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When a liquid mixtures is heated in a closed container, the mixture boils and starts forming vapor. The vapor will have different composition compared to the liquid. The vapors are obviously richer in more volatile component. If such vapors are collected and condensed, the condensed liquid will have different composition form original mixture and is richer in high volatile component. Read more
In order to separate binary mixtures using distillation process, there must be differences in volatilities of the components. The greater the difference, the easier it is to do so. A measure for this is termed relative volatility. Volatility is a way of expressing relationship between components of vapor Ya and that of liquid Xa in equilibrium. It is defined as ratio of partial pressure of component to mole fraction of component in liquid.
Equilibrium diagram (X-Y) diagram is simple diagram than boiling point diagram in which plot of temperature is eliminated. The boiling point diagram represents dynamic equilibrium of mole fraction of vapor and liquid phase at certain temperature at constant pressure. Boiling point diagram at constant pressure for binary mixture having component A and B is as below.
The equilibrium between the liquid and vapor phases of the system to be subjected to distillation is represented by vapor liquid equilibrium (VLE) diagram. Distillation columns are designed based on boiling point properties in the mixture being separated. Thus, the size particularly height of distillation column are determined by VLE data for the mixture.
Miscible liquid mixtures are of three types which can be revealed by plotting the vapor pressure against mole fraction of component.
1. First type of mixture:
The Vapor pressure curve exhibit minimum area. If we take mixture which has excess of ‘x’ (more volatile compound), we are somewhere at ‘C’ on the curve. On distillation, vapor will contain excess of ‘x’ and thus the remaining mixture will get richer in ‘y’. Finally we reach the point ‘D’ where vapor pressure is minimum and thus the boiling point is maximum. Here the mixture will distill unchanged in composition. Read more
Henry’s Law states that partial pressure on component on a solution is directly proportional to mole fraction in liquid.
Rault’s Law: Rault’s law states that at any particular constant temperature, partial pressure of one component of mixture is equal to mole fraction of that component multiplied by its vapor pressure in its pure form (state) at this temperature (saturation temperature).
Where,
