Molecular thermodynamics of biodiesel fuel compounds

Biodiesel fuel is a biodegradable clean energetic resource comprised by a mixture of monoalkyl esters of long chain fatty acids that can be obtained from a wide variety of raw materials. In terms of sulfur content, flash point, aromatic content, biodegradability and low emission of greenhouse-gases effect, biodiesel fuel is better than diesel fuel. In this paper we present a molecular-thermodynamic modeling of the phase equilibria of three very common long-chain alkylesters biodiesel compounds: methyl cis-9-octadecenoate (methyl oleate), methyl hexadecanoate (methyl palmitate) and methyl cis,cis,cis-9,12,15-octadecatrienoate (methyl linolenate). For all the cases, molecules are represented as chains of spherical segments that can associate due to the presence of short-ranged attractive sites. These attractive sites as well as the intermolecular interaction between monomer segments are modeled via square-well potentials of variable range (SW), following the Statistical Associating Fluid Theory for Potentials of Variable Range (SAFT-VR; A. Gil-Villegas, A. Galindo, P.J. Whitehead, S.J. Mills, G. Jackson, A.N. Burgess, J. Chem. Phys. 106(1997)4168–4186). The optimized values of the parameters for each pure component are obtained by fitting to vapor pressure and saturated liquid densities data, derived from the empirical Helmholtz free-energy reported recently by Huber et al. (M.L. Huber, E.W. Lemmon, A. Kazakov, L.S. Ott, T.J. Bruno, Energy Fuels 23(2009)3790–3797). Predictions are improved by the use of a discrete potential, instead of a SW potential, to represent segment–segment interactions, that can be tuned to give an optimized description of the liquid–vapor coexistence properties. The results obtained can be used to model reacting systems to produce biodiesel, based on esterification of fatty acids in presence of acid catalyst or on the transesterification with basic catalyst.