Thermodynamic modeling of ethanol/gasoline blends

T. M. SORIA; M. GONZALEZ PRIETO; S. PEREDA; S. B. BOTTINI

San Petesburgo

Simposio; 25th European Symposium on Applied Thermodynamics (ESAT 2011); 2011

Bioethanol fuel blends play nowadays an important role in countries like Brazil and USA. Moreover, policies worldwide promote blending of fossil- and bio-fuels to meet sustainable targets. Ethanol has a high impact in the final fuel properties. The high non-ideality of mixtures containing ethanol and hydrocarbons strongly affects the phase behavior of the blend and consequently has an impact on its storage, transportation and use in engines. French and Malone[1] discussed the effect of adding ethanol to gasoline, in several properties: i) volatility (i.e. Reid vapor pressure, ASTM D-86 Distillation, vapor-liquid ratio, and evaporative emissions), ii) liquid split (e.g. water tolerance and enhanced solubility of aromatic fuel components in groundwater). Thus, the design of new biofuel blends requires tools able to predict final fuel properties. Gasoline is a multicomponent mixture of mainly four families of hydrocarbons: normal-, branched- and cyclic-alkanes, together with aromatic hydrocarbons. Group contribution models are the best option to calculate the properties of mixtures containing a large number of similar compounds. In this case the number of required interaction parameters is dramatically reduced when compared with molecular models. In previous works, phase behavior of the family of normal-[2], branched-[3] and aromatic-[4] hydrocarbons in mixtures with water and alcohols were modeled with the group-contribution with association GCA-EoS equation of state. In the present work, the parameters required to include cycloalkanes have been determined. The GCA-EoS attractive contribution to the Helmholtz energy is based on the surface interaction between groups. Similar to UNIFAC, GCA-EoS requires the knowledge of the van der Waals area of each functional group in order to quantify this interaction. All cyclic paraffins are represented in UNIFAC by a single CH2 group, having the same area (q) and volume (r) as the CH2 in normal-alkanes. The values of r and q parameters were obtained by normalizing Bondi´s[5] calculations, based on the structure/geometry of each functional group forming organic molecules.