Trays, sieve flooding

As with bubble cap flood, sieve tray flood is the ratio of the Vload design (notated here as VN) to the maximum Vload of sieve trays. The Vload (notated here as VM) for sieve trays is factored by surface tension and by the actual cubic feet per second throughflow. 

Figure 8-123 illustrates a typical sieve tray capacity chart. Entrainment by jet flooding or limitation by downcomer flooding are two of the main capacity limiting factors. The liquid backup in the downcomer must balance the pressure drop across the tray, with the process balance [209]. 

For sieve trays Kister s [94, 184] final correlation is for the Souders-Brown flooding coefficient and is essentially independent of pressure. The Kister and Haas correlation [94] [184]... 

Figure 8-137. Flooding capacity, sieve trays weir height is less than 15% of tray spacing low- to non-foaming system hole area at least 10% hole sizes Ms-in. to M-in. dia. surface tension = 20 dynes/cm. Used by permission, Fair, J. R., Petro/Chem. Engineer, Sept (1961), p. 46, reproduced courtesy of Petroleum Engineer International, Dallas, Texas.

Fair [183] relates sieve trays and includes valve tray remarks to the extensive work done for bubble cap trays. Figure 8-137 and 8-139 show flooding data for 24-in. spacing of bubble cap trays from [81] and represents data well for 36-in. diameter columns, and is conservative for smaller columns. Fair s work has been corrected to 20 dynes/ cm surface tension by ... 

The calculated entrainment values may be as good or better than measured values [183]. Figure 8-139 illustrates comparison of entrainment between bubble cap and sieve trays. Fair [183] concludes that for vacuum to moderate pressure applications, sieve trays are advantageous from an entrainment-flooding stand-point. 

Example 8-37 Sieve Tray Splitter Design for Entrainment Flooding Using Fair s Method (used by permission [183])... 

Figure 8-140B. Sieve tray flooding, 6-in. tray spacing.

Figure 8-140. Studies of sieve tray and bubble cap tray flooding (24-in. tray spacing). (Note that the references listed on the illustrations in Figure 8-140 are from the original source, while Ref. 185 Is from this text.) Used by pennission. Fair, J. R., Petro/Chem Engineer, Sept. (1961) p. 45, reproduced courtesy Petroleum Engineer International Dallas, Texas.

Experimental flooding and entrainment data for sieve trays are not plentiful, and measurements are not precise. Accordingly, it has been necessary to relate correlations of flooding and entrainment to those of the well-knowm device, the bubble-cap tray. It appears that the two devices have about the same flooding limits, so long as usual design practice is followed. However, the sieve tray shows entrainment advantages, especially when used in vticuum and atmospheric service. 

Capps [188] compares valve and sieve tray performance as related to capacity and flooding. Also see sieve tray section presented earlier in this chapter. 

Fair, J. R, How to Predict Sieve Tray Entrainment and Flooding, Pelro/Chem Engr. SepL (1961), p. 45. 

Fair, J. R. (1961) Petro/Chem. Eng. 33 (Oct.) 45. How to predict sieve tray entrainment and flooding. 

The method of calculation introduced in this chapter not only allows an exact determination of the column diameter for nonpulsed sieve tray columns, but also allows a good estimation of the diameters of pulsed and stirred extractors. For the latter, however, more exact specific equations exist for the flooding point, see for example [1,4]. 

It is a characteristic of process equipment, that the best operation is reached, at neither a very high nor a very low loading. The intermediate equipment load that results in the most efficient operation is called the the best efficiency point. For distillation trays, the incipient flood point corresponds to the best efficiency point. We have correlated this best efficiency point, for valve and sieve trays, as compared to the measured pressure drops in many chemical plant and refinery distillation towers. We have derived the following formula ... 

All types of valve and sieve trays are always suffering from lost tray efficiency, as a result of both flooding and dumping. Such trays always have some entrained droplets of liquid, lifted by the flowing vapors, to the trays above. This tends to blow butane up into the lighter propane... 

Figure 18.5. Entrainment from sieve trays in the units mols liquid entrained/mol of liquid downflow LM, is the weight rate of flow of liquid and VMv is the weight rate of flow of vapor. The flooding correlation is Figure 13.32(b). [Fair and Matthews, Pet. Refiner 37(4), 153 (195S)].

The conditions of Example 13.15 will be used. This is the case of a standard sieve tray with 24 in. spacing and to operate at 80% of flooding. The entrainment correlation is Figure 18.4 for which the value of the abscissa was found to be... 

For sieve trays, the entrainment flood point can be predicted by using the method by Kister and Haas [Chem. Eng. Progr., 86(9), 63 (1990)]. The method is said to reproduce a large database of measured flood points to within 15 percent. (is,. is based on the net area. 

Example 9 Flooding, of a Distillation Trau An available sieve tray... 

Data specific to tray type must be established next, but these inputs will be discussed later. The data inputted for the next six prompts are the same for all tray types and are primarily for tray efficiency calculations. If tray efficiency or tray liquid residence time values are not desired, these inputs may be skipped (i.e., remain as zero values). However, for bubble cap and sieve trays, the SURF TENS DYN/CM prompt is for active area tray flood calculation. This value should therefore be inputted. 

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. 

Equation (3.91) is the jet flood equation. The chief difference between this equation and the entrainment flood equations, (3.88) through (3.90), is the area references. Equation (3.91) is based on the total sieve tray hole area for gas passage, and Eq. (3.88) through (3.90) are based on the tray active area. Again, the tray active area is simply the tower cross-sectional area less the total downcomer area. 

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