An equation for the estimation of alcohol-air diffusion coefficients for modelling evaporation losses in fuel systems

Abstract

Alcohols (not only ethanol but also methanol, propanol, butanol and pentanol) are receiving increasing attention as components for blending with conventional fuels (gasoline and diesel or biodiesel fuels) because they can be an efficient means to increase the renewable fraction of fuels and to reduce emissions. Mathematical models for predicting evaporation losses in storage and fuel distribution systems require information about diffusion coefficients, among other characteristic parameters of the alcohols and of the alcohol-air gaseous mixtures. Nine equations in the literature have been described and applied to the binary gaseous mixtures alcohol-air to estimate the diffusion coefficient. Other equations were discarded because they are not applicable to alcohol-air systems. The resulting diffusion coefficients were plotted against temperature for the five mentioned alcohols, and the results were compared with experimental data obtained from measurements with capillary glass tubes immersed in a controlled temperature glycerine bath at 25 °C, 30 °C, 40 °C and 50 °C. The diffusion coefficients were determined from the variation of the height of the liquid column and the vapour pressure. From comparing the standard deviations with respect to the experimental data, although the best fit for all alcohols was reached with the Arnold equation (0.0247 cm2/s), it was concluded that there is not a unique optimal formula for all alcohols. Additionally, all the revised equations underestimate the effect of temperature for methanol, ethanol and propanol. Specific logarithmic equations were proposed in this work for estimating the diffusion coefficient for each alcohol, and finally, a four-parameter equation was also proposed for the five alcohols studied, which provided excellent agreement (standard deviation 0.0045 cm2/s).