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

Total pressure is less
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

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Mass Transfer
Steam distillation is used to

Increase the efficiency of separation
Reduce the number of plates
Avoid thermal decomposition of a component
Increase the total pressure of distillation

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