Distillation is one of the most important mass transfer operations. And for binary distillation relative volatility is a very important parameter.
Relative Volatility can not only help us find difficulty in separation by distillation but also helps in finding several theoretical plates required for the given separation.
So I hope this will help you understand the importance of relative volatility, Now let's learn about it.
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Volatility is the ratio of the mole fraction of a component in vapor (indicated by y*) which is in equilibrium with liquid and the mole fraction of that component in liquid (indicated by x) at a specified (fixed) temperature.
Volatility is also called the vapor-liquid distribution coefficient and is represented by the K symbol (K value).
Here,
Now, Relative volatility is the ratio of the volatility of one component to the volatility of another component in a binary mixture. It is important to keep in mind that relative volatility is only valid for binary mixtures.
Relative Volatility of A concerning B = Volatility of Component A / Volatility of Component B
As you might get from the above equation that relative volatility is a dimensionless quantity.
Relative Volatility can not only help us find difficulty in separation by distillation but also helps in finding several theoretical plates required for the given separation.
So I hope this will help you understand the importance of relative volatility, Now let's learn about it.
Table of Content
What is Relative Volatility?
Before defining relative volatility, Let's understand what is volatility.Volatility is the ratio of the mole fraction of a component in vapor (indicated by y*) which is in equilibrium with liquid and the mole fraction of that component in liquid (indicated by x) at a specified (fixed) temperature.
The volatility of component A = y*/x
Volatility is also called the vapor-liquid distribution coefficient and is represented by the K symbol (K value).
Here,
- y* = Mole fraction of component A in vapor phase which is in equilibrium with the liquid at a specified temperature.
- x = Mole fraction of component A in liquid phase at a specified temperature
Now, Relative volatility is the ratio of the volatility of one component to the volatility of another component in a binary mixture. It is important to keep in mind that relative volatility is only valid for binary mixtures.
The Formula of Relative Volatility
For the mixture of two-component A (Lower Boiling Point Component) and B (Higher Boiling Point Component).Relative Volatility of A concerning B = Volatility of Component A / Volatility of Component B
(Relative Volatility)AB = (y*A/xA)/(y*B/xB)
As you might get from the above equation that relative volatility is a dimensionless quantity.
Importance of Relative Volatility
- Relative Volatility is helping us determine separation difficulty for any binary mixture by distillation. For mixtures with relative volatility with very large values are easy to separate by distillation. For this type of mixture, simple distillation can be the best choice.
- For very low relative volatility mixtures are difficult to separate by simple distillation.
- Azeotropic mixtures produce vapors that have the same composition as liquid. So, the azeotropic mixture posses relative volatility equal to one. So they are impossible to separate by simple distillation. These type of mixtures requires special types of distillation techniques such as azeotropic or extractive distillation.
- Also, relative volatility is used in many important equations such as the Fenske equation to find a minimum number of theoretical plates required for a given separation.
- Relative volatility is a very important quantity in designing distillation columns.