In modern diesel fuel a proportion of biodiesel is blended with petro-diesel to
reduce environmental impacts. However, it can adversely affect the operation of
nonwoven coalescing filter media when separating emulsified water from diesel
fuel. This can be due to factors such as increasing water content in the fuel, a
reduction in interfacial tension (IFT) between the water and diesel, the
formation of more stable emulsions, and the generation of smaller water
droplets. Standard water/diesel separation test methods such as SAE J1488 and
ISO 16332 use monoolein, a universal surface-active agent, to simulate the
effects of biodiesel on the fuel properties as part of water separation
efficiency studies. However, the extent to which diesel/monoolein and
diesel/biodiesel blends are comparable needs to be elucidated if the underlying
mechanisms affecting coalescence of very small water droplets in diesel fuel
with a low IFT are to be understood.
To address this challenge, test fuels composed of reference diesel (REF
diesel)/biodiesel and REF diesel/monoolein were experimentally studied to
determine fuel properties such as IFT, water content, and dynamic viscosity, as
well as online droplet size distributions with reference to IFT. It was found
that biodiesel and monoolein do not influence the IFT of water in fuel in a
comparable manner and resulting water droplet size distributions are
substantially different. Fuels blended with biodiesel exhibited higher viscosity
and water content than fuel freshly blended with monoolein. Online measurement
of water droplet sizes revealed substantially smaller water droplets in
biodiesel blends compared to monoolein blends at the same IFT measured using
offline tensiometry. These results may be instructive for the development of
standard test methods that simulate the effect of biodiesel blends in fuel-water
separation, as well as for improving the design of fuel-water separation
systems.
