Thermodynamics Research Center / ThermoML | Fluid Phase Equilibria

ABSTRACT

The phase equilibria of biodiesel systems, which are comprised of mixtures of triolein, methanol, carbon dioxide, methyl-oleate, glycerol, and water, are analyzed using PC-SAFT in ASPEN PLUS and SAFT-gamma Mie in gPROMS (gSAFT) for systems in the vicinity of their critical point. Pure-species density and liquid vapor pressure data were used to regress pure-species parameters for both variants of the SAFT equations. Binary vapor-liquid and liquid-liquid equilibrium data for the six species were used to obtain binary interaction parameters in PC-SAFT and cross parameters in SAFT-gamma Mie. Vapor-liquid and liquid-liquid equilibria data from the literature were supplemented with experimental data taken herein to obtain a complete set of binary parameters. Overall, SAFT-gamma Mie gave the best agreement with binary data, but had difficulty predicting liquid-liquid behavior in ternary systems. PC-SAFT showed good agreement with experimental data, especially for LLE systems. However, the Gibbs flash method in ASPEN PLUS yielded poor predictions for the triolein, methanol, and carbon dioxide mixture in the VLE and VLLE regions - but, addition of a tangent-plane-distance code revealed the prediction of unstable phases. Similarly, gSAFT in gPROMS, was unable to predict correct phase distributions, using parameters regressed in prior studies. Consequently, to examine the conversion of triolein to methyl-oleate observed experimentally at VLLE conditions (95 C, 9.65 MPa), RK-ASPEN (without binary interaction parameters) was used in a custom-written FORTRAN reactor program.