Temperature vs composition of binary mixture at constant pressure

temperature constant at t vapor pressure of pure light vapor pressure of pure light

Subcooled Liquid

x and y

(mole fraction light component)

Pressure vs. composition of binafy mixture at constant temperature a liquid phase present with composition z. At this high pressure, the liquid is called "subcooled."

If we now begin to drop the pressure, we will move down the vertical line drawn through composition z, shown in Figure 2.10. When pressure reaches the point labeled PBP, vapor will begin to appear. This therefore is called the bubble-point pressure of this mixture of composition z and at temperature T. The composition of this first bubble can be read off the y versus P curve by moving across horizontally at PBP.

As pressure is reduced further, more and more vapor is formed. Finally, at a pressure PDP all the liquid has vaporized. This is called the dew-point pressure of this mixture of composition z and at temperature T. The y versus P line is called the saturated-vapor line. At pressures below PDP only a single phase exists, "superheated vapor."

Binary Mixture

FIGURE 2.9

x vs. y for binary mixture

FIGURE 2.9

x vs. y for binary mixture

The same concepts can be visualized using constant-pressure T-xy diagrams (Figure 2.11). The mixture is superheated vapor at temperatures above the dew point Tdp and subcooled liquid at temperatures below the bubble point TBP.

Note that we can talk about either bubble-point temperature or bubble-point pressure, depending on which variable is fixed (isothermal or isobaric situations). The same is true for dew-point temperature and dew-point pressure.

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  • PHILIP
    How to draw dew point curve and bubble point curve in distillation?
    8 years ago

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