O • <»Ci OH*W»i — M-NfpTawC • >4 A - i'iLOwCxthC — * nCPT- >49 I # - I'.O 0UTANC -N tvHNC - Mil k A - iSO-iVUtl- M-tUHXC - 70U k ■ - - N-ByT*r»e - b

II 21 31 41

O • <»Ci OH*W»i — M-NfpTawC • >4 A - i'iLOwCxthC — * nCPT- >49 I # - I'.O 0UTANC -N tvHNC - Mil k A - iSO-iVUtl- M-tUHXC - 70U k ■ - - N-ByT*r»e - b

II 21 31 41

LIQUID RATS imzt'x) i I0a

Figure 6.9 Factors affecting the flood capacity factor. FRI sieve tray test data, Dr * 4 ft, S = 24 in, hw = 2 in, dH = 0.5 in, straight downcomers, AqIAt = 0.13. (a) Effect of liquid rate, Af = 0.08. (i>) Effect of fractional hole area. Cyclohexane-JV-heptane, 24 psia.

For the cyclohexane-N-heptane data, the likely flooding mechanism was spray entrainment flooding. For the butane data, the likely flooding mechanism was downcomer flooding. (Part a from M. Sakata and T. Yanagi, I. Ckem. E. Symp. Ser. 50, p. 3.2/21, 1979. Reprinted courtesy of The Institution of Chemical Engineers, UK Part b reprinted with permission from T. Yanagi and M. Sakata, Ind. Eng. Chern. Proc. Des. Dev., Vol. 21, p. 712, copyright (1982) American Chemical Society.)

incides to the decline of entrainment as liquid load is increased, which occurs in the spray regime (15,22-24,27; see Sec. 6.2.11). There is, however, some uncertainty as to whether the limit in this region is that of actual entrainment flooding, or that of excessive entrainment ("blowing").

■ Fractional hole area: CSB increases with fractional hole area (Fig. 6.9Ö). Roughly, when fractional hole area is between 0.05 and 0.08, an increase in fractional hole area of the order of 0.01 will enhance CSB by about 5 percent (1,18,19,26,28,29). When fractional hole area exceeds 0.1, the rate of increase of CSB with hole area is substantially lower (1,18,19,23,28,29).

■ Hole diameter: CgB increases as hole diameter is reduced. Roughly, CSB increases with the reciprocal of hole diameter to a power of 0.1 to 0.2 (14,15,26,28).

■ CSB is practically independent of pressure in distillation systems {21,25; see also cyclohexane-n-heptane curves on Fig. 6.9o). This suggests that CSB is, at the most, only a very weak function of physical properties. Further, it suggests that any physical property variations that accompany a change in distillation pressure (and, therefore, also in the equilibrium temperature), do not affect CSB. This contradicts trends predicted by some of the earlier flood correlations (18-20).

Fair's correlation (19, Fig. 6.10). The Fair flood has been the standard of the industry for entrainment flood prediction and was recommended by most designers (5,11,18,30-33). CSB is a function of the flow parameter Fu [Eq. (6.7)], tray spacing, surface tension, and fractional hole area. CSB is based on the net area ANt and is evaluated from Fig. 6.10. The flooding vapor velocity is calculated from

Figure 6.10 applies to a fractional hole area of 0.10 or greater. For fractional hole areas of 0.08 and 0.06, CSB from Fig. 6.10 should be multiplied by 0.9 and 0.8, respectively (18).

0 0

Post a comment