What are azeotropes, and how do they affect a distillation process? Azeotropic distillation is a process that can be used to separate azeotropes from other feed components. Discuss using a suitable example how this method can be applied to separate azeotropes during a distillation process.
The correct answer and explanation is :
Azeotropes and Their Effect on Distillation
An azeotrope is a mixture of two or more liquids that has a constant boiling point and retains its composition during phase changes, meaning it cannot be separated by simple distillation. This occurs due to specific interactions between the components, such as hydrogen bonding or dipole forces, which cause the mixture to behave as a single substance.
Azeotropes significantly impact distillation because they prevent the complete separation of components using conventional fractional distillation. Once the mixture reaches the azeotropic composition, further distillation does not change its composition, making it impossible to obtain pure components without special separation techniques.
Azeotropic Distillation for Separation
One effective method to break an azeotrope is azeotropic distillation, where an additional substance (called an entrainer) is introduced to alter the mixture’s volatility, enabling separation. This method is commonly applied to separate ethanol (C₂H₅OH) and water, which form an azeotrope at about 95.6% ethanol and 4.4% water (by weight) at standard pressure.
Example: Ethanol-Water Separation
To obtain pure ethanol beyond the azeotropic composition, an entrainer like benzene or cyclohexane is added. The process follows these steps:
- Addition of Entrainer: Cyclohexane is introduced into the distillation system, forming a new ternary azeotrope with water and ethanol.
- Distillation: The ternary azeotrope (water-ethanol-cyclohexane) distills off first because it has a lower boiling point than the ethanol-water azeotrope.
- Water Removal: The azeotrope separates into two liquid phases, with water in one phase and ethanol-cyclohexane in the other.
- Pure Ethanol Recovery: The cyclohexane is recycled, and nearly 100% pure ethanol is collected.
This method allows breaking the azeotropic barrier, making it possible to obtain pure ethanol suitable for industrial and pharmaceutical applications.
Here is an illustration of the azeotropic distillation process:
Here is an illustrative diagram showing the azeotropic distillation process for ethanol and water separation.