Glycerol (glycerin) is a high-boiling point liquid (boiling point ≈ 290°C) that is obtained as a by-product in the soap industry from spent-lye. Spent-lye is an aqueous mixture containing glycerol, salts, and other impurities. The challenge is to separate glycerol without decomposing it.

At its normal boiling point, glycerol may undergo partial decomposition due to excessive heat. Therefore, distillation under reduced pressure (also known as vacuum distillation) is used. Reducing the pressure lowers the boiling point of glycerol, allowing it to be distilled at a much lower temperature without decomposition.

For example, under reduced pressure, the boiling point of glycerol decreases significantly. If the pressure is reduced to about 20 mmHg, glycerol boils at around 180°C instead of 290°C.

The other methods are not suitable:

• Simple distillation: Used for liquids with sufficiently different boiling points, but glycerol may decompose at its high normal boiling point.
• Steam distillation: Used for immiscible liquids and volatile compounds; glycerol is miscible with water and non-volatile under steam distillation conditions.
• Fractional distillation: Used for separating miscible liquids with close boiling points; here, the key issue is decomposition, not proximity of boiling points.

Thus, distillation under reduced pressure is the correct technique as it prevents thermal decomposition of glycerol.

Related Topics

Distillation Techniques: Different distillation methods are used based on the properties of the mixture. Simple distillation is for large differences in boiling points, fractional for close boiling points, steam for immiscible volatile components, and vacuum for heat-sensitive compounds.

Vapor Pressure and Boiling Point: The boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure. Reducing external pressure (via vacuum) lowers the boiling point. This relationship is approximated by the Clausius-Clapeyron equation:

where $\Delta H_{vap}^{◉}$ is the enthalpy of vaporization, R is the gas constant, and P₁, P₂ are pressures at temperatures T₁, T₂ respectively.

Formulae

Clausius-Clapeyron Equation: As above, it describes how vapor pressure changes with temperature, explaining why boiling point decreases under reduced pressure.