Flood trays spray entrainment

At lower tray spacing, entrainment flooding may be related to lifting of the froth envelope and to froth rather than spray height. This correlation must not be extended to lower tray spacing. 

Spray entrainment flooding (Fig. 6.7a). At low liquid flow rates, trays operate in the spray regime, where most of the liquid on the tray is in the form of liquid drops (Figs, 6,25c and 6.276), As vapor velocity is raised, a condition is reached where the bulk of these drops are entrained into the tray above. The liquid accumulates on the tray above instead of flowing to the tray below,... 

Figure 6.7 Common flooding mechanisms in tray columns, (a) Spray entrainment flood (ft) froth ontrainment flood (c) downcomer backup flood Id) downcomer choke flood. (Parle a and ft reproduced from Dr. D. C. Hausch, Discussion of Paper Presented In the Fifth Session, Proceedings of the International Symposium on Distiliation, the Institution of Chemical Engineers (London), I960, reprinted courtesy of the Institution of Chemical Engineers, UK- Parts c and d from H. Z. Kister. Distillation Operation. Copyright C 1990 6y McGraw-Hill, Inc. reprinted by permission.)...

The factors affecting froth entrainment flooding differ from those effecting spray entrainment flooding. The critical variable is the distance between the top of the froth and the tray above (20,39,41), This... 

Capacity restriction mechanism(s). Column throughput is restricted by one of several different mechanisms. These include spray entrainment flooding, froth entrainment flooding, downcomer backup flooding, downcomer choke flooding, excessive entrainment and excessive pressure drop. Optimum tray and downcomer layouts vary with the mech-... 

When the tray spacing is large (> 18 to 24 in), the froth envelope seldom approaches the tray above. As vapor velocity is raised, a condition is reached when some of the froth inverts into spray. Flooding will then take place by the previously described spray entrainment mechanism. 

Sieve trays, spray regime. Several correlations for dear liquid hei t in the spray regime were proposed (29,35,85,87-91). Lockett (12) recommended Stichlmair s (29) correlation. Since then Kister and Haas (36) modified an earlier correlation by Jeronimo and Sawistowski (35) and used it successfully as a building block for correlating entrainment flooding and spray regime entrainment. The modified Jeronimo and Sawistowski correlation is... 

Entrainment occurs when spray or froth formed on one tray enters the gas passages in the tray above. In moderate amounts, entrainment will impair the countercurrent action and hence drastically decrease the efficiency. If it happens in excessive amounts, the condition is called priming and will eventually flood the downcomers. 

In the spray regime, flooding (usually called jet flooding) is caused by excessive entrainment of liquid from an active area to the tray above. It increases the tray pressure-drop, and the entrained liquid recirculates to the tray below. The larger liquid load in the downcomer and the increased tray-pressure-drop together cause the downcomer to overfill so the tray floods. 

Hole Sizes Small holes slightly enhance tray capacity when limited by entrainment flood. Reducing sieve hole diameters from 13 to 5 mm ( to in) at a fixed hole area typically enhances capacity by 3 to 8 percent, more at low liquid loads. Small holes are effective for reducing entrainment and enhancing capacity in the spray regime (Ql < 20 m3/hm of weir). Hole diameter has only a small effect on pressure drop, tray efficiency, and turndown. 

Entrainment (Jet) Flooding Froth or spray height rises with gas velocity. As the froth or spray approaches the tray above, some of the liquid is aspirated into the tray above as entrainment. Upon a further increase in gas flow rate, massive entrainment of the froth or spray begins, causing liquid accumulation and flood on the tray above. 

When Fair s correlation was developed, little was known about the difference between spray and froth entrainment flooding, and the data base used was small and included both types. The author compared predictions from Fair s correlation to a much wider data bank available at present. The correlation predicted most of these data well, perhaps somewhat on the conservative side. However, the correlation has been less successful ii. reliably predicting some of the effects (described above) of physical properties, operating variables, and tray geometry on entrainment flooding. 

Froth entrainment flooding is only encountered at close tray spacing. It can be excluded from any capacity checks when tray spacing exceeds 18 in (except when conditions are conducive to vapor cross flow, Sec. 6.2.4). Note that at low tray spacing (< 18 in), either spray or froth entrainment flooding can restrict capacity. 

---