Tray liquid level, factors

The sum of the crest height plus the weir height equals the depth of liquid on the tray deck. One might now ask, Is not the liquid level on the inlet side of the tray higher than the liquid level near the outlet weir While the answer is Yes, water does flow downhill, we design the tray to make this factor small enough to neglect. 

Column efficiency experimental data are usually compiled and saved and are used for estimating efficiencies of similar columns or for developing empirical correlations for predicting the efficiencies. When attempting to estimate a tray efficiency based on data available for another column, it is important to be aware of the factors that influence tray efficiency the most and take them into consideration. For instance, the tray dimensions should be comparable. Of particular importance are the tray diameter, tray spacing, number of passes, and liquid level on the tray. 

The experimental equipment has proved exceedingly valuable in elucidating many of the factors such as hydraulic gradient, weeping" characteristics, effect of vapor velocity on foam height, weir performance under foaming conditions, downcomer liquid levels, effect of deviation from verticality, etc., which affect the hydraulic performance of various types of trays. 

When straight or serrated segmental weirs are used in a column of circiilar cross secdion, a correction may be needed for the distorted pattern of flow at the ends of the weirs, depending on liquid flow rate. The correction factor F from Fig. 14-33 is used direcdly in Eq. (14-112) or Eq. (14-119). Even when circular downcomers are utilized, they are often fed by the overflow from a segmental weir. When the weir crest over a straight segmental weir is less than 6 mm V in), it is desirable to use a serrated (notched) weir to provide good liquid distribution. Inasmuch as fabrication standards permit the tray to be 3 mm Vh in) out of level, weir crests less than 6 mm V in) can result in maldistribution of hquid flow. 

We have yet to discuss the most important factor in determining the height of liquid in the downcomer. This is the pressure drop of the vapor flowing through the tray deck. Typically, 50 percent of the level in the downcomer is due to the flow of vapor through the trays. 

Figure 1.8 is a realistic picture as to what we would see if our towers were made of glass. In addition to the downcomers and tray decks containing froth or foam, there is a quantity of spray, or entrained liquid, lifted above the froth level on the tray deck. The force that generates this entrainment is the flow of vapor through the tower. The spray height of this entrained liquid is a function of two factors ... 

In most cases, the solvent is much less volatile than the feed components it is therefore present mainly in the liquid phase in the primary column. This is desirable, as it is the liquid-phase nonidealities which give rise to the greater separation factor between the feed components. However, there is a relatively small amount of solvent in the vapor phase, and to avoid excessive loss of this solvent with the top product in the primary column, sufficient trays are provided above the solvent addition plate to reduce the solvent concentration in the top product to an acceptable level. 

There are diree factors that limit the accuracy of the preceding analysis. The first of these relates to the phenomenon of inverse response discussed in Chapter 13. It is characteristic of valve tray columns and some sieve tray columns operating at low boilup rates. It exercises its most serious effect in those columns where base level is controlled via steam flow. If the level becomes too high, the level controller increases the steam flow. But this causes a momentary increase in base level due to the extra liquid coming down the column (also due to thermosyphon reboiler swell ). Without proper design the level controller can become very confused. This is discussed in detail in Chapter 16. 

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