Murphree Efficiency

The Murphree Tray Efficiency EM , is based on a semi-theoretical models that assumes that the vapour between trays is well-mixed (uniform composition), that the liquid in the downcomers is well-mixed (uniform composition), and that the liquid on the tray is well mixed and is of the same composition as the liquid in the downcomer leaving the tray.

It is defined for each tray according to the separation achieved on each tray. This can be based on either the liquid phase or the vapor phase. For a given component, it is equal to the change in actual concentration in the phase, divided by the change predicted by equilibrium condition. Referring to the Figure below, we have:

Tray Efficiency

 

Vapor phase: Murphree's efficiency - Gas phase


Liquid phase: Murphree's Efficiency - Liquid phase

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The Figure below showed that the vapour phase efficiency is the fraction of the vertical height (change in vapour phase mole fraction) between the operating line and the equilibrium curve.

Murphree Efficiency

Once the Murphree Efficiency is known, it can be easily be used on the McCabe-Thiele design, in the form of a "pseudo" equilibrium line. This is shown in the Figure below.

Application of Murphree Efficiency

 

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