Either isothermal or isobaric data can be represented by simply plotting liquid composition x versus vapor composition y. This a: versus y curve (see Figure 2.9) is called the "equilibrium line." This type of diagram is the most widely used on distillation.
Both T-xy and P-xy are useful in illustrating the concepts of bubble point, dew point, superheated vapor, and subcooled liquid. Consider the P-xy diagrams sketched in Figure 2.10. Suppose we have a mixture with composition z, and we hold it at the same temperature T for which the diagram was drawn. If we impose a very high pressure on the mixture, we will be above the x versus P curve (saturated-liquid line) and no vapor will be present. There will be only
(mole fraction light component)
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