Definition:
The P-xy and the T-xy are diagrams that represent the liquid and vapour equilibrium for a binary mixture. The component that is graphed is the most volatile one because is the one that will evaporate first during the distillation process. On the x-axis goes the mole fraction x,y (for liquid phase and vapour phase) and the y-axis is the temperature if its a P-xy or the pressure if its a T-xy.
These diagrams are useful to predict the composition of a phase in a binary mixture depending on the the temperature and the pressure of the system. The driagram indicates de molar fraction of the most volatile compound in the vapour (yi) or liquid phase (xi).
According to the statement that the sum of the molar fractions of all the components in a mixture is equal to one (1), the molar fraction of the other component can be calculated with the equation:
(Eq. 3-11)
Where
xi/j=molar fraction of component i/j in the liquid phase
yi/j=molar fraction of component i/j in the vapour phase
The P-xy and the T-xy are diagrams that represent the liquid and vapour equilibrium for a binary mixture. The component that is graphed is the most volatile one because is the one that will evaporate first during the distillation process. On the x-axis goes the mole fraction x,y (for liquid phase and vapour phase) and the y-axis is the temperature if its a P-xy or the pressure if its a T-xy.
These diagrams are useful to predict the composition of a phase in a binary mixture depending on the the temperature and the pressure of the system. The driagram indicates de molar fraction of the most volatile compound in the vapour (yi) or liquid phase (xi).
According to the statement that the sum of the molar fractions of all the components in a mixture is equal to one (1), the molar fraction of the other component can be calculated with the equation:
(Eq. 3-11)
Where
xi/j=molar fraction of component i/j in the liquid phase
yi/j=molar fraction of component i/j in the vapour phase
Figure 3.1. T-xy diagram example.
Figure 3.2. P-xy diagrams example.
Do you wonder what does the boundarie formed by the x-T (blue line) and the one by the y-T (green line) represent? Well, these two concepts will help you understand their meaning:
Bubble point: The temperature at which a liquid at an specific pressure forms the first vapour bubble. In other words, the temperature at which saturation occurs in the liquid phase. [2]
Dew point: The temperature at which the first drop of liquid condenses at an specific pressure or the temperature at which saturation occurs in the vapour phase. [2]
Solved problem
Based on the figure 3-3 , estimate the bubble point and composition of the vapour; and dew point and the liquid composition associated to a binary mixture of 70% benzene and 30% toluene @P=1atm.
Bubble point: The temperature at which a liquid at an specific pressure forms the first vapour bubble. In other words, the temperature at which saturation occurs in the liquid phase. [2]
Dew point: The temperature at which the first drop of liquid condenses at an specific pressure or the temperature at which saturation occurs in the vapour phase. [2]
Solved problem
Based on the figure 3-3 , estimate the bubble point and composition of the vapour; and dew point and the liquid composition associated to a binary mixture of 70% benzene and 30% toluene @P=1atm.
Figure 3.3. T-xy diagram. BenCene in a mixture with toluene. (Made in Aspen Plus)
Solution:
Remember: blue line represent the bubble line and the green line, the dew line.
First, we will locate the molar fraction of the benzene (0.7) on the x-axis and connect it to the blue line to find the bubble point, red circle (see Figure 3-4). And the temperature, can be located in the y-axis. The result is 86C approximately.
The composition of vapour phase is calculated locating the dew point at the temperature previously found (1). The vapour composition of benzene @86C is 0.86, so the toluene composition in the vapour is 0.14.
For the dew point: using 0.7 mole fraction, move from mole fraction to the green line(green circle). In this case we will have a temperature of 92.5C. The liquid composition at this temperature will be calculated moving from the dew point to the blue line. The result is 0.49 for benzene and 0.51 for toluene.
Remember: blue line represent the bubble line and the green line, the dew line.
First, we will locate the molar fraction of the benzene (0.7) on the x-axis and connect it to the blue line to find the bubble point, red circle (see Figure 3-4). And the temperature, can be located in the y-axis. The result is 86C approximately.
The composition of vapour phase is calculated locating the dew point at the temperature previously found (1). The vapour composition of benzene @86C is 0.86, so the toluene composition in the vapour is 0.14.
For the dew point: using 0.7 mole fraction, move from mole fraction to the green line(green circle). In this case we will have a temperature of 92.5C. The liquid composition at this temperature will be calculated moving from the dew point to the blue line. The result is 0.49 for benzene and 0.51 for toluene.
Figure 3.4. Problem solution
How does distillation columns work with equilibrium stages?
Distillation is the most widely used separation process in the chemical industry. It is also known as fractional distillation and it is used to separate liquid mixtures into two or more vapour or liquid products with different compositions.
This process is an equilibrium stage operation. In each stage, a vapour phase is contacted with a liquid phase and mass is from vapour to liquid and from liquid to vapour. The less volatile component concentrate in the liquid phase, and the most volatile concentrates in the vapour.
The product leaving from the top of the column is called the overhead product; and the product leaving from the bottom, the bottom product.
This column have been built with a set of “plates”, which of them is designed to promote contact between the vapour and the liquid stage, and every plates has a different temperature. [12]
Distillation is the most widely used separation process in the chemical industry. It is also known as fractional distillation and it is used to separate liquid mixtures into two or more vapour or liquid products with different compositions.
This process is an equilibrium stage operation. In each stage, a vapour phase is contacted with a liquid phase and mass is from vapour to liquid and from liquid to vapour. The less volatile component concentrate in the liquid phase, and the most volatile concentrates in the vapour.
The product leaving from the top of the column is called the overhead product; and the product leaving from the bottom, the bottom product.
This column have been built with a set of “plates”, which of them is designed to promote contact between the vapour and the liquid stage, and every plates has a different temperature. [12]
Figure 3.5. T-xy diagram showing theoretical plates for the distillation of Ethylene Glycol and naphthalene.
Azeotropes:
Definition:
An azeotrope is when within a system that is being distilled, the liquid and vapor phases have the same molar fraction. [3].
An azeotrope is when within a system that is being distilled, the liquid and vapor phases have the same molar fraction. [3].
Figure 3.6. T-xy diagram for the mixture formic acid-Toluene at P=1atm showing an azeotrope. Simultation done with ASPEN Plus using NRTL and UNIQUAC activity coefficient models.
Azeotropic Distillation:
Azeotropic distillation is a type of multicomponent fractionation and which is used for separating binary mixtures. This method involves the addition of a third component which is not present in the mixture. By adding this third component, which is a separation occurs altering the volatilities of the components and thus increase its volatility between them.
This type of distillation is usually divided into two types: homogeneous and heterogeneous, where the first refers to the broken phase does not appear in the liquid and the second, the two liquid phases exist in the same region of space composition.
Azeotropic distillation is a type of multicomponent fractionation and which is used for separating binary mixtures. This method involves the addition of a third component which is not present in the mixture. By adding this third component, which is a separation occurs altering the volatilities of the components and thus increase its volatility between them.
This type of distillation is usually divided into two types: homogeneous and heterogeneous, where the first refers to the broken phase does not appear in the liquid and the second, the two liquid phases exist in the same region of space composition.
2015. Ana Botello, Jessica Moctezuma, Lenia Sánchez, Mariana Muñoz, María José Morales