Mass Transfer
In extractive distillation, solvent is

Of high viscosity to give high tray efficiency
Of high volatility
Present in overhead stream
Added to alter the relative volatility of the mixture

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Mass Transfer
In case of steam distillation, the steam leaving the liquid is not completely saturated with distillate vapour, because

Saturated steam is used for steam distillation
Mixing of steam with the material being vaporised is not so intimate as to result in equilibrium condition
Temperature is less
Total pressure is less

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