A phase diagram (or phase envelope) is the P-T projection of the phase diagram of a multicomponent system of fixed composition. It is made up of two parts: (1) bubble point line separating the liquid phase from the mixed phase, and (2) a dew point line separating the mixed phase from the gas phase. The lines within the envelope represent different volume percentages of liquid and gas.
As we reduce pressure from an all liquid region, we’ll begin to evolve gas once we hit the bubble point line. As we continue to reduce pressure, we get more and more gas, and less liquid, until we hit the dew point line. Below that point, we produce only gas.
We still have a critical point, but it no longer has the same definition as in the pure component diagram. Notice in this diagram that there are temperatures and pressures higher than the critical point at which 2 phases can exist. The critical point is now defined as the pressure and temperature at which the properties (composition, density, viscosity, etc.) of the liquid and gas converge. The cricondenbar is the highest pressure at which any two phases can exist, while the cricondentherm is the highest temperature at which any two phases can exist.