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
To get high tray efficiency

Time of contact between the two phases should be less
Gas velocity should be very low
Interfacial surface between liquid and gas phase should be large
Liquid entrainment should be severe

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