High-pressure phase behavior of binary systems of heptadecane and octadecane with carbon dioxide or ethane.

Phase behavior of four binary systems of heptadecane and octadecane with CO2 and ethane was investigated in the temperature range from 270 to 320 K and pressures up to 30 MPa. The observed phase boundaries correspond to the three-phase coexisting lines with some nonvariant points of quadruple points and critical endpoints. Phase density inversion between two liquid phase was also observed in the n-alkanes with CO2 systems. The phase density inversion points form a straight line in the pressure–temperature diagram regardless of the carbon number of the n-alkanes. In the homologous series of n-alkane + CO2 systems, the “odd-even effect” of carbon numbers of n-alkanes has been observed in the relation of the triple point of n-alkane and the quadruple point.