Phase equilibrium measurements and thermodynamic modelling for the system (CO2 + ethyl palmitate + ethanol) at high pressures

This work reports phase equilibrium measurements for the binary {CO2(1) + ethyl palmitate(2)} and ternary {CO2(1) + ethyl palmitate(2) + ethanol(3)} systems at high pressures. There is currently great interest in biodiesel production processes involving supercritical and/or pressurized solvents, such as non-catalytic supercritical biodiesel production and enzyme-catalysed biodiesel production. Also, supercritical CO2 can offer an interesting alternative for glycerol separation in the biodiesel purification step in a water-free process. In this context, the main goal of this work was to investigate the phase behaviour of binary and ternary systems involving CO2, a pure constituent of biodiesel ethyl palmitate and ethanol. Experiments were carried out in a high-pressure variable-volume view cell with operating temperatures ranging from (303.15 to 353.15) K and pressures up to 21 MPa. The CO2 mole fraction ranged from 0.5033 to 0.9913 for the binary {CO2(1) + ethyl palmitate(2)} system and from 0.4436 to 0.9712 for ternary system {CO2(1) + ethyl palmitate(2) + ethanol(3)} system with ethyl ester to ethanol molar ratios of (1:6), (1:3), and (1:1). For the systems investigated, vapour–liquid (VL), liquid–liquid (LL) and vapour–liquid–liquid (VLL) phase transitions were observed. The experimental data sets were successfully modeled using the Peng–Robinson equation of state with the classical van der Waals quadratic (PR-vdW2) and Wong–Sandler (PR–WS) mixing rules. The PR–WS showed good performance in the prediction of the phase transition for the ternary systems based on the binary system data.

Ethyl palmitate and biodiesel comparison in a pressure–composition diagram for the systems (CO2 + ethyl palmitate + biodiesel), at different temperatures.Figure optionsDownload as PowerPoint slideHighlights
► We measured VLE, LLE, and VLLE for the system (CO2 + ethyl palmitate + ethanol).
► The saturation pressures were obtained using a variable-volume view cell.
► Phase envelope of (CO2 + ethyl palmitate) is different that (CO2 + soybean oil biodiesel).
► The experimental data were modeled using PR-vdW2 and PR–WS equations of state.