Glycerol is separated in soap industries by:

Step 1: Understand the Industrial Context

In soap manufacturing, animal fats or vegetable oils are hydrolysed with concentrated NaOH (saponification). The by-product of this process is glycerol (propane-1,2,3-triol), which remains mixed with spent-lye (aqueous NaOH + NaCl + water).

Step 2: Properties of Glycerol Relevant to Separation

• Glycerol (bp = 290°C) has a very high boiling point
• Glycerol is heat-sensitive — it decomposes (forms acrolein) at its normal boiling point
• Glycerol is miscible with water and cannot be separated by extraction or steam distillation

Step 3: Evaluate Each Method

(a) Differential extraction ❌ Glycerol is highly water-soluble (miscible with water). It has poor solubility in common organic solvents. Differential extraction would not work efficiently.

(b) Fractional distillation ❌ At 290°C, glycerol decomposes. Fractional distillation at normal pressure would destroy the product.

(c) Distillation under reduced pressure ✅ Reducing the pressure lowers glycerol's boiling point (e.g., to ~150–180°C at 15 mmHg). At this lower temperature, glycerol distils without decomposing. This is the standard industrial method.

(d) Steam distillation ❌ Glycerol is miscible with water — one of the key conditions for steam distillation (water-immiscibility) is NOT met.

Step 4: Conclusion

Answer: (c) Distillation under reduced pressure

Key Points to Remember:

• Glycerol: bp = 290°C, heat-sensitive → always use vacuum distillation
• Glycerol is water-miscible → steam distillation NOT applicable
• This is the industrial standard: saponification → soap + spent-lye → vacuum distillation → pure glycerol
• Other examples requiring vacuum distillation: glucose (decomposes >200°C), aniline dyes