Trays, sieve entrainment

Figure 6.17 Entrainment, sieve trays [44]. (From Petro/ Chemical Engineering, with permission.)...

Sieve trays installed with holes behind false downcomer. Entrained liquid overhead. Limited lean oil flow. Vessel manufacturer error. 

Entrainment about three times that of perforated type plate or sieve tray. Jet-action accompanies bubbling. 

Flexitray Type A Figure 8-72 valve lift w/srp edge orifice, round Koch Higher than Sieve Tray. Lower entrain than Sieve Wide range Low Good... 

Column diameter for a particular service is a function of the physical properties of the vapor and liquid at the tray conditions, efficiency and capacity characteristics of the contacting mechanism (bubble trays, sieve trays, etc.) as represented by velocity effects including entrainment, and the pressure of the operation. Unfortunately the interrelationship of these is not clearly understood. Therefore, diameters are determined by relations correlated by empirical factors. The factors influencing bubble cap and similar devices, sieve tray and perforated plate columns are somewhat different. 

Figure 8-121. Sieve tray entrainment correction. Used by permission, Hunt, C. D A., Hanson, D. N., and Wilke, C. R., The American Institute of Chemical Engineers, Chemical Engineers Journal, V. 1 (1955), p. 441, all rights reserved.

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]. 

As tray spacing increases, entrainment reduces in quantity, but does increase with the sieve tray hole diameter [183, 184], but generally increases with reduction in hole... 

Figure 8-139. Entrainment comparison sieve trays vs. bubble caps for 24-in. tray spacing. Note BCT = Bubble Cap Tray ST = Sieve Tray FP = Flow Parameter. Used by permission, Fair, J. R., Petro-Chem Engineer, Sept. (1961), p. 45, reproduced courtesy of Petroleum Engineer International, Dallas, Texas.

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. 

Data are not available to distinguish between the entrainment of sieve and perforated trays without downcomers. The relation of Himt et al. [33] given for sieve trays is recommended, and should apply quite well. 

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 extent of entrainment of the liquid by the vapour rising over a plate has been studied by many workers. The entrainment has been found to vary with the vapour velocity in the slot or perforation, and the spacing used. Strang 60-1, using an air-water system, found that entrainment was small until a critical vapour velocity was reached, above which it increased rapidly. Similar results from Peavy and Baker 6 11 and Colburn 62 have shown the effect on tray efficiency, which is not seriously affected until the entrainment exceeds 0.1 kmol of liquid per kmol of vapour. The entrainment on sieve trays is discussed in Section 11.10.4. 

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)].

Quantity of Entrainment on the Basis of Sieve Tray Correlations... 

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... 

Dual-Flow Trays These are sieve trays with no downcomers (Fig. 14-27b). Liquid continuously weeps through the holes, hence their low efficiency. At peak loads they are typically 5 to 10 percent less efficient than sieve or valve trays, but as the gas rate is reduced, the efficiency gap rapidly widens, giving poor turndown. The absence of downcomers gives dual-flow trays more area, and therefore greater capacity, less entrainment, and less pressure drop, than conventional trays. Their pressure drop is further reduced by their large fractional hole area (typically 18 to 30 percent of the tower area). However, this low pressure drop also renders dual-flow trays prone to gas and liquid maldistribution. 

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. 

Specifying the need for a tray-type column, the type of tray must be determined. Sieve trays are considered most appropriate for this application. They offer a simple and inexpensive construction with low pressure drop (if the hydraulic design is adequate). Bubble cap and valve-type trays offer advantages in controlling liquid droplet entrainment, but pose significant difficulties for installation of cooling coils. 

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 Moderate Moderate High, about 3 timas higher than sieve trays (4) Low to moderate... 

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,... 

---