Density Measurements and PC-SAFT Modeling for Beef Tallow Fatty Acid Methyl Esters (FAME Mixture) + Alkane up to 70 MPa

Biodiesel is mainly constituted by fatty acid alkyl esters, and the effect of adding alkane compounds is of great importance in order to analyze the changes in the thermophysical properties under high pressure that is of relevance in diesel engines. This work is aimed at the study of density behavior of two systems formed by beef tallow fatty acid methyl esters (FAME mixture) + alkane (decane or dodecane) in the temperature range of 303-393 K and in the pressure range of 0.2-68.29 MPa. Experiments were carried out by means of two vibrating tube densimeters with an expanded uncertainty in density of 0.7 kg·m^-3. Transesterification of waste beef tallow by using methanol at supercritical conditions was used to obtain biodiesel. The FAME mixture was constituted by methyl tetradecanoate, methyl hexadecanoate, methyl octadecenoate, and methyl (Z)-octadec-9-enoate. Density behavior was correlated using the Tammann-Tait equation and modeled with the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state. Density correlation deviations using the Tammann-Tait equation were found to be less than 0.05% and 4.15 kg·m^-3 for the average absolute deviation (AAD) and standard deviation (STD), respectively. The PC-SAFT modeling yielded an AAD = 0.11% and an STD = 1.15 kg·m^-3. The behavior for density and the derived thermodynamic properties were explained in terms of the fatty acid methyl ester content.