Entrainment flooding, sieve trays

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)].

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]. 

Kister and Haas [184] recommend using 25 dynes/cm in Equation 8-286 when the actual surface tension is a 25 dynes/cm. This correlation is reported [94, 184] to give better effects of physical properties, and predicts most sieve and valve tray entrainment flood data to 15 to 20%, respectively. 

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])... 

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. 

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. 

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... 

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... 

Hole Sizes Small holes slightly enhance tray capacity when limited by entrainment flood. Reducing sieve hole diameters from 13 to 5 mm ( to in) at a fixed hole area typically enhances capacity by 3 to 8 percent, more at low liquid loads. Small holes are effective for reducing entrainment and enhancing capacity in the spray regime (Ql < 20 m3/hm of weir). Hole diameter has only a small effect on pressure drop, tray efficiency, and turndown. 

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. 

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. 

Entrainment flooding is predicted by an updated version of the Souders and Brown correlation. The most popular is Fair s (1961) correlation (Fig. 20), which is suitable for sieve, valve, and bubble-cap trays. Fair s correlation gives the maximum gas velocity as a function of the flow parameter (L/G)V(Pg/Pl), tray spacing, physical properties, and fractional hole area. 

The Smith et al. correlation (20, Fig. 6.11). This is another early entrainment flooding prediction method that has sometimes been recommended (11). Compared to Fair s correlation, the Smith et al. correlation is claimed (20) to be less conservative. It was derived from a small base of field data for sieve, valve, and bubble-cap trays. Similar to Fair s correlation, Smith s correlation uses CSB versus a flow parameter plot, but here the dependence of CSB on the flow parameter is weaker, and there is no surface tension correction factor. CSB and flood 316 both based on the net area AN, and are evaluated from Fig. 6.11 and Eq. (6,9), respectively. The height over the weir, how, is obtained from Eq, (6.49). 

It was shown to predict most of the presently published sieve-tray and valve-tray entrainment flood data to within 15 and 20 percent, respectively. This is an improvement compared to the previous correlations above,... 

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

Valve trays. Manufacturer literature contains correlations for entrainment flooding (7-9). The three sieve tray correlations above are also applicable to valve trays. Of the three, the author recommends the Kister and Haas (15) correlation because it was specifically extended for valve trays and because it possesses the advantages listed above. When applying this correlation to valve trays, is the fractional hole... 

---