Valve trays, efficiency

Trays operate within a hydraulic envelope. At excessively high vapor rates, liquid is carried upward from one tray to the next (essentially back mixing the liquid phase in the tower). For valve trays and sieve trays,. i capacity limit can be reached at low vapor rates when liquid falls through the rray floor rather than being forced across the active area into tlic downcomers. Because the liquid does not flow across the trays, it rass.scs contact with the vapor, and the separation efficiency drops dramatically. ... 

Nye Tray, 10-20% increased tray (over sieve or valve) capacity and good efficiency. More capacity from existing column. Improved inlet area for sieve or valve tray with greater area for vapor-liquid disengagement. 

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)].

Figure 13.42. Efficiency of Glitsch V-l valve trays on isobut-ane/butane and cyclohexane/n-heptane as a function of vapor density and percent of flood, measured by Fractionation Research Inc. (Glitsch Inc., Bulletin 160, Dallas, TX, 1958).

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. 

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. 

Important Note Bubble cap HHD factor is equivalent to the DPntAYi dry pressure drop of valve trays. The bubble cap tray total pressure drop factor DPTRay is equivalent to the HDC2 factor of valve-type trays. You may therefore substitute these bubble cap values in the ETF efficiency equations as given for valve trays to determine bubble cap tray efficiency. 

In valve trays, the perforations are equipped with valve units (Fig. 19). At high gas rates, the gas force opens the valves, thus providing area for gas flow. At low gas rates, there is insufficient force to keep many of the valves open, and these close, preventing the liquid from weeping. Sieve and valve trays show comparable capacity, efficiency, and... 

Table 6.1 compares the main tray types. The comparison is general and assumes the trays are properly designed, installed, and operated. Sieve and valve trays have comparable capacity, efficiency, entrain-... 

The O Connell correlation was based on data for bubble-cap trays, and it was stated (131) to predict 90 percent of the efficiency data within 10 percent, both for distillation and absorption. For sieve and valve trays, its predictions are likely to be slightly conservative (151). Ludwig (4) warns that O Connell s absorber correlation (Fig. 7.55, sometimes predicts efficiencies that are too high He believes that it can be used for stripping of gases from rich oils and for absorbers provided care is exercised not to accept too high values. 

Using valve trays. Their sideways gas movement alleviates liquid channeling (Sec. 7.3.2, item 9). On the basis of eliminating the stagnant regions, Biddulph (168) expects valve tray efficiencies in... 

This means both vapor and liquid loadB are raised and lowered simultaneously. Increasing vapor rate reduces efficiency, while increasing liquid rates raises efficiency. The two effects normally cancel each other, and efficiency is practically independent of load changes (assuming no excessive entrainment or weeping). Figure 7.106 shows a typical dependence of tray efficiency on vapor and liquid loads for a commercial-scale distillation column. Anderson et al. (97) show a similar dependence for several different valve trays. 

Value, meaning of, 268 Valve trays, 651-652, 656, 681-686 allowable velocities for, 656-661 cost of, 709 efficiency of, 661-667 pressure drop over, 667, 671-673 VMves, cost of, 511-513 Variability of data, 743-745 Variance, 743... 

Distillation columns were simulated and designed with the CHEMCAD-SCDS method using the Soave-Redlich-Kwong equation of state. Reflux ratio for C-601 was set at 1.5 Rmin - For C-602, C-603, C-604, and C-605 it was 1.2 Runn. Cooling water was available with an inlet temperature of 29°C and an outlet temperature of 35°C. Plate efficiency of the valve trays was assumed to be... 

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