Phase behaviour measurements for the system (carbon dioxide + biodiesel + ethanol) at high pressures

This work reports phase equilibrium measurements for binary system {CO2(1) + biodiesel(2)} and ternary system {CO2(1) + biodiesel(2) + ethanol(3)}. The biodiesel (ethyl esters) used in this work was produced from soybean oil, purified and characterised following the standard specification for subsequent use. Nowadays, great interest in biodiesel production processes at supercritical and/or pressurised solvents is observed, such as, non-catalytic supercritical biodiesel production and enzyme-catalyzed biodiesel production, besides the supercritical CO2 can be an interesting alternative to glycerol separation in the biodiesel purification step. Towards this, the main goal of this work is to study the phase behaviour at high pressure for the binary and ternary systems involving CO2, biodiesel and ethanol. Experiments were carried out in a high pressure variable-volume view cell with operating temperatures ranging from (303.15 to 343.15) K and pressures up to 25 MPa. The CO2 molar fraction ranged from 0.4213 to 0.9855 for the system {CO2(1) + biodiesel(2)}, 0.4263 to 0.9781 for the system {CO2(1) + biodiesel(2) + ethanol(3)} with a biodiesel to ethanol molar ratio of (1:3), and 0.4317 to 0.9787 for the system {CO2(1) + biodiesel(2) + ethanol(3)} with a biodiesel to ethanol molar ratio of (1:8). 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 modelled using the Peng–Robinson equation of state with the classical van der Waals quadratic mixing rule (PR-vdW2) and Wong–Sandler (PR-WS) mixing rule, which provided a better fitting to experimental data.

Comparison between ethyl and methyl esters in a pressure-composition of {CO2(1) + biodiesel(2)} at 303.15 K (triangles), 323.15 K (squares) and 343.15 K (circles). Open symbols are ethyl biodiesel (this work) and closed symbols are methyl biodiesel data by Pinto et al. [1].Figure optionsDownload as PowerPoint slideHighlights
► We measured phase behaviour for the system involving {CO2 + biodiesel + ethanol}.
► The saturation pressures were obtained using a variable-volume view cell.
► The experimental data were modelled using PR-vdW2 and PR-WS equations of state.