Tray Flooding Mechanisms

Flooding is excessive accumulation of liquid inside the column. This accumulation is generally caused by one of the following mechanisms. 

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, 

When the tray spacing is large ( 18 to 24 in), the froth envelope seldom approaches the tray above. As vapor velocity is raised, a con- 

At high liquid rates ( 6 gpm/in of outlet weir), high ratio ( 2.5) of flow-path length to tray spacing, and a high fractional hole area ( 11 percent), cross flow of vapor in opposite direction to the liquid can build up froth near tray inlet and center. The froth buildup raises the 

Froth entrainment flooding (Fig. 14.1b). At higher liquid rates, the dispersion on the tray is in the form of a froth (Fig. 6.1a). When vapor flow rate 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. 

Downcomer backup flooding (Fig. 14.1c). Aerated liquid is backed up into the downcomer because of tray pressure drop, liquid height on the tray, and frictional losses in the downcomer apron. All of these increase when liquid flow rate is raised, while tray pressure drop also increases when vapor flow rate is raised. When the backup of aerated liquid in the downcomer exceeds the tray spacing, liquid accumulates on the tray above, causing downcomer backup flooding. 

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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.)...

Flooding- mechanism Entrainment (jet) flood only Tray typee Sieve or valve trays only Pressure 1,5-500 psia (Note 1)... 

The flooding mechanism is complex, depending on the type of tray. The UV ratio and the column s pressure are the key factors. As illustrated in Fig. 16.5 at low pressures the entrainment flooding is the main limiting factor of column capacity, while at higher pressures the flooding in downcomer should be taken into consideration. More details about this issue can be found in Kister (1992). 

Having estimated the number of trays, the column diameter can then be estimated. This is usually estimated with reference to the flood point for the column. The flood point occurs when the relative flowrates of the vapor and liquid are such that the liquid can no longer flow down the column in such a way as to allow efficient operation of the column14. For plate columns, there are a number of different mechanisms that can create flooding, but in one way or another, either14 ... 

In the froth regime, which is between the spray and emulsion ones, flooding may be by either mechanism, depending on the tray spacing and the particular combination of vapour and liquid loads. 

Flooding is an excessive accumulation of liquid inside a column. Flood symptoms include a rapid rise in pressure drop (the accumulating liquid increases the liquid head on the trays), liquid carryover from the column top, reduction in bottom flow rate (the accumulating liquid does not reach the tower bottom), and instability (accumulation is non-steady-state). This liquid accumulation is generally induced by one of the following mechanisms. 

Steve Tray Capacity at Flooding The capacity of a sieve tray is determined by hydraulic mechanisms involved in flooding and is not... 

Ultimately, purely mechanical difficulties arise. High pressure drop may lead directly to flooding. With a large pressure difference in the space between trays, the level of liquid leaving a tray at relatively low pressure and entering one of high pressure must necessarily assume an elevated position in the downspouts, as shown in Figure 4.6. As the pressure difference increases due to an increased rate of flow of the gas, the level in the downspout will rise further to permit the liquid to enter the lower tray. Ultimately, the liquid level in the downcomer may reach that on the tray above and the liquid will fill the entire space between the trays. The tower is then flooded, the tray efficiency falls to a very low value, the flow of gas is erratic, and liquid may be forced out of the gas exit pipe at the top of the tower. 

The downcomer in Fig. 4.12a is likely to lose its seal whenever its liquid height drops below tray level. When the seal is lost, vapor fium the tray ascends the downcomer, which may cause flooding, cycling, and/or poor separation. Downcomer unsealing by this mechanism is most likely to occur when the liquid drawn constitutes a laige portion of the downcomer liquid flow, when the quantity drawn tends to fluctuate, and/or when excessive leakage takes place due to tray weeping or draw pan leaks. 

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-... 

Another mechanical problem often encountered in valve trays is "sticking of the valve. Sticking occurs because of buildup of sludge and/or corrosion products while the valve is in contact with the tray floor. Sticking reduces the open area of the tray and can initiate premature flooding. 

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