Froth entrainment flooding

The entrainment flooding prediction methods described here are based primarily on spray entrainment flooding. Considerations unique to froth entrainment flooding can be found elsewhere (Kister, Disfiliation Design, McGraw-Hill, New York, 1992). 

Froth entrainment flooding (Fig. 6.7b). At higher liquid flow rates, the dispersion on the tray is in the form of a froth (Figs. 6.25c and 6.27a), When vapor velocity is raised, froth height increases. When tray spacing is small, the froth envelope approaches the tray above, As this surface approaches the tray above, entrainment rapidly increases, causing liquid accumulation on the tray above. 

Entrainment flooding can be classified into spray entrainment flooding and froth entrainment flooding. Spray entrainment flooding is far more common. Froth entrainment flooding is only encountered when... 

Conditions favor vapor cross flow These are the only conditions under which froth entrainment flooding is likely to be encountered when tray spacing exceeds 18 in. 

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. 

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... 

Froth entrainment flood can be predicted using the Fair or the Smith et al. correlations. Both included froth entrainment flood data in their data base. Testing of Fair s correlation (20,41) against a handful of more recent data suggests that it gives conservative froth entrainment flood predictions. The Smith et al. correlation is claimed 120) to be less conservative. The Kister and Haas correlation is unsuitable for froth entrainment flood prediction. 

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. 

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-... 

Rgure 14.1 Common flooding mechanisms in tray columns, (c) Spray entrainment flood (6) froth entrainment flood (c) downcomer backup flood id) downcomer choke flood. PaHs a and b reproduced from Dr. D. C. Hausch, "Discussion of Papers Presented in the Fifth Session/ Proceedings of the International Symposium on Distillation, the Institution of Chemical Engineersj London, 1960 reprinted courtesy of the Institution of Chemical Engineers, UK.)... 

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