Droplet Evaporation of Alcohol-Biodiesel Blends

Abstract

Biodiesel has been proposed as a substitute for diesel given that biodiesel has lower net average greenhouse gas emissions than diesel. Additionally, alcohol may be added to biodiesel to improve biodiesel’s performance in a diesel engine as well to reduce engine emissions. This work will study the droplet evaporation process of alcohol-biodiesel blends. Due to alcohol’s polar nature and the fatty acid methyl esters’s (FAME) slightly polar nature, an appropriate method must be chosen to represent the evaporation process of a non-ideal mixture. The vapour-liquid equilibria was modelled in two ways: the first method uses only Raoult’s Law, while the second method uses Raoult’s law modified with activity coefficients calculated using the UNIFAC method. The comparison of the modelled results with experimental vapour-liquid equilibria data has shown that activity coefficients calculated using the UNIFAC method are able to accurately represent alcohol-biodiesel systems. Droplet evaporation experiments have been performed for biodiesel-propanol and biodiesel-pentanol blends at temperatures of 450°C and 700°C with the alcohol concentrations of 5%, 10%, 15%, and 20%. Additionally, the droplet evaporation was numerically modelled using two different models to represent the liquid state: a model with a well-mixed liquid phase and a model which includes component diffusion in the liquid phase. Comparing the experimental droplet temperatures to the numerical models has shown that the diffusion-limited model best represents the droplet evaporation process, suggesting that some of the alcohol components remain in the center of the droplet even when the droplet temperature is greater than the boiling temperature of the alcohol. This was further confirmed by observations of bubbling within the droplet during evaporation of the biodiesel-alcohol blends, in which there were both small bubbles and large bubbles forming. The formation of large bubbles has shown to correspond with the difference between experimental droplet diameter and the diffusion-limited model’s droplet diameter.