Trays, fractionating sieve

Bubble-cap trays may be operated over a far wider range of vapor flows, without loss of tray efficiency. It is the author s experience that bubble-cap trays fractionate better in commercial service than do perforated (valve, or sieve) trays. Why, then, are bubble-cap trays rarely used in a modern distillation ... 

Trays, fractionating assembly of sieve trays, 428 bubblecap, 428,430-433 capacity, F-factor, 429 capacity, Jersey Critical, 432 capacity, Souders-Brown, 432 cartridge, 428 design data sheet, 429 dualflow, 426 efficiency, 439-456 Linde, 430 ripple, 426 sieve, 428,429 turbogrid, 426 types, 426 valve. 429.430.432 Trickle reactors, 576, 607 Tridiagonal matrix, 407 Trommels, 335... 

Testing of plates and other devices is carried out by Fractionation Research, Inc. for industrial sponsors. Some of the test data for sieve plates have been published for the cyclohexane//i-heptane and isobu-tane//i-butane systems. Representative data are shown in Fig. 14-43. These are taken from Sakata and Yanagi Jn.stn. Chem. Engis. Symp. See. No. 56, 3.2/21 (1979)] and Yanagi and Sakata [Jnd. Eng. Chem. Proc. Des. Devel, 21, 712 (1982)]. The column diameter was 1.2 m, tray spacing was 600 mm, and weir height was 50 mm. 

Fractionating coiumn totai cross section vapor veiocity 1.0-1.5 Sieve tray hoie velocity to avoid weeping >12... 

Souders-Brown. The Souders-Brown method (References 1, 2) is based on bubble caps, but is handy for modem trays since the effect of surface tension can be evaluated and factors are included to compare various fractionator and absorber services. These same factors may be found to apply for comparing the services when using valve or sieve trays. A copy of the Souders-Brown C factor chart is shown in Reference 2. 

Large fractional hole area, long flow path relative to tray spacing and high liquid flow rate are the key factors leading to the formation or intensification of vapor cross-flow channeling on sieve and valve trays. 

Fractionating column total cross section vapor velocity Sieve tray hole velocity to avoid 1.0-1.5... 

Packed beds also seem to have a better turndown capability than valve or sieve trays, at low vapor flows. On the other hand, many packed fractionators seem quite intolerant of reduced liquid or reflux flow rates. This is typically a sign of an improperly designed distributor in the packed fractionator. 

Figure 13.41. Efficiencies of some fractionations with several types of trays as a function of vapor factor F = u fp or linear velocity, (a) Data of methanol/water in a column 3.2 ft dia [data of Kastanek, Huml, and Braun, Inst. Chem. Eng. Symp. Ser. 32(5), 100 (1969)]. (b) System cyclohexane/w-heptane in a 4 ft dia sieve column [Sakata and Yanagi, Inst. Chem. Eng. Symp. Ser. 56, 3.2/21 (1979)] valve tray data (Bulletin 160, Glitsch Inc., 1967). (c) Methanol/water [Standart et al., Br. Chem. Eng. 11, 1370 (1966) Sep. Sci. 2, 439 (1967). (d) Styrene/ethylbenzene at lOOTorr [Billet and Raichle, Chem. Ing. Tech. 38, 825 (1966) 40, 377 (1968)]. (e) Ethanol/water (Kirschbaum, Destillier und Rektifiziertechnik, Springer, Berlin, 1969). (f) Methanol/water [Kastanek, Huml, and Braun, Inst. Chem. Eng. Symp. Ser. 32, 5.100, (1969)].

Fractional hole area Af The ratio of hole area to bubbling area (sieve trays) or slot area to bubbling area (valve trays). 

Fractional Hole Area Typical sieve and fixed valve tray hole areas are 8 to 12 percent of the bubbling areas. Smaller fractional hole... 

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. 

The terms in Eqs. (14-123) to (14-126) are in English units and are explained in the Nomenclature. For sieve trays, m= 1.94 and C = 0.79. Note that the constants are a slight revision of those presented in the original paper (C. L. Hsieh, private communication, 1991). Clear liquid height is calculated from Colwell s correlation [Eqs. (14-115) to (14-122)]. The Hsieh and McNulty correlation applies to trays with 9 percent and larger fractional hole area. For trays with smaller hole area, Hsieh and McNulty expect the weeping rate to be smaller than predicted. 

Low dry tray pressure drop. On sieve and fixed valve trays, this means high (>11 percent) fractional hole area. On moving valve trays, this means venturi valves (smooth orifices) or long-legged valves (>15 percent slot area). On all trays, the channeling tendency and severity escalate rapidly as the dry pressure drop diminishes (e.g., as fractional hole area increases). 

Solution Table 14-12 presents measurements by Billet (loc. cit.) for ethyl-benzene-styrene under similar pressure with sieve and valve trays. The column diameter and tray spacing in Billets tests were close to those in Example 9. Since both have single-pass trays, the flow path lengths are similar. The fractional hole area (14 percent in Example 9) is close to that in Table 14-12 (12.3 percent for the tested sieve trays, 14 to 15 percent for standard valve trays). So the values in Table 14-12 should be directly applicable, that is, 70 to 85 percent. So a conservative estimate would be 70 percent. The actual efficiency should be about 5 to 10 percent higher. 

APtotai Total pressure drop across sieve-tray plate, s Fraction of open area on sieve plate. 

Vmin Minimum velocity required for passage of absorption gas through sieve-tray holes, w Weight fraction of nitric acid solution. 

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. 

Fractional hole area (sieve trays). Eight to 10% is generally considered optimum. Higher area may enhance capacity at the expense of more weeping at low gas flow rates. 

Figure 6.1 Continued) Common tray types, (c) Sieve tray id) dual flow tray. [Pari e courtesy ofGlitsch, Inc,] [Peal d courtesy of Fractionation Research Inc. (FRI)-l...

Figure 6.9 Factors affecting the flood capacity factor. FRI sieve tray test data, DT 4 ft, S = 24 in, hw = 2 in, dH = 0,5 in, straight downcomers, AJAT = 0.13. (a) Effect of liquid rate, Af = 0,08, (i>) Effect of fractional hole area. Cydohexane-Af-heptane, 24 psia.

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... 

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