The two-component system with respect to a wide range of phase equilibrium data, including the vapor-liquid equilibrium data, how to accurately estimate the phase equilibrium from existing phase equilibrium data, the separation of the components using the same, the design of the purification device or the control method, apparatus, and their design or control programs designed to provide an apparatus of the device. To the two-component threshold rating points X represented by the below using a balanced sample data (22) (24), (25) calculate the infinite dilution pressure gradient, Y ₁ and Y ₂ shown in, and (wherein, P _c1 is , two-component system the critical pressure of the component 1 according to the, p _1s, p _2s, respectively, the vapor pressure of the component 1 and component 2 in the temperature T, γ _i ^∞ is an infinite dilution for component i in the liquid phase hwalryang represents a coefficient), by correlating the obtained X and Y ₁ and Y _2, XY ₁ correlated with the new infinite dilution from the correlation of XY ₂ hwalryang coefficient γ ₁ ^∞, γ ₂ ^∞ or two-component parameter a, calculating a B, and the resulting by γ ₁ ^∞, γ ₂ ^∞ or a, B, and estimates the phase equilibrium. The components of the distillation column, such as by using a value obtained by separation, carried out in the design, the control of the purification unit. As it reaches the critical point X: (22) infinite dilution pressure gradient, Y ₁ and _{Y 2: (24) (25} )