[curiouscat]

Consider a pair of compounds A & B whose Normal BPs differ by 15 C.

Now if I have another pair of compounds C & D also with the same difference in BPs will both separations necessarily be equally hard? Same difference in BPs but not identical BPs.

i.e. "equally hard" meaning Same degree of separation achievable in identical number of distillation stages.

[AWK]

For some mixtures you cannot separate compounds in distillation process.

[Enthalpy]

Rationale attempt:

• The efficiency of a separation step depends on the ratio of vapour pressure of the compounds at identical temperature.
• At its normal boiling point, the vapour pressure of each compound is 1atm.
• A bigger heat of vaporization lets the partial vapour pressure of a compound changer faster with the temperature.
• So the efficiency of a separation step depends not only on the difference of the normal boiling points but also on the heats of vaporization.

This is only reasoning. Experience may tell something different.

As well, and still because of the heats of vaporization, distillation can be easier or harder depending on the chosen operating pressure.

[Corribus]

The way the substances behave as mixtures may not be identical. E.g., if A and B have very strong interactions with each other, and C and D don't.

[Yggdrasil]

Boiling point does not tell the whole story.  For example, water and methylcyclohexane differ in boiling point by 1^oC, but separating them is trivial.  Water and ethanol have quite different boiling points, but because they form an azeotrope, separation via distillation is not straightforward.

[curiouscat]

For some mixtures you cannot separate compounds in distillation process.

Indeed. Let's leave azeotropes aside for the moment.