Page 173 - Biofuels Refining and Performance

156 Chapter Five

compounds, which have higher CNs (Table 5.1) than unsaturated fatty
compounds, are among the major compounds removed by winterization.
Thus the CN of biodiesel decreases during winterization. Loss of mate-
rial was reduced when winterization was carried out in presence of cold-
flow improvers or solvents such as hexane and isopropanol [39].
In other work [40], tertiary fatty amines and amides have been
reported to be effective in enhancing the ignition quality of biodiesel
without negatively affecting the low-temperature properties. Also, sat-
increased the PP substan-
urated fatty alcohols of chain lengths C 12
tially. Ethyl laurate weakly decreased the PP.

5.4 Oxidative Stability
Oxidative stability of biodiesel has been the subject of considerable
research [41–62]. This issue affects biodiesel primarily during extended
storage. The influence of parameters such as presence of air, heat, traces
of metal, antioxidants, and peroxides as well as nature of the storage
container was investigated in the aforementioned studies. Generally, fac-
tors such as the presence of air, elevated temperatures, or the presence
of metals facilitate oxidation. Studies performed with the automated oil
stability index (OSI) method have confirmed the catalyzing effect of
metals on oxidation; however, the influence of the compound structure
of the fatty esters, especially unsaturation as discussed below, was even
greater [52]. Numerous other methods, including not only wet-chemical
ones such as the acid value and peroxide value, but also pressurized dif-
ferential scanning calorimetry, nuclear magnetic resonance (NMR), and
so forth, have been applied in oxidation studies of biodiesel.
Two simple methods for assessing the quality of stored biodiesel are
the acid value and viscosity since both increase continuously with
increasing fuel degradation, i.e., deteriorating fuel quality. The peroxide
value is less suitable because it reaches a maximum and then can
decrease again due to the formation of secondary oxidation products [48].
Autoxidation occurs due to the presence of double bonds in the chains
of many fatty compounds. Autoxidation of unsaturated fatty compounds
proceeds with different rates, depending on the number and position of
double bonds [63]. Especially the positions allylic to double bonds are
susceptible to oxidation. The bis-allylic positions in common polyun-
saturated fatty acids, such as linoleic acid (double bonds at .C-9 and
.C-12, giving one bis-allylic position at C-11) and linolenic acid (double
bonds at .C-9, .C-12, and C-15, giving two bis-allylic positions at C-11
and C-14), are even more prone to autoxidation than the allylic positions.
The relative rates of oxidation given in the literature [63] are 1 for
oleates (methyl, ethyl esters), 41 for linoleates, and 98 for linolenates.
This is essential because most biodiesel fuels contain significant amounts