Mass Transfer
In an operating distillation column, the

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
Driving force for the liquid flow is its specific weight
Vapors and liquids are at their dew point and bubble point respectively

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Mass Transfer
In a counter-current liquid extractor

Only one of the liquids may be pumped at any desired rate
Both liquids can have any desired flow rate
Both liquids flow at fixed rate
Liquid's flow rate depends upon the temperature and pressure

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