Fractional distillation is a process to separate mixtures of volatile liquids and miscible liquids. It is commonly used to separate various components (difference in boiling point is less than 25oC) of a mixture depending upon their boiling points.
If a liquid is heated in closed container, some molecules present at the surface of the liquid evaporate into the space above the liquid. After vaporization, some of these molecules present in the vapor condense back into the liquid. The space above the liquid is occupied by molecules of vaporized liquid, the vapor pressure above the liquid (vapor pressure of a liquid) rises until it reaches a certain value. When the pressure stabilizes, the rates of evaporation and condensation become equal and the system reaches equilibrium.
Increase in temperature increases equilibrium vapor pressure of a liquid. As the temperature of a liquid rises, more molecules get vaporized. Consequently, the equilibrium vapor pressure increases. When the vapor pressure of a liquid is equal to the pressure of the atmosphere above the liquid, it boils. The normal boiling point of a substance is defined as the temperature at which the vapor pressure of that substance equals standard atmospheric pressure, 760 mmHg. If the barometric pressure is less than 760 mmHg, the temperature at which a substance boils will be less than the normal boiling point.
To compare observed boiling point to the one reported in literature, it is often required to compensate for differences in pressure. The following formula provides a good estimate to correct an observed boiling point to a normal boiling point.
∆t = 0.00012 (760 – p) (t + 273)
Where, ∆t = is the correction in degrees centigrade to be added to the observed boiling point t at barometric pressure p (in mmHg).
A basic method can also be used for estimating boiling points near 760 mm Hg i.e. the observed boiling point will differ from the normal boiling point by 0.5° for every 10mm Hg difference in pressure. Miscible liquids mix well together. These mixtures obey Dalton’s law of partial pressure, according to which, the vapor pressure above a mixture is equal to the sum of vapor pressures of the individual components.
PTotal = PA + PB
Where, PA and PB are the partial pressures of components A and B, respectively. The difference in the behavior of the two types of mixtures on distillation is due to differences in their partial pressures.
In a mixture of miscible substances, partial pressure of a component depends on the vapor pressure of pure component and relative amount of the component. According to Raoult’s law, the partial pressure of a component in an ideal solution is equal to the vapor pressure of the pure component multiplied by its mole fraction.
The mole fraction of component A and B, XA and XB respectively, which is defined as
XA = number of moles of A / number of moles of A + number of moles of B
As the vapor pressures of pure components increase with temperature, the vapor pressure of each molecule above mixture also increases. When the vapor pressure of the mixture reaches 760 mmHg, it starts boiling. For ideal solutions, i.e., solutions that obey Raoult’s law, boiling point of the mixture will be between boiling points of the pure components.
Increasing surface area increases the purity of the distillate due to increase in distillation process. As vapor travels up the column it condenses on surface and then re-evaporates. The process repeats causing vapor to purify at each step. Units for measuring how many times vapor enters the cycle of condensation and re-evaporation are called the theoretical plates.
Distillation flask is a round-bottom flask and is sometimes referred to as “the pot”. The liquid to be distilled must fill the distillation flask to 1/2 to 2/3 of its capacity. For even boiling of liquid, boiling chips or magnetic stirrer is added to the liquid before heating it.
The adapter connects distillation flask, condenser, and thermometer. It is often referred as a “distillation head”. The position of a thermometer is adjusted so that the bulb below the adapter side arm is connected to the condenser.
Condenser cools the vapor to liquid and directs this condensate to the receiving flask. The most common type of condenser used is the water-jacketed type.
Fractionating column is a piece of glassware used to separate vaporized mixtures of liquid compounds with close volatilities. These columns help to separate the mixture by allowing the mixed vapors to cool, condense and vaporize again. Columns can be filled with glass or plastic beads. Filled columns improve the separation between the liquids being distilled.
The receiving flask
Container to collect the condensed vapor is called the receiver. It may be round-bottom flask, an Erlenmeyer flask, a bottle, or a graduated cylinder.
A technique, called Steam distillation, is used to purify high boiling organic compounds by distilling immiscible mixture of impure compound and water. For example, a perfume industry uses this technique to purify many essential oils.
Applications of Fractional Distillation
Various applications of fractional distillation are mentioned below:
- Fractional distillation is used for the concentration of heavy isotopes of oxygenthe oil industry, fractional distillation is used to separate the compounds in crude oil. For example, in petroleum refining (Figure 3), gasoline gets separated first, followed by kerosene, heating oil and lubricating oil depending on their boiling points.
- It is used to purify alcohol and to make stronger alcoholic drinks. It is also used for solvent recycling, extraction of essential oils and purification of fragrances in perfume industries.
- Fractional distillation is used for separating salt from seawater as well as pure water from seawater.