Flood point concept and traps

Rood-point definition. In 1966, Silvey and Keller (54) listed 10 different flood point definitions that have been used by different literature sources. A recent survey (59) listed twice that many. As Silvey and Keller pointed out (54) the existence of so many definitions puts into question what constitutes flooding in a packed tower, and at what gas rate it occurB. Symptoms used to identify flood in these definitions include appearance of liquid on top of the bed, excessive entrainment, a sharp rise in pressure drop, a sharp rise in liquid holdup, and a sharp drop in efficiency. An examination of the definitions shows some conflicts in inferring the flood point from even a single symptom. For instance, one definition sets the flood point when "slight splashing" occurs, while another sets it when a higher degree of entrainment ("marked spraying") is experienced. Many definitions use inconcise terms such as "appreciable amounts," or "a very high value," thus leaving room for subjectivity. Other definitions use arbitrary criteria such as "Vfe-in liquid buildup on top of the bed" or a "pressure drop of 2 in of water per foot of packing." Finally, Kunesh (51) points out that flooding is an inherently unstable condition, and that an observer using a single definition consistently may report different results for two experimental runs because of variations of such factors as an increase in boilup and the stability of pressure control.

Based on the above, some designers argued (5,15,57) that the traditional approach of using the flood point as the upper capacity limit of a packed column is best abandoned. They recommended shifting to alternate criteria such as maximum operational capacity or maximum permissible pressure drop. Others (3,37,41,51,58-60) stick with the flood-point criterion while recognizing its limitations. There are strong, practical reasons for retaining the flood point as the prime criterion for the upper capacity limit.

First and foremost, the alternative capacity criteria—namely the maximum operational capacity and the maximum pressure drop— have been demonstrated to be less reliable than the flood point (see Sees. 8.2.4 and 8.2.5). Bolles and Fair (55), MacDougall (58), and

Kister and Gill (60,60o) demonstrated that despite differences in definitions, flood-point data compared quite well to correlation predictions. Both Kister and Gill (60,60a) and MacDougall (58) show that flood data from various sources (using various definitions) can be correlated to with ± 10 to 15 percent accuracy. It was also demonstrated that the flood point can be predicted far more reliably than packing pressure drop (55,58) and maximum operational capacity (60).

Second, flood point data are easy to find, while maximum operational capacity data are less abundant. Generating maximum operational capacity data requires expensive efficiency measurements, while flood-point determination requires far less expensive tests.

Third, maximum operational capacity data are practically nonexistent at high liquid rates. Efficiency measurements are usually performed at or close to total reflux (liquid to vapor mass ratios of about unity) in order to prevent pinching from impairing data accuracy. In order to obtain data at high liquid rates, liquid to vapor mass ratios of the order of 2 to 3 or more are usually required.

Which definition to use? The surveys of Kister and Gill (59,60a) suggest that most flood-point definitions describe the point of flooding initiation ("incipient flooding"; point C on Figs. 8.15 and 8.16a). There are only three exceptions—"The slope of the pressure drop curve (plotted against gas velocity) goes to infinity"; "the gas velocity at which efficiency goes to zero"; and "The pressure drop reaches 2 in of water per foot of packing." These exceptions describe fully developed flood conditions. The different incipient flooding definitions gave surprisingly little scatter of flood-point data (for a given packing under similar operating conditions). It follows that any definition describing flooding initiation should be satisfactory.

The author believes that due to the variations in the predominant symptom with the system and the packing, the use of multiple symptoms is most appropriate. The author prefers the following definition by Bravo and Fair (50), "A region of rapidly increasing pressure drop with simultaneous loss of mass transfer efficiency. Heavy entrain-ment is also recognized as a symptom of this region." An almost identical definition was presented earlier by Billet (56).

8.2.4 Maximum operational capacity (MOC): concept and traps

The maximum operational capacity or throughput is defined (15) as the "Maximum vapor rate that provides normal efficiency of a packing" (i.e., point F in Fig. 8.16a).

The MOC is clear-cut in Fig. 8.16a. On the other hand, locating the

MOC in Fig. 8.16d is difficult and leaves a lot of room for subjectivity. Further, MOC determination is sensitive to the accuracy of efficiency measurements—factors such as liquid and vapor distribution, sampling procedure, end effects at the top and bottom of the packings, and the availability of a large number of efficiency points near the MOC, where conditions may not be steady-state. For instance, for one set of published experimental data reported by Eckert and Walter (61), MOC increased by about 10 percent due to reducing packed height from 10 to 5 ft.

The MOC is a useful upper-capacity criterion, specifically related to packing efficiency. However, the MOC is far from the panacea for resolving the uncertainties inherent in the flood-point concept, as it substitutes those for new uncertainties.

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