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

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

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

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