Heterogeneous phase and volumetric behavior of the methane n‐heptane system at low temperatures

The phase behavior of the methane‐n‐heptane system was determined at temperatures from −200° to 340°F. at pressures up to 1,500 lb./sq. in. abs. The system was found to have a miscibility gap in the liquid‐vapor region. The compositions of phases along the three phase (L1—L2—V) locus indicate that the L1 phase is approximately 0.64 mole fraction methane and that the L2 and V phases are substantially pure methane phases. The quadruple point (L1—L2—S1—V) was found at a temperature of −154.4°F. and a pressure of 339 lb./sq. in. abs. A singular point (type k) at which the L2 phase is in critical identity with the vapor phase was found at −114.6°F. and at a pressure of 694 lb./sq. in. abs. The fugacity of methane at constant temperature as a function of the composition of dissolved methane was found to be linear in the low‐temperature range. The plot of Henry's law constant expressed as fugacity mole fraction methane ratio vs. reciprocal absolute temperature was linear in the temperature range from −130° to approximately 100°F. These data indicate that the solution thermodynamics of methane‐n‐heptane is particularly simple in the low‐temperature range.