A predictive thermodynamic model for the Brazilian gasoline

In Brazil, as in many other countries, the fuel quality control is made according to standard assays, which depend on some of the fuel properties, such as specific gravity, Reid vapor pressure and distillation curve. These properties are intimately related to the fuel composition and their prediction relies on the knowledge of its components characteristics. In Brazil, 25% (v/v) of anhydrous ethanol is added to the mixture of hydrocarbons that is usually called gasoline, according to the local regulations. In this work, the composition of the gasoline was obtained by gas chromatography as a small series of pseudo-components. Given the gasoline composition, its volatility properties and specific gravity were calculated using the Peng-Robinson equation of state with the Fisher-Gmehling mixing rule. The model provided excellent predictions for specific gravity, Reid vapor pressure and most of the distillation curve. However, important deviations are observed at the end of the distillation curve (90+%). These deviations are due to chemical alterations of the fuel caused by the high temperatures occurring at the end of the distillation curve. The model, as proposed, is efficient to forecast the fuel properties; as a consequence, it can be applied as a tool in the study of gasoline formulations and as an alternative tool in quality control.