Identify the substance

Glycerol ($\mathrm{C_3H_5(OH)_3}$) is a high-boiling, viscous liquid.

Recall its behavior on heating

Glycerol has a very high boiling point (about $290^\circ\mathrm{C}$) and may decompose if heated strongly at atmospheric pressure.

Method used in soap industry

In soap manufacture, glycerol is obtained from the spent lye. Since glycerol is heat-sensitive and has a high boiling point, it is separated and purified by distillation under reduced pressure (vacuum distillation).

Under reduced pressure, the boiling point decreases, so glycerol can be distilled without significant decomposition.

Check the options

• A: Fractional distillation — not preferred because glycerol’s high boiling point causes difficulty at normal pressure.
• B: Distillation under reduced pressure — correct.
• C: Differential extraction — not the standard industrial separation method here.
• D: Steam distillation — used for steam-volatile compounds; glycerol is not steam-volatile.

Conclusion

Therefore, glycerol is separated in soap industries by: $\boxed{\text{Distillation under reduced pressure}}$