Predicting the temperature-dependent viscosity of vegetable oil/diesel reverse microemulsion fuels

Vegetable oils are of considerable interest for potential diesel fuel replacement applications, as a result of their non-toxic nature and the fact that they can be produced from renewable sources. However, vegetable oils cannot be used directly for diesel replacement because of their high viscosities and resulting potential to cause engine problems. A number of different methods have been developed to reduce the viscosity of vegetable oils for diesel replacement. The work described here focuses on a promising, but less-studied approach: the use of surfactants to stabilize reverse microemulsions in fuels containing vegetable oils. The objective of the work was to explore the ability of one of the more widely used viscosity prediction models, the Chevron model, coupled with temperature dependent viscosity relationships for the individual fuel components, to predict the viscosities of reverse microemulsion fuels. Experiments were conducted with canola, algae or palm kernel oil, combined with No. 2 diesel and ethyl alcohol. Oleylamine surfactant and octyl alcohol cosurfactant were used to stabilize the reverse microemulsions. Measurements were conducted at 5, 10, 25 and 40 degrees C. Selected experiments also explored the effects of added water. Results of the work show that the model provides very good prediction of viscosities across nearly all systems and temperatures studied. Only in systems with very high water content (4%) were viscosities underpredicted. This result suggests that the presence of microemulsion droplets has a minimal impact on viscosity of reverse microemulsion fuels in the absence of significant water, while the composition of the fluid continuum largely dictates the viscosity of the resulting fuel. The prediction approach used here could be valuable for formulation of reverse microemulsion fuels for optimal viscosity. (C) 2013 Elsevier Ltd. All rights reserved.