Distillation and Absorption Tray Efficiencies

In all the previous discussions on theoretical trays or stages in distillation, we assumed that the vapor leaving a tray was in equilibrium with the liquid leaving. However, if the time of contact and the degree of mixing on the tray is insufficient, the streams will not be in equilibrium. As a result the efficiency of the stage or tray is not 100%. This means that we must use more actual trays for a given separation than the theoretical number of trays determined by calculation. The discussions in this section apply to both absorption and distillation tray towers. 

Three types of tray or plate efllciency are used overall tray efficiency Eq, Murphree tray efficiency E , and point or local tray efllciency E ff (sometimes called Murphree point efficiency). These will be considered individually. 

For example, if eight theoretical steps are needed and the overall efficiency is 60%, the number of theoretical trays is eight minus a reboiler, or seven trays. The actual number of trays is 7/0.60, or 11.7 trays. 

Two empirical correlations for absorption and distillation overall tray efficiencies in commercial towers are available for standard tray designs (OI). For hydrocarbon distillation these values range from about 50 to 85% and for hydrocarbon absorption from about 10 to 50%. These correlations should only be used for approximate estimates. 

Murphree tray efficiency. The Murphree tray efficiency is defined as follows  

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