Tricritical phenomena in ‘‘quasibinary’’ mixtures of hydrocarbons. I. Methane systems

Temperature, pressure, and composition measurements are reported for the three coexisting phases in the region of the tricritical points of two ternary mixtures of hydrocarbons: methane+n-pentane+2,3-dimethylbutane (I) and methane+2,2-dimethylbutane+2,3-dimethylbutane (II). These systems satisfy a ’’quasibinary’’ approximation in which the proportions of the two higher hydrocarbons [expressed as Z = xC/(xB+xC)] are the same in each of the three phases, so that the phase equilibrium is equivalent to that in a binary system with a solute of average properties. Least-squares analysis does not locate the tricritical point with high precision, but all the measurements are consistent with the predictions of the phenomenological theory that the exponents governing the approach to the tricritical point are the ’’classical’’ mean-field values. If, in accord with this view, we write the difference between the upper and lower critical-end-point temperatures (i.e., extent of the three-phase region) as Tu−Tl = B(Z−Zt)3/2, then the coordinates of the two tricritical points are (I) Zt = 0.28, Tt = 202.82 K, pt = 6.058 MPa and (II) Zt = 0.09, Tt = 201.60 K, pt = 5.938 MPa, xA = 0.965.