Phase Diagram for Pure Component and Multi-component Mixture

Phase diagram, which is a plot of pressure versus temperature, shows the conditions under which the various phases exist.

Phase Diagram for Pure Component

The vapor-pressure line in figure below separates the conditions, for which the substance is liquid, from the conditions for which it is vapor. Pressure temperature points that fall exactly on the line indicate conditions for which vapor and liquid coexist. C is the critical point and Tc represents the critical temperature. It is the temperature above which the gas and liquid cannot coexist, regardless of the pressure. Pc represents the critical pressure. It is the pressure above which liquid and gas cannot coexist, regardless of the temperature.

The change from one phase to another can be accomplished by changing either pressure, temperature, or both. For example, by increasing the temperature, liquid can be transformed to vapor phase. The vapor-pressure line will be the bubble-point line since the first gas bubble will appear at that temperature. If initially in the vapor phase, by lowering the temperature the vapor phase can be transformed into liquid phase. The vapor-pressure line will be the dew-point line since the first liquid drop appear at that temperature. For a single-component system, bubble-points and dew-points are represented by the same vapor-pressure line. Whether it is a bubble-point or a dew-point line depends on whether the liquid is changing into gas phase or the gas phase is changing into liquid phase respectively. We can effect the same change by lowering the pressure or increasing the temperature. By lowering the pressure, we can cross the vapor-pressure line and go from liquid into vapor phase, whereas, by increasing the temperature, we can cross the line and also go from liquid phase into vapor phase.

Multi-component Mixture

The phase behavior of multi-component mixtures is more complicated than a single-component system. In this case, instead of a single line representing the vapor-pressure curve, there is a broad region in which the two phases coexist (phase envelope). This phase envelope is bounded on one side by a bubble-point line and the other side by a dew-point line. If the pressure is reduced at a constant temperature starting in a pure liquid state, the bubble-point represents the pressure at which the first bubble of vapor appears. If the pressure is reduced further, eventually, at dew-point, the last drop of liquid will disappear. The two lines join at the critical point. The critical point, in a multi-component mixture, is defined as the point at which all properties of the liquid and the gas become identical.

Figure above illustrates the highest temperature (cricondentherm) at which the two phases can coexist. This is represented by CT in the figure. The figure also illustrates the highest pressure (cricondenbar) at which the two phases can coexist. This is represented by Cb in figure. Unlike single-component systems, the critical point in neither the highest pressure nor the highest temperature at which the two phases can coexist.

The bubble-point line in the figure represents 100 % liquid. Above the bubble-point line, pure liquid exists. The lines within the two-phase envelope represent percentages of liquid in two phases as the pressure is reduced.