Mass Transfer
If a two phase system is in physical equilibrium; then it means that, the

Total pressure throughout the liquid phase is equal to that throughout the vapor phase
Temperature of the liquid phase is equal to that of the vapor phase
All of these
Escaping tendency of each component from the liquid phase to the vapor phase is exactly equal to that from vapor phase to liquid phase

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Mass Transfer
To get high tray efficiency

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

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