Chapter 4: Introduction to Separation Methods

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4.2:

Distillation: Vapor–Liquid Equilibria

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Distillation: Vapor–Liquid Equilibria

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Distillation separates miscible liquids with significantly different boiling points that reflect different vapor–liquid equilibrium relationships, which represent how the various components are distributed between the vapor and liquid phases. When the mixture is heated, the first vapor bubble appears at the bubble point of the more volatile component. The vapor is condensed to a liquid, called a distillate, and collected separately from the mixture. The early distillate is rich in the more volatile component because the less volatile component still favors the liquid phase. The temperature needed to maintain boiling increases as the more volatile component boils off. So, as the distillation progresses, the evaporation rate of the less volatile component increases, making the middle distillate a more even mixture. The final distillate is rich in the less volatile component.

4.2:

Distillation: Vapor–Liquid Equilibria

Distillation is a separation technique that takes advantage of the boiling point properties of disparate elements in a mixture. To perform distillation, we begin by heating a miscible mixture of two liquids with a significant difference in boiling points (at least 20°C). As the solution heats up and reaches the bubble point of the more volatile component, some molecules of the more volatile component transition into the gas phase and travel upward into the condenser, which is a glass tube with separate inner and outer sections. The vapor travels into the inner section of the condenser, where it is condensed to liquid by the cold water flowing in the outer section. This condensed vapor, the distillate, is collected in a graduated cylinder or test tube. As the distillation progresses, the composition of the distillate changes. The distillate is rich in the more volatile component in the early phases of distillation, becomes more evenly composed of the two components in the middle of distillation, and becomes dominated by the less volatile component near the end of distillation. The compositional change is accompanied by a rise in the temperature needed to boil the solution. The vapor-liquid equilibrium diagram shows the change in the composition of the liquid in the flask and that of the distillate over the distillation.

A vapor-liquid equilibrium diagram is a plot of the equilibrium temperature as a function of the mole fraction of the components of a binary mixture. The x-axis represents the mole fraction of either of the two components in the mixture, and the y-axis represents the temperature. These plots are available in the literature for common mixtures and can be used to determine the boiling points of a mixture according to the mole fractions.

They are also used to determine the composition of liquid and vapor at any given temperature during a distillation experiment. Determining the composition helps identify the temperature at which the distillation will end.