Mass Transfer
In an operating distillation column, the

Driving force for the liquid flow is its specific weight
Highest temperature is encountered at the top of the column
Vapors and liquids are at their dew point and bubble point respectively
Driving force for the vapor flow is the pressure drop, as the pressure decreases gradually from the bottom to the top of the column

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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
Total pressure is less
Temperature is less

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