(88e) Modeling the Minimum Entrainment on Sieve Trays | AIChE

(88e) Modeling the Minimum Entrainment on Sieve Trays


Ohe, S. - Presenter, Science University of Tokyo

Entrainment on a distillation tray affects efficiency, pressure drop and the capacity of the distillation columns. Entrainment on a sieve tray increases with the increase of vapor load at constant liquid load. However, at constant vapor load, entrainment behaves somewhat differently. It has a minimum value. This phenomenon was uncovered by systematic study on a wide range of trays and systems at FRI (Fractionation Research Incorporated). Many arguments have been given for this phenomenon. Of these, the dominant one seems to be that the minimum is caused by the transition of vapor liquid contact from spray to froth on the tray. This study, however, argues from an entirely different point of view. An assumption is made that the longer the residence time of the liquid on the tray, the greater the chances of generation of entrainment, The reason is that the tray specification, especially the height of the outlet weir, affects the phenomenon of minimum entrainment. It was found that data released by FRI exhibited this new relationship between minimum entrainment and the newly defined minimum residence time at constant vapor rate and increasing liquid rate for sieve tray. The liquid load at which the entrainment becomes minimum at constant vapor load operation coincides with the liquid load at which the residence time of liquid on the tray becomes minimum. In the newly defined residence time, the definition of volume is the product of the bubbling area and the observed clear liquid height. The observed clear liquid height has a significant effect on the residence time on the tray. The clear liquid height as reported by FRI was obtained from a series of experiments for pressure drop study. This afforded an opportunity to predict the minimum of entrainment from only the value of clear liquid height. This relation can be applied to predict the minimum entrainment, if the clear liquid height is known. We have examined several correlation methods for determining clear liquid height. Among them, we focused on Bennett?s correlation, because it is the most recent of those examined. Like most other correlations, Bennett?s is also based on Francis?s equation. However, Francis?s correlation itself is not adequate, due to an anomaly in its derivation: namely, the value of the exponent. Francis's correlated between clear liquid height and liquid load using an exponent with a value of 2/3. However, in actual fact this fraction does not fit the observed data. On checking, we determined that the value should be greater than unity.


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