Flood trays froth entrainment

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.)... 

The action on this type of tray seems to produce fewer jets of liquid froth than a bubble cap tray. The entrainment from the surface of the bubbling liquid-froth mixture is less (about K) than a bubble cap tray for the same superficial tower velocity and tray spacing. Generally the trays will flood before capacity reaches a limitation set by 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. 

Equation (3.89) is the sieve tray liquid entrainment flood gas loading equation. Equation (3.89) sets the maximum gas rate VM- At a higher Vm, excess gas-liquid froth buildup would reach the tray above and recycle liquid to it. This liquid recycle would build up to a point at which the liquid would block any vapor passage, resulting in a flooded column and costly shutdown. 

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. 

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 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-... 

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 fact, the operation of a downcomer at the point of flooding can be simply illustrated froth from the active area flows over the weir onto the downcomer froth. The bulk of the vapour disengagement occurs probably in a very short time, and only the small bubbles in the froth are entrained downwards. Within the downcomer, the bubbles coalesce until they are large enough to rise out from the froth. If coalescence is not fast enough, some of the vapour is carried down to the tray below. Even with this simpler picture, the process is not easily modelled. 

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. 

Section 6.2.11 recommends the use of Fair s entrainment correlation in the froth (and emulsion) regime. This section also states that entrainment in the emulsion regime is unlikely to be a problem. Fair s correlation (Fig. 6.16) predicts fractional entrainment (pound of entrained liquid per pound of liquid) of 0.0 ll and 0.0075 at 80 percent of flood for the top and bottom trays, respectively. Even at 90 percent of... 

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