Backup 1 Tray Efficiency

Swept-back weirs (Fig. 6.76) are sometimes used at high liquid loads. They extend the weir length, which in turn lowers the effective liquid load (gallons per minute per inch of weir length), without changing tray or downcomer area. Swept-back weirs reduce tray pressure drop and downcomer backup, improve liquid distribution on the tray, and improve tray efficiency by inducing liquid flow into peripheral stagnant zones. However, the above improvements are usually small. 

Weir height. This parameter sets the level of liquid on the tray in the froth and emulsion regimes (Fig. 17a,b). The higher the level, the better is the contact and the efficiency at the expense of a greater liquid backup in the downcomer. Typical absorption weir heights are 2-3 in. (50-75 mm). 

The clear liquid height, or the liquid holdup, is the height to which the aerated mass would collapse in the absence of vapor flow. The clear liquid height gives a measure of the liquid level on the tray, and is used in efficiency, flooding, pressure drop, downcomer backup, weep-... 

Save the file. Feel free to explore the other possible alternatives in the Data Browser when the distillation block is open. If you have time, look at Convergence and Tray Rating. Downcomer backup (under profiles in tray rating) is useful. We will look at Efficiency in section VI below. 

With relatively high outlet weir height hw, the clear liquid height h increases and vapor and liquid contact time increases. This improves distillation efficiency. However, a too high outlet weir height could affect the downcomer backup and tray capacity. 

This is like unsealing a tray s downcomer in a distillation tower. If the bottom edge of the downcomer from a tray is above the top edge of the outlet weir on the tray below, then vapor can blow up through the unsealed downcomer. This will prevent the internal reflux ft om draining down the column. Tower flooding and loss of product separation efficiency will result. This is called liquid flooding or excessive downcomer backup due to loss of the dovmcomer liquid seal. 

Note, though, that jet flood may be caused by excessive downcomer backup. It is simple to see in a glass column separating colored water from clear methanol how tray separation efficiency is reduced as soon as the spray height equals the tray spacing. And while this observation of the onset of incipient flood is straightforward in a transparent tower, how do we observe the incipient flooding point in a commercial distillation tower ... 

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