Azeotropic distillation (Dean-Stark)

What is an azeotrope?

An azeotrope is a mixture of two or more components whose proportions are such that the vapor produced by partial evaporation has the same composition as the liquid. Therefore, they behave in distillation as if they were a pure compound (see Table 1).

When a mixture is at the azeotropic point (azeotropic mixture), it cannot be distilled or separated into its components.

Therefore, when an acetotrope is to be separated by distillation, an additional third component (called "entrainer") is added to the mixture. The effect achieved is to change the volatility of one of the components of the azeotrope to a greater extent than the other, allowing the separation to take place.

Examples of azeotropes

There are a multitude of combinations between two solvents that result in an azeotropic mixture, and Table 1 summarizes the most common ones.

Table 1: Diferentes azeótropos formados por disolventes más comunes.
Compound ACompound BAzeotrope
B.p. (ºC)% (weight)B.p. (ºC)% (weight)B.p. (ºC)
H2O (100)1.3Diethyl ether (34.5)98.734.2
H2O (100)1.4Pentane (36.1)98.634.6
MeOH (64.7)12.1Acetone (56.1)87.955.5
MeOH (64.7)72.5Toluene (110.7)27.563.5
EtOH (78.3)68.0Toluene (110.7)32.076.7
H2O (100)13.5Toluene (110.7)86.584.1

What is azeotropic distillation?

Azeotropic distillation is a useful procedure for removing a liquid from a reaction crude by co-distillation with an immiscible organic solvent.

This technique is often used in equilibrium reactions where water is generated as a by-product of the reaction. The removal of water will shift the equilibrium of the reaction to the product side.

azeotropic distillation Dean-Stark equipment

Azeotrope water/toluene separation

If the reaction is carried out for example with toluene, which is less dense than water, the vapor in the reflux condenser will consist of an azeotropic mixture of toluene and water. When this mixture condenses, it falls into the so-called Dean-Stark apparatus, forming two phases, the upper layer consisting of toluene and the lower one of water.

azeotropic distillation Dean-Stark equipment

When the liquid level in the Dean-Stark trap reaches the top of the side arm, the toluene flows back into the reaction flask. Water can be removed through a tap at the bottom of the Dean-Stark trap.

The Dean-Stark apparatus Was invented in 1920 by U.S. Chemists Ernest Woodward Dean (1888-1959) and David Dewey Stark (1893-1979), applied to the determination of water in petroleum.

Application examples

The azeotropic distillation is also used in dehydration equilibrium reactions. Because some of them present not only an equilibrium that reverses the reaction when water appears, but the process is fast. Examples of this type of reactions are the formation of dioxolanes from aldehydes as shown in the scheme.

application example azeotropic distillation dehydration of aldehydes to give dioxolanes

When water is removed from the reaction by azeotropic distillation, the equilibrium shifts favorably towards the formation of the products.

Breaking azeotropes

Azeotropes of low boiling point, cannot be completely purified by distillation, to obtain the most volatile component. Alternatively, the pure most volatile component can be separated by breaking the azeotrope.

This is achieved by a method other than distillation, using molecular sieves. The use of molecular sieves is used to break the EtOH/H2O (96%) azeotrope and obtain the anhydrous alcohol, with the water remaining absorbed in the molecular sieves. The sieves can be reused later by dehydration using an oven.

Summary

Azeotropic distillation is a special technique in which another component is added to break intermolecular interactions, resulting in a hetereogenous mixture (as it produces two phases with immiscible liquids and new azeotrope). For example, toluene (better than benzene which is carcinogenic) can be added to an azeotropic mixture of ethanol/water, to separate the water from the azeotrope.

The Dean-Stark apparatus can be used to perform azeotropic distillations in the laboratory, and is also particularly suitable for separating water from a reaction, e.g. in toluene/water mixtures.