Phase behaviour of high molecular mass methyl esters in supercritical ethane

High pressure phase equilibria measurements of long chain saturated methyl esters (methyl decanoate, methyl dodecanoate, methyl tetradecanoate, methyl hexadecanoate, methyl octadecanoate and methyl docosanoate) in supercritical ethane have been measured for temperatures between 312 K and 355 K and for methyl ester mass fractions from 0.65 to 0.018. The results show a generally linear relationship between the phase equilibria pressure and the temperature with complete mutual solubility above 17 MPa. No three-phase regions or temperature inversions (change in sign of gradient of temperature–pressure relationship at constant composition) were observed while maxima in the phase transition pressures were found for methyl ester mass fractions usually between 0.2 and 0.3. When the phase transition pressure is plotted as a function of carbon number at constant temperature and mass fraction methyl ester, a linear relationship is realised. This linear relationship can be used to determine the solubility of any saturated methyl ester with between 10 and 22 carbon atoms, with limited extrapolation being possible. Thermodynamic modelling of the measured phase equilibria with the sPC-SAFT and Peng Robinson equations of state showed that, even with the use of an interaction parameter, these equations of state are not able to model the phase behaviour and the linear pressure–carbon numbers provide more accurate predictions.

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► Phase equilibria of long chain methyl ester in supercritical ethane.
► Increase in temperature leads to linear increase in phase transition pressure.
► Increase in molecular mass of methyl ester leads to linear increase in phase transition pressure.
► Peng Robinson and simplified PC-SAFT equation of state not able to model systems accurately.