In the previous analysis, we have assumed that the vapor leaving each tray was in equilibrium with the liquid leaving the same tray. This analysis assumed that the trays are operating at 100% efficiency. However, in actual practice, the trays are not perfect, i.e. there are deviations from ideal conditions.
Except when temperature changes significantly (for example, in presence of exothermic chemical reaction) from tray to tray, the assumption that vapour and liquid leaving the trays are at the same temperature is often reasonable. The equilibrium with respect to mass transfer, however, is not often valid. This can be due to:
Insufficient time of contact
(2) Insufficient degree of mixing
Recall: When we used the McCabe-Thiele Method for continuous distillation operation, we already noted that not all triangles are of the same size.
Therefore in actual operation the vapor and liquid streams leaving a tray are not in equilibrium. To achieve the same degree of separation desired, more trays will have to be added to compensate for the lack of perfect separability. The actual number of trays required can be adjusted using the tray efficiency.
We will look at 2 types of tray efficiency: Overall Efficiency and Murphree Efficiency.
[ For more info on various types of tray efficiencies, see Chps. 7, 8 and 9, "Distillation Tray Fundamentals", M.J. Lockett ]
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