The effect of the addition of water, propane, or docosane on the vapour–liquid and solid–fluid equilibria in asymmetric binary n-alkane mixtures

In this paper experimental results on vapour–liquid and solid–fluid phase equilibria for three types of ternary mixtures are reported. The ternary systems studied were methane+water+hexadecane, methane+propane+tetracosane, and methane+docosane+tetracosane. The influence of an excess water phase on the vapour–liquid critical curve of the methane+hexadecane system is determined and found to be negligible in a temperature range from 290 to 350 K. The effect of the addition of propane on the phase behaviour of two mixtures of methane+tetracosane was studied. On addition of propane (xC3=0.15) a decrease of the bubble- and dew-point pressures of up to 40 MPa was observed. Also a shift of the solid+fluid/fluid)boundary curve towards lower temperatures at constant pressure of approximately 6 K was found. For 20 mixtures of the ternary system methane+docosane+tetracosane, the solid–fluid and vapour–liquid equilibria were studied at temperatures up to 350 K and at pressures up to 200 MPa. The different isobaric solid-solubility curves show discontinuities at a relative solute mole fraction of tetracosane of 0.15 and 0.5. This most likely indicates the presence of different mixed crystal structures in the solid phases precipitating from the ternary mixtures at high pressures.