Conventional Gibbs energy for the ternary system ethylene, normal butane, carbon dioxide plotted vs. mole fraction of the first two components, with temperature and pressure set to give the equimolar ternary critical composition. Gibbs rolling plane is shown marking off phase-equilibrium states (blue-yellow boundaries) with the following approximate compositions:

liquid (red cross)
vapor (green cross)
C2H4
0.28
0.26
nC4H10
0.30
0.37
CO2 (by difference)
0.42
0.37

This drawing extends only a small distance into the two-phase region (toward the viewer) so that the curvature change for the unstable states (concave upward instead of downward) is too small to be seen from the viewer’s angle, even in the full-size view.
Iowa State University