Distillation columns froth contacting

The trays in a real distillation column are not equilibrium stages. In other words, the vapor above the froth is not in equilibrium with the liquid leaving the tray. The degree of separation on a real tray is, of course, determined as much by mass and energy transfer between the phases being contacted on a tray as it is by thermodynamic equilibrium considerations. This is where an efficiency comes into the picture. 

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