Biomedical Engineering Reference
In-Depth Information
Table 8. Free fatty acid content in biodiesel feedstock
3.2. Viscosity
Viscosity is the, “Measure of the internal friction or resistance of an oil to flow....The
most common method for designation of viscosity is kinematic viscosity...” [39] Biodiesel is
typically much more viscous than petrodiesel. Diesel has a kinematic viscosity of 1.3-4.1 at
40°C while biodiesel is 4.0 to 6.0. Viscosity is an important chemical property to observe
because too high a viscosity can damage the engine over long periods of time, while too low
can result in power loss due to leakage [4]. Biodiesel should never reach the minimum
viscosity of 1.9, so the latter should not be an issue [4].
3.3. Flash Point
Flash point is, “the lowest temperature at which a liquid will generate sufficient vapor to
flash (ignite) when exposed to a source of ignition.” [39] For biodiesel, the flash point
standard is set well above that for petro-diesel. This is for fire safety reasons. Since methanol
is used in the manufacturing process, and can reduce the flash point of biodiesel significantly
if even trace residue is left, it is recommended to have a flash point of 150°C to ensure that all
the methanol is burned off. Biodiesel's flash point ranges from 100 to 170°C. The minimum
flash point for petrodiesel is only 70°C [4].
3.4. Cold Flow Properties: Cloud Point and Pour Point
One of the downsides of biodiesel is its high cloud point and pour point. This means that
in cold temperature conditions, biodiesel will begin to solidify or gel at warmer temperatures
than diesel. Therefore, the lower the cloud point, the better. Cloud point is, “the temperature
at which small solid crystals are first visually observed as the fuel is cooled.” Fuels can often
continue to be used below the cloud point, but some can quickly reach the pour point [4].
Pour point is, “the temperature at which the fuel contains so many agglomerated crystals it is
essentially a gel and will no longer flow.” [4] Table 9 shows the different cloud and pour
points of biodiesel derived from different feedstocks. Soybean derived biodiesel, the most
common in the US has the highest cloud point of those shown.
