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
Vapors and liquids are at their dew point and bubble point respectively
Highest temperature is encountered at the top of the column
Driving force for the liquid flow is its specific weight

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Mass Transfer
Blowdown in a cooling tower

Means discarding a small fraction of circulating water to prevent and limit the concentration of salt and impurities
All of these
Is undesirable
Increases the scale forming tendencies of water

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