Dumping/weeping

Sieve Plates. The conventional sieve or perforated plate is inexpensive and the simplest of the devices normally used. The contacting orifices in the conventional sieve plate are holes that measure 1 to 12 mm diameter and exhibit ratios of open area to active area ranging from 1 20 to 1 7. If the open area is too small, the pressure drop across the plate is excessive if the open area is too large, the Hquid weeps or dumps through the holes. 

Dumping As gas velocity is lowered below the weep point, the fraction of liquid weeping increases until all the liquid fed to the tray weeps through the holes and none reaches the downcomer. This is the dump point, or the seal point. The dump point is well below the range of acceptable operation of distillation trays. Below the dump point, tray efficiency is slashed, and mass transfer is extremely poor. Operation below the dump point can be accompanied by severe hydraulic instability due to unsealing of downcomers. 

Above the weep point, W = 0 and fw = 0. At the dump point (Sec. 6.2.14], all liquid weeps from the tray, and fw 1.0. It was stated (50,70) that when weeping across the tray is uniform, the decrease in tray efficiency is usually considered acceptable with a weep fraction of up to 0.1. 

The most extensive studies on dumping were reported by Prince and Chan (77-79). The Chan and Prince dump-point correlation (Fig. 6,20 was recommended by Chase (30), The author has also had favorable experience with the correlation under conditions widely different from those used in its derivation. Alternatively, the dump point can be predicted from a weep rate correlation by setting the weep rate equal to 100 percent of the liquid entering the tray. However, little has been reported by either Lockett and Banik (56) or Hsieh and McNulty <63 about the reliability of dump-point predictions from their correlation. 

Weep point check. If the trays operate above their weep points at turndown conditions, checks of the fractional weep and dump point may not be needed. The weep point check will be performed using Fair s correlation (Sec. 6.2.12). 

Comment. Figure 6.13 predicts weeping at turndown conditions in both the top and bottom sections, It is necessary to check for dumping and for fraction weep,... 

Comment. Figure 6.20 predicts that at turndown, neither top nor bottom section will dump. However, both sections will operate close to the dump point, and excessive weeping is likely. This is consistent with the prediction from Fair s weep correlation (above), which indicates that at turndown both sections operate well below the weep point. 

Comment. The calculation converged on the first trial to within reasonable accuracy. The calculation showed that the weep fraction is small (<0.07), As per Sec. 6.2.12, a weep fraction of up to 0.1 can be tolerated. This small weep fraction appears inconsistent with what would be expected based on the proximity of the operating point to the predicted dump point and on the large gap between the operating point and the predicted weep point. 

The example reflects the prime difficulty often encountered by tray designers inconsistent predictions from different correlations. The three entrainment flood correlations used gave predictions that widely differed the differences were up to 50 to 60 percent- Another inconsistency was in the weep-dump prediction. These inconsistencias stem from the empiricism associated with prediction methods. Our understanding of tray hydrodynamics has still a very long way to go before it can provide us with models that reliably predict tray performance from first principles. 

FIGURE 8.12 Estimate of excessive weeping. Note Operating points above the respective lines represent salf designs. A point below the line may indicate uncertainty, but not necessarily a dumping station. (From Chemical Engineering, McGraw-Hill, 1977.)... 

FIGURE 12.35 Chart for estimating the vapor load at the point of efficiency drop-off. The drop-off can be due to weeping or dumping. (B. D. Smith, 1963. Design of Equilibrium Stage Processes, New York McGraw-Hill.)... 

Often, a question is asked about the design turndown ratio of a device. It is practical to express this as a ratio of the design loading to the loading at the point of efficiency drop-off, whether represented by physical weeping or dumping. Fignre 12.35 represents this lower limit. 

Accordingly, can operate down to 50% of design rate without serious weeping or dumping. 

The most important requirement for obtaining satisfactory efficiencies is that the plates operate properly. Adequate and intimate contact between vapor and liquid is essential. Any misoperation of the column, such as excessive foaming or entrainment, poor vapor distribution, or short-circuiting, weeping, or dumping of liquid, lowers the plate efficiency. 

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