Note: we recommend you to take a look at the Vapour Pressure section before. Also, if you haven't understood the topic about the Antoine Equation.
TXY diagram
TXY diagrams are graphic representations of the phase equilibrium in a binary system. In this type of diagrams the temperature is plotted as a function of the composition of the most volatile compound.
TXY diagrams are built are constant pressure. It's important to mention that in the TXY diagrams the x axis represents both the composition in the liquid and the composition in the vapour for, again, the most volatile component of the mixture.
The areas outside of enclosed regions represent a single phase, while the areas enclosed by the lines represent two phases in equilibrium, in this case we are going to consider a vapour-liquid equilibrim. Look at the next example of a typical TXY diagram:
TXY diagrams are built are constant pressure. It's important to mention that in the TXY diagrams the x axis represents both the composition in the liquid and the composition in the vapour for, again, the most volatile component of the mixture.
The areas outside of enclosed regions represent a single phase, while the areas enclosed by the lines represent two phases in equilibrium, in this case we are going to consider a vapour-liquid equilibrim. Look at the next example of a typical TXY diagram:
Everything above the red line will be vapour, while everything under the blue line will be liquid. The blue line represents the bubble line, that is, the temperature at which you will be able to see the first bubble. On the other hand, the red line is the dew point, in other words, it represents the point at which the vapour will start to condensate.
For example: Consider that the last TXY diagram was real for a mixture of benzene and toluene. The most volatile compound in this case is benzene, therefore we will be plotting it's composition in the x axis. For a mixture containing 12%mol of benzene, the bubble point would be 370°C and at this temperature, the composition of the vapour would be 40%mol benzene. It's quite easy right?
In a similar way we can have PXY diagrams in which the temperature is constant but the pressure varies. In this case the bubble line is above the dew line. If you analyse it more deeply it makes sense because liquids exist at high pressures, while vapours exist at low pressures.
To make calculations with the diagram and the VLE, the next equations are important to mention. They are useful to obtain the values of the fraction in vapor and liquid of the most volatile component.
Now it's your turn, the next file contains both a TXY and PXY diagram for benzene and toluene.
You can click on it if you are interested in learning more about how these type of diagrams are built. |
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in case you would like to watch what you have learned in a computational simulator of Txy and Pxy diagrams. (CDF Player for Wolfram)
References.
Vapour Liquid Equilibria. IRA A. Fulton College. Engineering and Technology [Online] https://www.et.byu.edu/~rowley/ChEn273/Topics/Mass_Balances/Multiphase_Systems/Vapor_Liquid_Equilibrium/VLE.htm. Consulted 28/10/16.
Curious mind curious brain [Online] http://curiousmindcuriousbrain.com/. Consulted 14/10/16.