Mass Transfer
In an operating distillation column, the

Driving force for the liquid flow is its specific weight
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
Highest temperature is encountered at the top of the column

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Mass Transfer
To get high tray efficiency

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

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