Mass Transfer
When the solvent dissolves very little of solute, then

Large quantity of solvent is required to extract the solute
Solvent of low latent heat of vaporisation should be used
Solvent of low freezing point should be used
Very small quantity of solvent is required

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Mass Transfer
To get high tray efficiency

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

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