect the PP than the CP or they have only a slight
influence on the CP. The CP, however, is more important than the PP
for improving low-temperature flow properties [35].
The use of branched esters such as isopropyl, isobutyl, and 2-butyl
esters instead of methyl esters [36, 37] is another approach for improv-
ing the low-temperature properties of biodiesel. Branched esters have
lower melting points in the neat form (Table 5.1). These esters showed
(crystallization onset temperature), as determined by dif-
a lower T CO
ferential scanning calorimetry (DSC) for the isopropyl esters of soybean
oil (SBO) by 7–11 C and for the 2-butyl esters of SBO by 12–14 C [36].
The CPs and PPs were also lowered by the branched-chain esters. For
example, the CP of isopropyl soyate was given as 9 C [7] and that of
2-butyl soyate as 12 C [36]. In comparison, the CP of methyl soyate is
around 0 C [32]. However, in terms of economics, only isopropyl esters
appear attractive as branched-chain esters, although even they are
more expensive than methyl esters. Branching in the ester chain does
not have any negative effect on the CN of these compounds, as dis-
cussed above.
Winterization [35, 38, 39] is based on the lower melting points of
unsaturated fatty compounds than saturated compounds (Table 5.1).
This method removes by filtration the solids formed during the cooling
of the vegetable oil esters, leaving a mixture with a higher content of
unsaturated fatty esters and thus with lower CP and PP. This procedure
can be repeated to further reduce the CPs and PPs. Saturated fatty