CHAPTEr 18: Vapor-Liquid Equilibrium
It is easy to predict an ideal behavior of mixtures, but what happens in reality?
REALITY: In reality, each mixture behaves in a specific way, making it difficult to predict its phase change tendency.
This behavior is shown in different diagrams:
Txy diagram: This diagram shows the vapor-liquid equilibrium in the mixture with a variation of the temperature in function of the composition of acetone at a constant pressure. At temperatures bellow the blue line, the mixture is liquid, between both lines it is at a vapor-liquid equilibrium and above the green line it is vapor. This means that the blue line represents the boiling point (it evaporates from liquid to vapor) and the green line represents the dew line (it condenses from vapor to a liquid-vapor mixture). This diagram is useful for calculations (using the lever rule) that involve a change in composition.
Pxy diagram: This diagram is very similar to the Txy diagram, instead of keeping P constant, the pressure changes in function of composition and the temperature remains constant.
Ya vs Xa diagram: This diagram shows the behavior of the vapor composition of acetone in function of its liquid composition. It shows a curve above a reference line, the distance between these lines is represented by the difference in boiling points of the components.
Activity coefficient vs Xa diagram: For an ideal solution, the activity coefficient has a constant value of 1, since it has no deviation.
REALITY: In reality, each mixture behaves in a specific way, making it difficult to predict its phase change tendency.
This behavior is shown in different diagrams:
Txy diagram: This diagram shows the vapor-liquid equilibrium in the mixture with a variation of the temperature in function of the composition of acetone at a constant pressure. At temperatures bellow the blue line, the mixture is liquid, between both lines it is at a vapor-liquid equilibrium and above the green line it is vapor. This means that the blue line represents the boiling point (it evaporates from liquid to vapor) and the green line represents the dew line (it condenses from vapor to a liquid-vapor mixture). This diagram is useful for calculations (using the lever rule) that involve a change in composition.
Pxy diagram: This diagram is very similar to the Txy diagram, instead of keeping P constant, the pressure changes in function of composition and the temperature remains constant.
Ya vs Xa diagram: This diagram shows the behavior of the vapor composition of acetone in function of its liquid composition. It shows a curve above a reference line, the distance between these lines is represented by the difference in boiling points of the components.
Activity coefficient vs Xa diagram: For an ideal solution, the activity coefficient has a constant value of 1, since it has no deviation.
Example by thermocuates
Equilibrium diagrams for a mixture Acetone-Water
For the acetone-water mixture Txy diagram there is no clear positive nor negative type of deviation, actually its behavior is similiar to an ideal one.
In the same way as the Txy diagram, there is no clear positive nor negative type of deviation, actually its behavior is similiar to an ideal one.
In this case, the difference in boiling points is very clear, meaning that acetone tends to evaporate.
In this case, the diagram shows a positive deviation that indicates repulsion intermolecular forces. This is one of the reasons that makes this type of mixture easy to separate, by having repulsion forces, acetone is going to evaporate more easily.
distillation
Assuming an acetone composition of 0.45, how many distillation stages are needed in order to obtain the highest purity?
According to the Txy diagram, as we can see in the black line, the highest purity we can reach is approximately 0.83. The number of stages needed to reach that purity, is one.
MIT. OpenCourseware. Thermodynamics and kinetics. 2008